Modern ventilation modes. Artificial ventilation of the lungs in some clinical conditions The ventilator mode is inhaled by a person

Artificial ventilation (Controlled mechanical ventilation - CMV) - a method by which impaired lung functions are restored and maintained - ventilation and gas exchange.

There are many known methods of mechanical ventilation - from the simplest (“mouth to mouth” », “mouth to nose”, using a breathing bag, manual) to complex - mechanical ventilation with precise adjustment of all breathing parameters. The most widely used methods of mechanical ventilation are in which, using a respirator, a gas mixture with a given volume or with a given pressure is introduced into the patient’s respiratory tract. This creates positive pressure in the airways and lungs. After the end of artificial inhalation, the supply of the gas mixture to the lungs stops and exhalation occurs, during which the pressure decreases. These methods are called Intermittent positive pressure ventilation(Intermittent positive pressure ventilation - IPPV). During spontaneous inspiration, contraction of the respiratory muscles reduces the intrathoracic pressure and makes it below atmospheric pressure, and air enters the lungs. The volume of gas entering the lungs with each breath is determined by the amount of negative pressure in the airways and depends on the strength of the respiratory muscles, the rigidity and compliance of the lungs and chest. During spontaneous exhalation, the pressure in the airways becomes weakly positive. Thus, inhalation during spontaneous (independent) breathing occurs at negative pressure, and exhalation occurs at positive pressure in the airways. The so-called average intrathoracic pressure during spontaneous breathing, calculated by the area above and below the zero line of atmospheric pressure, will be equal to 0 during the entire respiratory cycle (Fig. 4.1; 4.2). With intermittent positive pressure ventilation, the average intrathoracic pressure will be positive, since both phases of the respiratory cycle - inhalation and exhalation - are carried out with positive pressure.

Physiological aspects of mechanical ventilation.

Compared to spontaneous breathing, during mechanical ventilation there is an inversion of respiratory phases due to an increase in pressure in the airways during inspiration. Considering mechanical ventilation as a physiological process, it can be noted that it is accompanied by changes in the airways of pressure, volume and flow of inhaled gas over time. By the time inhalation is complete, the volume and pressure curves in the lungs reach their maximum values.

The shape of the inspiratory flow curve plays a certain role:

• constant flow (not changing during the entire inhalation phase);
• decreasing - maximum velocity at the beginning of inspiration (ramp-shaped curve);
• increasing - maximum speed at the end of inspiration;
• sinusoidal flow - maximum speed in the middle of inspiration.

Graphic recording of pressure, volume and flow of inhaled gas allows you to visualize the benefits various types devices, select certain modes and evaluate changes in respiratory mechanics during mechanical ventilation. The type of inhaled gas flow curve determines the pressure in the airways. The highest pressure (P peak) is created with increasing flow at the end of inspiration. This shape of the flow curve, like the sinusoidal one, is rarely used in modern respirators. The greatest benefits are created by a decreasing flow with a ramp curve, especially with assisted ventilation (AVL). This type of curve contributes to the best distribution of inhaled gas in the lungs when the ventilation-perfusion relationship in them is disrupted.

The intrapulmonary distribution of inhaled gas during mechanical ventilation and spontaneous breathing is different. During mechanical ventilation, the peripheral segments of the lungs are ventilated less intensively than the peribronchial areas; dead space increases; rhythmic changes in volumes or pressures cause more intense ventilation of air-filled areas of the lungs and hypoventilation of other parts. Nevertheless, the lungs of a healthy person are well ventilated under a wide variety of spontaneous breathing parameters.

In pathological conditions requiring mechanical ventilation, the conditions for the distribution of inhaled gas are initially unfavorable. Mechanical ventilation in these cases can reduce the unevenness of ventilation and improve the distribution of inspired gas. However, it must be remembered that inadequately selected mechanical ventilation parameters can lead to an increase in ventilation unevenness, a pronounced increase in physiological dead space, a decrease in the effectiveness of the procedure, damage to the pulmonary epithelium and surfactant, atelectasis and an increase in pulmonary shunt. Increased airway pressure can lead to decreased MVR and hypotension. This negative effect often occurs when hypovolemia is not corrected.

Transmural pressure (RTm) determined by the difference in pressure in the alveoli (P alve) and intrathoracic vessels (Fig. 4.3). During mechanical ventilation, the introduction of any DO gas mixture into healthy lungs will normally lead to an increase in P alv. At the same time, this pressure is transmitted to the pulmonary capillaries (Pc). P alv quickly balances out with Pc, these indicators become equal. Rtm will be equal to 0. If the compliance of the lungs due to edema or other pulmonary pathology is limited, the introduction of the same volume of gas mixture into the lungs will lead to an increase in P alv. The transfer of positive pressure to the pulmonary capillaries will be limited and Pc will increase by a smaller amount. Thus, the difference in pressure P alv and Pc will be positive. Rtm on the surface of the alveolar-capillary membrane will lead to compression of the cardiac and intrathoracic vessels. At zero Rtm, the diameter of these vessels will not change [Marino P., 1998].

Indications for mechanical ventilation.

mechanical ventilation in various modifications indicated in all cases where there are acute respiratory disorders leading to hypoxemia and (or) hypercapnia and respiratory acidosis. The classic criteria for transferring patients to mechanical ventilation are RaO 2< 50 мм рт.ст. при оксигенотерапии, РаСО 2 >60 mmHg and pH< 7,3. Анализ газового состава ар­териальной крови - наиболее точный метод оценки функции легких, но, к сожалению, не всегда возможен, особенно в экстренных ситуациях. В этих случаях показаниями к ИВЛ служат Clinical signs acute respiratory disorders: severe shortness of breath accompanied by cyanosis; severe tachypnea or bradypnea; participation of the auxiliary respiratory muscles of the chest and anterior abdominal wall in the act of breathing; pathological breathing rhythms. Transferring the patient to mechanical ventilation is necessary in case of respiratory failure accompanied by agitation, and even more so in case of coma, sallow skin color, increased sweating or changes in the size of the pupils. Determination of respiratory reserves is important in the treatment of ARF. When they decrease critically (BEFORE<5 мл/кг, ЖЕЛ<15 мл/кг, ФЖЕЛ<10 мл/кг, ОМП/ДО>60%) requires mechanical ventilation.

Extremely urgent indications for mechanical ventilation are apnea, agonal breathing, severe hypoventilation and circulatory arrest.

Artificial ventilation of the lungs is carried out:

• in all cases of severe shock, hemodynamic instability, progressive pulmonary edema and respiratory failure caused by bronchopulmonary infection;
• in case of traumatic brain injury with signs of impaired breathing and/or consciousness (indications have been expanded due to the need to treat cerebral edema with hyperventilation and sufficient oxygen supply);
• with severe trauma to the chest and lungs, leading to respiratory failure and hypoxia;
• in case of drug overdose and poisoning with sedatives (immediately, since even minor hypoxia and hypoventilation worsen the prognosis);
• if conservative therapy for acute respiratory failure caused by status asthmaticus or exacerbation of COPD is ineffective;
• with ARDS (the main landmark is a drop in PaO 2, which is not eliminated by oxygen therapy);
• patients with hypoventilation syndrome (central origin or disorders of neuromuscular transmission), as well as if muscle relaxation is necessary (status epilepticus, tetanus, convulsions, etc.).

Prolonged tracheal intubation.

Long-term mechanical ventilation through an endotracheal tube is possible for 5-7 days or more. Both orotracheal and nasotracheal intubation are used. For long-term mechanical ventilation, the latter is preferable, since it is easier to tolerate by the patient and does not limit the intake of water and food. Oral intubation is usually performed for emergency reasons (coma, cardiac arrest, etc.). With oral intubation there is a higher risk of damage to the teeth and larynx and aspiration. Possible complications of nasotracheal intubation may be: nosebleeds, insertion of a tube into the esophagus, sinusitis due to compression of the bones of the nasal sinuses. Maintaining patency of the nasal tube is more difficult, since it is longer and narrower than the oral one. The endotracheal tube must be changed at least every 72 hours. All endotracheal tubes are equipped with cuffs, the inflation of which creates a tight seal between the apparatus and the lungs. However, it should be remembered that insufficiently inflated cuffs lead to leakage of the gas mixture and a decrease in the volume of ventilation set by the doctor on the respirator.

A more dangerous complication may be aspiration of secretions from the oropharynx into the lower respiratory tract. Soft, easy-to-squeeze cuffs designed to minimize the risk of tracheal necrosis do not eliminate the risk of aspiration! Inflating the cuffs must be done very carefully until there is no air leakage. With high pressure in the cuff, necrosis of the tracheal mucosa is possible. When choosing endotracheal tubes, preference should be given to tubes with an elliptical cuff with a larger tracheal occlusion surface.

The timing of replacing an endotracheal tube with a tracheostomy tube should be determined strictly individually. Our experience confirms the possibility of long-term intubation (up to 2-3 weeks). However, after the first 5-7 days, it is necessary to weigh all the indications and contraindications for tracheostomy. If the period of mechanical ventilation is expected to end in the near future, you can leave the tube for a few more days. If extubation is not possible in the near future due to the patient’s serious condition, a tracheostomy should be performed.

Tracheostomy.

In cases of prolonged mechanical ventilation, if sanitation of the tracheobronchial tree is difficult and the patient’s activity is reduced, the question inevitably arises of performing mechanical ventilation through a tracheostomy. Tracheostomy should be treated as a major surgical procedure. Preliminary tracheal intubation is one of the important conditions for the safety of the operation.

Tracheostomy is usually performed under general anesthesia. Before the operation, it is necessary to prepare a laryngoscope and a set of endotracheal tubes, an Ambu bag, and suction. After inserting the cannula into the trachea, the contents are sucked out, the sealing cuff is inflated until gas leakage stops during inhalation, and the lungs are auscultated. It is not recommended to inflate the cuff if spontaneous breathing is maintained and there is no threat of aspiration. The cannula is usually replaced every 2-4 days. It is advisable to postpone the first change of the cannula until the canal is formed by the 5-7th day.

The procedure is carried out carefully, having an intubation kit ready. Changing the cannula is safe if provisional sutures are placed on the tracheal wall during tracheostomy. Pulling these sutures makes the procedure much easier. The tracheostomy wound is treated with an antiseptic solution and a sterile bandage is applied. The secretion from the trachea is sucked out every hour, more often if necessary. The vacuum pressure in the suction system should be no more than 150 mm Hg. To suction the secretion, a plastic catheter 40 cm long with one hole at the end is used. The catheter is connected to a Y-shaped connector, suction is connected, then the catheter is inserted through an intubation or tracheostomy tube into the right bronchus, the free opening of the Y-shaped connector is closed and the catheter is removed with a rotational movement. The duration of suction should not exceed 5-10 s. Then the procedure is repeated for the left bronchus.

Stopping ventilation while secretions are being suctioned may worsen hypoxemia and hypercapnia. To eliminate these undesirable phenomena, a method of suctioning secretions from the trachea without stopping mechanical ventilation or replacing it with high-frequency ventilation (HFIV) has been proposed.

Non-invasive methods of ventilation.

Tracheal intubation and mechanical ventilation in the treatment of ARF have been considered standard procedures over the past four decades. However, tracheal intubation is associated with complications such as nosocomial pneumonia, sinusitis, injuries of the larynx and trachea, stenosis, and bleeding from the upper respiratory tract. Mechanical ventilation with tracheal intubation is called invasive methods of treating ARF.

At the end of the 80s of the 20th century, for long-term ventilation of the lungs in patients with a persistently severe form of respiratory failure due to neuromuscular diseases, kyphoscoliosis, idiopathic central hypoventilation, a new method of respiratory support was proposed - non-invasive, or auxiliary, mechanical ventilation using nasal and face masks (VIVL). ). IVL does not require the use of artificial airways - tracheal intubation, tracheostomy, which significantly reduces the risk of infectious and “mechanical” complications. In the 90s, the first reports appeared on the use of IVL in patients with ARF. Researchers noted the high efficiency of the method.

The use of IVL in patients with COPD contributed to a reduction in deaths, a reduction in the length of stay of patients in the hospital, and a reduction in the need for tracheal intubation. However, the indications for long-term IVL cannot be considered definitively established. The criteria for selecting patients for IVL in ARF are not unified.

Mechanical ventilation modes

Volume controlled ventilation(volume, or traditional, mechanical ventilation - Conventional ventilation) is the most common method in which a specified DO is introduced into the lungs during inhalation using a respirator. In this case, depending on the design features of the respirator, you can set the DO or MOB, or both indicators. RR and airway pressure are arbitrary values. If, for example, the MOB value is 10 l, and the DO value is 0.5 l, then the RR will be 10: 0.5 = 20 per minute. In some respirators, the respiratory rate is set independently of other parameters and is usually 16-20 per minute. The pressure in the airways during inspiration, in particular its maximum peak (Ppeak) value, depends on the volumetric volume, the shape of the flow curve, the duration of inspiration, airway resistance and the compliance of the lungs and chest. Switching from inhalation to exhalation is carried out after the end of the inhalation time at a given RR or after introducing a given RR into the lungs. Exhalation occurs after the respirator valve opens passively under the influence of elastic traction of the lungs and chest (Fig. 4.4).

DO is set at the rate of 10-15, more often 10-13 ml/kg body weight. An inappropriately chosen DO significantly affects gas exchange and maximum pressure during the inhalation phase. With an inadequately small DO, part of the alveoli is not ventilated, as a result of which atelectatic foci are formed, causing intrapulmonary shunt and arterial hypoxemia. Too much BP leads to a significant increase in airway pressure during inspiration, which can cause pulmonary barotrauma. An important adjustable parameter of mechanical ventilation is the inhalation/exhalation time ratio, which largely determines the average pressure in the airways during the entire respiratory cycle. A longer inhalation provides better distribution of gas in the lungs during pathological processes accompanied by uneven ventilation. Lengthening the expiratory phase is often necessary in case of broncho-obstructive diseases that reduce the expiratory rate. Therefore, modern respirators have the ability to regulate the time of inhalation and exhalation (T i and T E) within a wide range. In volumetric respirators, T i modes are more often used: T e = 1: 1; 1: 1.5 and 1: 2. These modes help improve gas exchange, increase PaO 2 and make it possible to reduce the fraction of inhaled oxygen (IOX). The relative lengthening of the inspiratory time allows, without reducing the tidal volume, to reduce the P peak during inspiration, which is important for the prevention of pulmonary barotrauma. During mechanical ventilation, a mode with an inspiratory plateau is also widely used, achieved by interrupting the flow after the end of inspiration (Fig. 4.5). This mode is recommended for long-term mechanical ventilation. The duration of the inspiratory plateau can be set arbitrarily. Its recommended parameters are 0.3-0.4 s or 10-20% of the duration of the respiratory cycle. This plateau also improves the distribution of the gas mixture in the lungs and reduces the risk of barotrauma. The pressure at the end of the plateau actually corresponds to the so-called elastic pressure, it is considered equal to the alveolar pressure. The difference between P peak and P plateau is equal to the resistive pressure. In this case, it becomes possible to determine during mechanical ventilation the approximate value of the extensibility of the lungs - chest system, but for this you need to know the flow speed [Kassil V.L. et al., 1997].

The choice of MOB can be approximate or carried out under the control of arterial blood gas levels. Due to the fact that PaO 2 can be influenced by a large number of factors, the adequacy of mechanical ventilation is determined by PaCO 2 . Both with controlled ventilation and in the case of an approximate establishment of MOB, moderate hyperventilation is preferred, maintaining PaCO 2 at a level of 30 mm Hg. (4 kPa). The advantages of such tactics can be defined as follows: hyperventilation is less dangerous than hypoventilation; with a higher MOB there is less risk of lung collapse; in case of hypocapnia, synchronization of the device with the patient is facilitated; hypocapnia and alkalosis are more favorable for the action of certain pharmacological agents; in conditions of reduced PaCO 2 the danger of cardiac arrhythmias decreases.

Given that hyperventilation is a routine technique, one should be aware of the danger of a significant decrease in MVR and cerebral blood flow due to hypocapnia. A drop in PaCO 2 below the physiological norm suppresses the stimulus for spontaneous breathing and can cause unreasonably prolonged mechanical ventilation. In patients with chronic acidosis, hypocapnia leads to depletion of the bicarbonate buffer and delayed recovery after mechanical ventilation. In high-risk patients, maintaining appropriate MOB and PaCO 2 is vital and should only be carried out under strict laboratory and clinical control.

Long-term mechanical ventilation with constant DO makes the lungs less elastic. Due to the increase in the volume of residual air in the lungs, the ratio of the values ​​of DO and FRC changes. Improving the conditions of ventilation and gas exchange is achieved by periodically deepening breathing. To overcome the monotony of ventilation, respirators provide a mode that periodically inflates the lungs. The latter helps to improve the physical characteristics of the lungs and, first of all, increase their extensibility. When introducing an additional volume of a gas mixture into the lungs, the danger of barotrauma should be remembered. In the intensive care unit, lung inflation is usually done using a large Ambu bag.

The effect of mechanical ventilation with intermittent positive pressure and passive expiration on cardiac activity.

Mechanical ventilation with intermittent positive pressure and passive expiration has a complex effect on the cardiovascular system. During the inspiratory phase, increased intrathoracic pressure is created and venous flow to the right atrium is reduced if the pressure in the chest is equal to the venous pressure. Intermittent positive pressure with balanced alveolocapillary pressure does not increase transmural pressure and does not change afterload on the right ventricle. If the transmural pressure increases during lung inflation, the load on the pulmonary arteries increases and the afterload on the right ventricle increases.

Moderate positive intrathoracic pressure increases venous inflow to the left ventricle because it promotes the flow of blood from the pulmonary veins into the left atrium. Positive intrathoracic pressure also reduces left ventricular afterload and results in increased cardiac output (CO).

If chest pressure is very high, left ventricular filling pressure may decrease due to increased afterload on the right ventricle. This can lead to overdistension of the right ventricle, shift of the interventricular septum to the left, and decreased filling volume of the left ventricle.

Intravascular volume has a great influence on the state of pre- and afterload. With hypovolemia and low central venous pressure (CVP), increased intrathoracic pressure leads to a more pronounced decrease in venous inflow to the lungs. CO also decreases, which depends on inadequate filling of the left ventricle. An excessive increase in intrathoracic pressure, even with normal intravascular volume, reduces the diastolic filling of both ventricles and CO.

Thus, if PPD is carried out under conditions of normovolemia and the selected modes are not accompanied by an increase in transmural capillary pressure in the lungs, then there is no negative effect of the method on the activity of the heart. Moreover, the possibility of increased CO and BP systems should be taken into account during cardiopulmonary resuscitation (CPR). Inflating the lungs manually with a sharply reduced CO and zero blood pressure contributes to an increase in CO and a rise in BP [Marino P., 1998].

mechanical ventilation With positive pressure V end exhalation (PEEP)

(Continuous positive pressure ventilation - CPPV - Positive end-expiratory pressure - PEEP). In this mode, the pressure in the airways during the final phase of exhalation does not decrease to 0, but is maintained at a given level (Fig. 4.6). PEEP is achieved using a special unit built into modern respirators. A large amount of clinical material has been accumulated indicating the effectiveness of this method. PEEP is used in the treatment of ARF associated with severe pulmonary diseases (ARDS, common pneumonia, chronic obstructive pulmonary diseases in the acute stage) and pulmonary edema. However, it has been proven that PEEP does not reduce and may even increase the amount of extravascular water in the lungs. At the same time, the PEEP mode promotes a more physiological distribution of the gas mixture in the lungs, reducing venous shunt, improving the mechanical properties of the lungs and oxygen transport. There is evidence that PEEP restores surfactant activity and reduces its bronchoalveolar clearance.

When choosing the PEEP mode, you should keep in mind that it can significantly reduce CO. The higher the final pressure, the more significant the effect of this regime on hemodynamics. A decrease in CO can occur at a PEEP of 7 cm water column. and more, which depends on the compensatory capabilities of the cardiovascular system. Increasing pressure to 12 cm water column. contributes to a significant increase in the load on the right ventricle and an increase in pulmonary hypertension. The negative effects of PEEP may largely depend on errors in its use. You should not immediately create a high level of PEEP. The recommended initial PEEP level is 2-6 cm of water column. Increasing end-expiratory pressure should be carried out gradually, “step by step” and in the absence of the desired effect from the set value. Increase PEEP by 2-3 cm of water column. no more than every 15-20 minutes. PEEP is especially carefully increased after 12 cm of water column. The safest level of the indicator is 6-8 cm of water column, but this does not mean that this mode is optimal in every situation. With a large venous shunt and severe arterial hypoxemia, a higher level of PEEP with a VFC of 0.5 or higher may be required. In each specific case, the PEEP value is selected individually! A prerequisite is a dynamic study of arterial blood gases, pH and central hemodynamic parameters: cardiac index, filling pressure of the right and left ventricles and total peripheral resistance. In this case, the compliance of the lungs should also be taken into account.

PEEP promotes the “opening” of non-functioning alveoli and atelectatic areas, resulting in improved ventilation of alveoli that were insufficiently ventilated or not ventilated at all and in which blood shunting occurred. The positive effect of PEEP is due to an increase in the functional residual capacity and compliance of the lungs, an improvement in ventilation-perfusion relationships in the lungs and a decrease in the alveolar-arterial oxygen difference.

The correctness of the PEEP level can be determined by the following main indicators:

• no negative effect on blood circulation;
• increased lung compliance;
• reduction of pulmonary shunt.

The main indication for PEEP is arterial hypoxemia, which is not eliminated by other modes of mechanical ventilation.

Characteristics of ventilation modes with volume regulation:

• the most important parameters of ventilation (DO and MOB), as well as the ratio of the duration of inhalation and exhalation, are established by the doctor;
• precise control of the adequacy of ventilation with the selected FiO 2 is carried out by analyzing the gas composition of arterial blood;
• established volumes of ventilation, regardless of the physical characteristics of the lungs, do not guarantee optimal distribution of the gas mixture and uniform ventilation of the lungs;
• To improve ventilation-perfusion relationships, periodic inflation of the lungs or mechanical ventilation in the PEEP mode is recommended.

Pressure controlled ventilation during the inspiratory phase - a widespread mode. One of the modes of ventilation, which has become increasingly popular in recent years, is pressure-controlled ventilation with inverse inhalation: exhalation time ratio (PC-IRV). This method is used for severe lung lesions (common pneumonia, ARDS), requiring a more careful approach to respiratory therapy. It is possible to improve the distribution of the gas mixture in the lungs with a lower risk of barotrauma by lengthening the inspiratory phase within the respiratory cycle under the control of a given pressure. Increasing the inspiratory/expiratory ratio to 4:1 reduces the difference between peak airway pressure and alveolar pressure. Ventilation of the alveoli occurs during inhalation, and during the short phase of exhalation, the pressure in the alveoli does not decrease to 0 and they do not collapse. The pressure amplitude with this ventilation mode is less than with PEEP. The most important advantage of pressure-controlled ventilation is the ability to control peak pressure. The use of ventilation with regulation according to DO does not create this possibility. A given DO is accompanied by an unregulated peak alveolar pressure and can lead to overinflation of non-collapsed alveoli and their damage, while some of the alveoli will not be adequately ventilated. An attempt to reduce P alv by reducing DO to 6-7 ml/kg and correspondingly increasing RR does not create conditions for uniform distribution of the gas mixture in the lungs. Thus, the main advantage of mechanical ventilation with regulation by pressure indicators and an increase in the duration of inspiration is the possibility of complete oxygenation of arterial blood at lower tidal volumes than with volumetric ventilation (Fig. 4.7; 4.8).

Characteristic features of mechanical ventilation with adjustable pressure and inverted inhalation/exhalation ratio:

• the maximum pressure level Ppeak and ventilation frequency are set by the doctor;
• P peak and transpulmonary pressure are lower than with volumetric ventilation;
• the duration of inhalation is longer than the duration of exhalation;
• distribution of the inhaled gas mixture and oxygenation of arterial blood is better than with volumetric ventilation;
• positive pressure is created throughout the entire respiratory cycle;
• during exhalation, positive pressure is created, the level of which is determined by the duration of exhalation - the higher the pressure, the shorter the exhalation;
• ventilation of the lungs can be carried out with a lower DO than with volumetric ventilation [Kassil V.L. et al., 1997].

Assisted ventilation

Assisted controlled mechanical ventilation - ACMV, or AssCMV - mechanical support for the patient’s spontaneous breathing. During the onset of spontaneous inspiration, the ventilator delivers artificial breath. Pressure drop in the respiratory tract by 1-2 cm water column. during the beginning of inhalation, it affects the trigger system of the device, and it begins to supply the released DO, reducing the work of the respiratory muscles. VIVL allows you to set the necessary, most optimal RR for a given patient.

Adaptive method of IVL.

This method of performing mechanical ventilation is that the ventilation frequency, as well as other parameters (DO, the ratio of the duration of inhalation and exhalation), are carefully adapted (“adjusted”) to the spontaneous breathing of the patient. Based on the preliminary parameters of the patient's breathing, they usually set the initial frequency of the respiratory cycles of the device to be 2-3 more than the frequency of the patient's spontaneous breathing, and the device's BP is 30-40% higher than the patient's own BP at rest. The patient's adaptation is easier with an inhalation/exhalation ratio = 1:1.3, using a PEEP of 4-6 cm H2O. and when an additional inhalation valve is included in the RO-5 respirator circuit, allowing the entry of atmospheric air when the instrumental and spontaneous respiratory cycles do not coincide. The initial period of adaptation is carried out with two to three short-term sessions of VIVL (VNVL) for 15-30 minutes with 10-minute breaks. During breaks, taking into account the patient’s subjective sensations and the degree of respiratory comfort, ventilation is adjusted. Adaptation is considered sufficient when there is no resistance to inhalation, and chest excursions coincide with the phases of the artificial respiratory cycle.

Trigger method of IVL

carried out using special respirator components (“trigger block” or “response” system). The trigger block is designed to switch the dispenser from inhalation to exhalation (or vice versa) due to the patient's respiratory effort.

The operation of the trigger system is determined by two main parameters: the sensitivity of the trigger and the “response” speed of the respirator. The sensitivity of the unit is determined by the smallest amount of flow or negative pressure required to operate the respirator switching device. If the sensitivity of the device is low (for example, 4-6 cm H2O), it will require too much effort on the part of the patient to initiate assisted breathing. With increased sensitivity, the respirator, on the contrary, can react to random causes. The flow sensitive trigger unit should respond to a flow of 5-10 ml/s. If the trigger block is sensitive to negative pressure, then the vacuum response of the device should be 0.25-0.5 cm of water column. [Yurevich V.M., 1997]. Such speed and vacuum during inspiration can be created by a weakened patient. In all cases, the trigger system must be adjustable to create better conditions for the patient's adaptation.

Trigger systems in various respirators are regulated by pressure (pressure triggering), flow rate (flow triggering, flow by) or by volume triggering (volume triggering). The inertia of the trigger block is determined by the “delay time”. The latter should not exceed 0.05-0.1 s. The auxiliary inhalation should occur at the beginning, and not at the end, of the patient’s inhalation and, in any case, should coincide with his inhalation.

A combination of mechanical ventilation and IVL is possible.

Artificially assisted ventilation

(Assist/Control ventilation - Ass/CMV, or A/CMV) - a combination of mechanical ventilation and mechanical ventilation. The essence of the method is that the patient is subjected to traditional mechanical ventilation with up to 10-12 ml/kg, but the frequency is set so that it provides minute ventilation within 80% of the required one. In this case, the trigger system must be turned on. If the design of the device allows, then use the pressure support mode. This method has gained great popularity in recent years, especially when the patient adapts to mechanical ventilation and when the respirator is turned off.

Since the MOB is slightly lower than required, the patient attempts to breathe independently, and the trigger system provides additional breaths. This combination of mechanical ventilation and IVL is widely used in clinical practice.

It is advisable to use artificially assisted ventilation with traditional mechanical ventilation for gradual training and restoration of respiratory muscle function. The combination of mechanical ventilation and mechanical ventilation is widely used both during the adaptation of patients to mechanical ventilation and mechanical ventilation modes, and during the period of switching off the respirator after long-term mechanical ventilation.

Support breathing pressure

(Pressure support ventilation - PSV, or PS). This mode of triggered ventilation is based on the fact that a positive constant pressure is created in the system between the device and the patient’s airways. When a patient tries to inhale, the trigger system is activated, which responds to a decrease in pressure in the circuit below a preset PEEP level. It is important that during the period of inspiration, as well as during the entire respiratory cycle, there are no episodes of even a short-term decrease in pressure in the respiratory tract below atmospheric pressure. When an attempt is made to exhale and the pressure in the circuit increases above the set value, the inspiratory flow is interrupted and the patient exhales. The airway pressure quickly decreases to the PEEP level.

The (PSV) regimen is usually well tolerated by patients. This is due to the fact that pressure support for breathing improves alveolar ventilation when intravascular water content in the lungs is increased. Each of the patient’s attempts to inhale leads to an increase in the gas flow supplied by the respirator, the speed of which depends on the patient’s share of participation in the act of breathing. DO with pressure support is directly proportional to the set pressure. In this mode, oxygen consumption and energy consumption are reduced, and the positive effects of mechanical ventilation clearly predominate. Of particular interest is the principle of proportional assisted ventilation, which consists in the fact that during a vigorous inspiration, the patient's volumetric flow rate increases at the very beginning of inspiration, and the set pressure is achieved faster. If the inspiratory attempt is weak, then the flow continues almost until the end of the inhalation phase and the set pressure is reached later.

The Bird-8400-ST respirator has a Pressure Support modification that provides the specified DO.

Pressure Support Ventilation (PSV) Features:

• the P peak level is set by the doctor and the value of V t depends on it;
• a constant positive pressure is created in the apparatus-respiratory tract system of the patient;
• for each independent breath of the patient, the device responds by changing the volumetric flow rate, which is adjusted automatically and depends on the patient’s inspiratory effort;
• The respiratory rate and the duration of the phases of the respiratory cycle depend on the patient’s breathing, but within certain limits they can be regulated by the doctor;
• the method is easily compatible with mechanical ventilation and PPVL.

When a patient tries to inhale, after 35-40 ms the respirator begins to supply a flow of gas mixture into the airways until a certain set pressure is reached, which is maintained throughout the patient’s entire inhalation phase. The peak flow rate occurs at the beginning of the inspiratory phase, which does not lead to a flow deficit. Modern respirators are equipped with a microprocessor system that analyzes the shape of the curve and the flow rate and selects the most optimal mode for a given patient. Pressure breathing support in the described mode and with some modifications is used in respirators “Bird 8400 ST”, “Servo-ventilator 900 C”, “Engstrom-Erika”, “Purittan-Bennet 7200”, etc.

Intermittent forced ventilation (IPVV)

(Intermittent mandatory ventilation - IMV) is a method of assisted ventilation in which the patient breathes independently through a respirator circuit, but at randomly specified intervals one mechanical breath is taken with a given DO (Fig. 4.9). As a rule, synchronized PPV (Synchronized intermittent mandatory ventilation - SIMV) is used, i.e. the beginning of the instrumental inhalation coincides with the beginning of the patient’s spontaneous inhalation. In this mode, the patient himself performs the main work of breathing, which depends on the frequency of the patient’s spontaneous breathing, and in the intervals between breaths, inhalation is carried out using the trigger system. These intervals can be adjusted arbitrarily by the doctor; mechanical inhalation is carried out after 2, 4, 8, etc. the patient's next attempts. With PPV, a decrease in pressure in the respiratory tract is not allowed and PEEP must be used to support breathing. Each independent breath of the patient is accompanied by pressure support, and against this background, a mechanical breath occurs with a certain frequency [Kassil V.L. et al., 1997].

Main characteristics of PPVL:

• assisted ventilation is combined with mechanical inhalation at a given DO;
• the respiratory rate depends on the frequency of the patient’s inspiratory attempts, but it can also be adjusted by the doctor;
• MOB is the sum of spontaneous breathing and MO of mandatory breaths; the doctor can regulate the patient’s breathing work by changing the frequency of forced breaths; the method may be compatible with pressure ventilation support and other IVL methods.

High frequency ventilation

High-frequency ventilation is considered to be ventilation with a frequency of respiratory cycles of more than 60 per minute. This value was chosen because at the specified frequency of switching the phases of respiratory cycles, the main property of HF mechanical ventilation is manifested - constant positive pressure (CPP) in the respiratory tract. Naturally, the frequency limits from which this property manifests itself are quite wide and depend on the MOB, the compliance of the lungs and chest, the speed and method of insufflation of the respiratory mixture and other reasons. However, in the vast majority of cases, it is at a frequency of respiratory cycles of 60 per minute that PPD is created in the patient’s respiratory tract. This value is convenient for converting the ventilation frequency into hertz, which is useful for calculations in higher ranges and comparison of the results obtained with foreign analogues. The frequency range of respiratory cycles is very wide - from 60 to 7200 per minute (1-120 Hz), however, the upper limit of the frequency of HF ventilation is considered to be 300 per minute (5 Hz). At higher frequencies, it is inappropriate to use passive mechanical switching of the phases of the respiratory cycles due to large losses of DO during switching; it becomes necessary to use active methods of interrupting the injected gas or generating its oscillations. In addition, when the frequency of HF mechanical ventilation is above 5 Hz, the amplitude pressure values ​​in the trachea become practically insignificant [Molchanov I.V., 1989].

The reason for the formation of PPD in the respiratory tract during HF mechanical ventilation is the effect of “interrupted exhalation”. It is obvious that, with other parameters unchanged, an increase in respiratory cycles leads to an increase in constant positive and maximum pressures with a decrease in the pressure amplitude in the airways. An increase or decrease in DO causes corresponding changes in pressure. Shortening the inspiratory time leads to a decrease in POP and an increase in maximum and amplitude pressure in the airways.

Currently, the three most common methods of HF ventilation are volumetric, oscillatory and jet.

Volumetric HF ventilation (High frequency positive pressure ventilation - HFPPV) with a given flow or a given DO is often referred to as HF positive pressure ventilation. The frequency of respiratory cycles is usually 60-110 per minute, the duration of the insufflation phase does not exceed 30% of the cycle duration. Alveolar ventilation is achieved at reduced DO and the specified frequency. The FRC increases, conditions are created for the uniform distribution of the respiratory mixture in the lungs (Fig. 4.10).

In general, volumetric HF mechanical ventilation cannot replace traditional mechanical ventilation and is of limited use: during lung operations with the presence of bronchopleural fistulas, to facilitate the adaptation of patients to other modes of mechanical ventilation , when the respirator is turned off.

Oscillatory HF ventilation (High frequency oscillation - HFO, HFLO) is a modification of apneic “diffusion” breathing. Despite the absence of respiratory movements, this method achieves high oxygenation of arterial blood, but the elimination of CO 2 is impaired, which leads to respiratory acidosis. It is used for apnea and the impossibility of rapid tracheal intubation in order to eliminate hypoxia.

Jet HF ventilation (High frequency jet ventilation - HFJV) is the most common method. In this case, three parameters are regulated: ventilation frequency, operating pressure, i.e. the pressure of the respiratory mixture supplied to the patient's hose and the inhalation/exhalation ratio.

There are two main methods of HF ventilation: injection and transcatheter. The injection method is based on the Venturi effect: a stream of oxygen supplied under a pressure of 1-4 kgf/cm 2 through the injection cannula creates a vacuum around the latter, as a result of which atmospheric air is sucked in. Using connectors, the injector is connected to the endotracheal tube. Through the additional injector pipe, atmospheric air is sucked in and the exhaled gas mixture is discharged. This makes it possible to implement jet HF ventilation with a leaky breathing circuit.

Barotrauma of the lungs

Barotrauma during mechanical ventilation is lung damage caused by increased pressure in the respiratory tract. It is worth pointing out two main mechanisms that cause barotrauma: 1) overinflation of the lungs; 2) uneven ventilation against the background of an altered lung structure.

During barotrauma, air can enter the interstitium, mediastinum, neck tissue, cause pleural rupture, and even penetrate into the abdominal cavity. Barotrauma is a serious complication that can be fatal. The most important condition for the prevention of barotrauma is monitoring of respiratory biomechanics, careful auscultation of the lungs, and periodic X-ray monitoring of the chest. If a complication occurs, early diagnosis is necessary. Delay in diagnosing pneumothorax significantly worsens the prognosis!

Clinical signs of pneumothorax may be absent or nonspecific. Auscultation of the lungs during mechanical ventilation often does not reveal changes in breathing. The most common signs are sudden hypotension and tachycardia. Palpation of air under the skin of the neck or upper chest is a pathognomonic symptom of pulmonary barotrauma. If barotrauma is suspected, an urgent chest x-ray is necessary. An early symptom of barotrauma is the identification of interstitial pulmonary emphysema, which should be considered a harbinger of pneumothorax. In a vertical position, the air is usually localized in the apical part of the pulmonary field, and in a horizontal position, in the anterior costophrenic groove at the base of the lung.

When performing mechanical ventilation, pneumothorax is dangerous due to the possibility of compression of the lungs, large vessels and heart. Therefore, detected pneumothorax requires immediate drainage of the pleural cavity. It is better to inflate the lungs without using suction, using the Bullau method, since the negative pressure created in the pleural cavity can exceed the transpulmonary pressure and increase the speed of air flow from the lung into the pleural cavity. However, as experience shows, in some cases it is necessary to apply dosed negative pressure in the pleural cavity for better expansion of the lungs.

Ventilation withdrawal methods

The restoration of spontaneous breathing after prolonged mechanical ventilation is accompanied not only by the resumption of respiratory muscle activity, but also by a return to normal ratios of intrathoracic pressure fluctuations. Changes in pleural pressure from positive to negative values ​​lead to important hemodynamic changes: venous return increases, but afterload on the left ventricle also increases, and systolic stroke volume may fall as a result. Rapid removal of the respirator may cause cardiac dysfunction. Stopping mechanical ventilation is possible only after eliminating the causes that caused the development of ARF. In this case, many other factors must be taken into account: the general condition of the patient, neurological status, hemodynamic parameters, water and electrolyte balance and, most importantly, the ability to maintain adequate gas exchange during spontaneous breathing.

The method of transferring patients after long-term mechanical ventilation to spontaneous breathing with “weaning” from the respirator is a complex multi-stage procedure, including many technical techniques - physical therapy, respiratory muscle training, physiotherapy for the chest area, nutrition, early activation of patients, etc. [Gologorsky V. A. et al., 1994].

There are three methods of canceling mechanical ventilation: 1) using PPVL; 2) using a T-shaped connector or T-shaped method; 3) using IVL sessions.

1. Intermittent forced ventilation. This method provides the patient with a certain level of mechanical ventilation and allows the patient to breathe independently in the intervals between the use of the respirator. The periods of mechanical ventilation are gradually reduced and the periods of spontaneous breathing are increased. Finally, the duration of mechanical ventilation is reduced until it is completely stopped. This technique is unsafe for the patient, since spontaneous breathing is not supported by anything.
2. T-shaped method. In these cases, periods of mechanical ventilation alternate with sessions of spontaneous breathing through a T-insert connector with the respirator running. Oxygen-enriched air comes from the respirator, preventing atmospheric and exhaled air from entering the patient’s lungs. Even with good clinical indicators, the first period of spontaneous breathing should not exceed 1-2 hours, after which mechanical ventilation should be resumed for 4-5 hours to ensure the patient’s rest. By increasing the frequency and duration of spontaneous ventilation, the latter is stopped for the entire day, and then for the whole day. The T-shaped method allows you to more accurately determine indicators of pulmonary function during dosed spontaneous breathing. This method is superior to PPVL in terms of the effectiveness of restoring the strength and performance of the respiratory muscles.
3. Assisted respiratory support method. In connection with the emergence of various methods of mechanical ventilation, it has become possible to use them during the period of weaning patients from mechanical ventilation. Among these methods, the most important is IVL, which can be combined with PEEP and HF ventilation modes.

The trigger mode of ventilation is usually used. Numerous descriptions of methods published under different names make it difficult to understand their functional differences and capabilities.

The use of assisted ventilation sessions in trigger mode improves respiratory function and stabilizes blood circulation. DO increases, RR decreases, RaO 2 levels increase.

Through repeated use of IVL with systematic alternation with IVL in PEEP modes and with spontaneous breathing, it is possible to achieve normalization of the respiratory function of the lungs and gradually “wean” the patient from respiratory care. The number of IVL sessions can be different and depends on the dynamics of the underlying pathological process and the severity of pulmonary changes. The IVL mode with PEEP provides an optimal level of ventilation and gas exchange, does not depress cardiac activity and is well tolerated by patients. These techniques can be supplemented with HF ventilation sessions. Unlike HF mechanical ventilation, which creates only a short-term positive effect, IVL modes improve lung function and have an undoubted advantage over other methods of canceling mechanical ventilation.

Features of nursing

Patients undergoing mechanical ventilation should be under continuous monitoring. Monitoring of blood circulation indicators and blood gas composition is especially necessary. The use of alarm systems is shown. It is customary to measure exhaled volume using dry spirometers and ventilometers. High-speed analyzers of oxygen and carbon dioxide (capnograph), as well as electrodes for recording transcutaneous PO 2 and PCO 2, greatly facilitate obtaining the most important information about the state of gas exchange. Currently, monitor monitoring of such characteristics as the shape of pressure and gas flow curves in the respiratory tract is used. Their information content makes it possible to optimize mechanical ventilation modes, select the most favorable parameters and predict therapy.

New perspectives on respiratory therapy

Currently, there is a trend towards the use of pressocyclic modes of auxiliary and forced ventilation. In these modes, unlike traditional ones, the DO value is reduced to 5-7 ml/kg (instead of 10-15 ml/kg body weight), positive pressure in the respiratory tract is maintained by increasing the flow and changing the time ratio of the inhalation and exhalation phases. In this case, the maximum P peak is 35 cm water column. This is due to the fact that spirographic determination of the values ​​of DO and MOD is associated with possible errors caused by artificially induced spontaneous hyperventilation. In studies using inductive plethysmography, it was found that the values ​​of DR and MOR are smaller, which served as the basis for reducing DR with the developed methods of mechanical ventilation.

Modes of artificial ventilation

• Airway pressure release ventilation - APRV - ventilation of the lungs with periodic reduction of pressure in the inhalation tract.
• Assist control ventilation - ACV - auxiliary controlled ventilation (VUVL).
• Assisted controlled mechanical ventilation - ACMV (AssCMV) artificially assisted ventilation.
• Biphasic positive airway pressure - BIPAP - ventilation with two phases of positive airway pressure (BPAP) modification of mechanical ventilation and IVL.
• Continuous distending pressure - CDP - spontaneous breathing with constantly positive pressure in the respiratory tract (CPAP).
• Controlled mechanical ventilation - CMV - controlled (artificial) ventilation.
• Continuous positive air-way pressure - CPAP - spontaneous breathing with positive airway pressure (CPAP).
• Continuous positive pressure ventilation - CPPV - ventilation with positive end-expiratory pressure (PEEP, Positive end-expirator psessure - PEEP).
• Conventional ventilation - traditional (conventional) ventilation.
• Extended mandatory minute volume (ventilation) - EMMV - PPVL with automatic provision of a given MOU.
• High frequency jet ventilation - HFJV - high-frequency injection (jet) ventilation - HF IVL.
• High frequency oscillation - HFO (HFLO) - high frequency oscillation (oscillatory HF ventilation).
• High frequency positive pressure ventilation - HFPPV - HF ventilation under positive pressure, volume controlled.
• Intermittent mandatory ventilation - IMV - forced intermittent ventilation (PPVL).
• Intermittent positive negative pressure ventilation - IPNPV - ventilation with negative pressure on exhalation (with active exhalation).
• Intermittent positive pressure ventilation - IPPV - ventilation of the lungs with intermittent positive pressure.
• Intratracheal pulmonary ventilation - ITPV - intratracheal pulmonary ventilation.
• Inverse ratio ventilation - IRV - ventilation with a reverse (inverted) inhalation:exhalation ratio (more than 1:1).
• Low frequency positive pressure ventilation - LFPPV - low frequency ventilation (bradypnoic).
• Mechanical ventilation - MV - mechanical ventilation (MV).
• Proportional assist ventilation - PAV - proportional assisted ventilation (VVL), a modification of pressure ventilation support.
• Prolonged mechanical ventilation - PMV - prolonged mechanical ventilation.
• Pressure limit ventilation - PLV - pressure-limited ventilation during inhalation.
• Spontaneous breathing - S.B. - independent breathing.
• Synchronized intermittent mandatory ventilation - SIMV - synchronized forced intermittent ventilation (SPPVL).

Ventilation modes are determined by the method of switching from exhalation to inhalation, as well as the possibility of combining respiratory support with spontaneous breathing (Table 50-3 and Fig. 50-1). Most modern ventilators allow ventilation in several modes, and in devices with microprocessor control these modes can be combined.

A. Forced ventilation (Controlled Mechanical Ventilation): In this mode, the device switches from exhalation to inhalation after a specified period of time. This period of time determines the frequency of instrumental breaths. Tidal volume, frequency of instrumental inhalations and minute volume of breathing are constant, regardless of attempts to independently inhale. Spontaneous breathing is not provided. Setting a limit on inspiratory pressure prevents barotrauma of the lungs. It is advisable to carry out forced ventilation in the absence of attempts at spontaneous breathing. If the patient is awake and trying to breathe, then it is necessary to administer sedatives and muscle relaxants.

B. Assist-Control Ventilation: Installing a pressure sensor in the breathing circuit allows you to use a spontaneous inspiration attempt to trigger a mechanical inspiration. By adjusting the sensitivity of the sensor, you can select the depth of spontaneous inhalation required to start (more often the vacuum value in the breathing circuit is set). The device is set to the minimum fixed

TABLE 50-3.Ventilation modes

breathing frequency, but each attempt to inhale independently (the vacuum created by the patient must be no less than the specified one) triggers a mechanical inhalation. In the absence of spontaneous inhalation attempts, the device operates in forced mode.

B. Intermittent Mandatory Ventilation: This mode allows for spontaneous breathing. The main physiological benefit is a decrease in mean airway pressure(Table 50-4). In addition to the ability to breathe independently through the ventilator, a certain number of mechanical breaths is set (i.e., the minimum guaranteed tidal volume is set). If the specified frequency of mechanical breaths is high (10-12/min), then the ventilator provides almost the entire minute volume of breathing. On the contrary, if the specified frequency of mechanical breaths is low (1-2/min), then the ventilator provides only a minimum of respiratory support, and most of the minute breathing volume is provided by the patient’s spontaneous breathing. The frequency of mechanical inhalations is selected in such a way as to ensure normal PaCO 2. This mode has become widespread when transferring a patient from mechanical ventilation to spontaneous breathing. With synchronized intermittent forced ventilation, mechanical inspiration coincides, if possible, with the beginning of spontaneous inspiration. Correct synchronization prevents the imposition of a mechanical inspiration in the middle of a spontaneous one, which leads to a significant increase in tidal volume. Limitation of in-

Rice. 50-1. Airway pressure curves for different ventilation modes

TABLE 50-4.Advantages of synchronized intermittent forced ventilation

spiratory pressure protects the lungs from barotrauma.

The circuit of the device providing intermittent forced ventilation provides a continuous supply of the respiratory mixture, which is necessary for independent breathing in the intervals between mechanical breaths. Modern devices allow for synchronized intermittent forced ventilation, while older models for this need to be equipped with a parallel circuit, a constant flow system of the respiratory mixture, or an inhalation valve that works “on demand”. Regardless of the system, proper function of the guide valves and sufficient gas flow rates are necessary to prevent increased work of breathing, especially when positive end-expiratory pressure (PEEP) is used.

D. Mechanical ventilation with guaranteed minute volume of breathing (Mandatory Minute Ventilation): The patient breathes independently and also receives mechanical breaths; Exhaled minute volume is continuously monitored. The device operates in such a way that spontaneous and instrumental breaths add up to a given minute volume of breathing. The effectiveness of this regimen for transferring from mechanical ventilation to spontaneous breathing remains to be determined.

D. Pressure maintenance ventilation; synonym: Pressure Support Ventilation: Pressure support ventilation is used while spontaneous breathing is maintained; it is designed to increase tidal volume, as well as overcome increased resistance caused by the endotracheal tube, breathing circuit (hoses, connectors, humidifier) ​​and apparatus (pneumatic circuit, valves). With each attempt to independently inhale, the device blows into the airways a flow of respiratory mixture, the volumetric velocity of which is sufficient to achieve the specified inspiratory pressure. When the inspiratory flow decreases to a certain level, the ventilator switches from inhalation to exhalation through a negative feedback mechanism, and the pressure in the airways decreases to the original level. The only parameter that can be set is the inspiratory pressure. The respiratory rate is determined by the patient, while the tidal volume can fluctuate significantly depending on the inspiratory flow, the mechanical properties of the lungs and the force of spontaneous inspiration (i.e., the vacuum created). A low level of set inspiratory pressure (5-15 cm H2O) is usually sufficient to overcome any resistance caused by the breathing apparatus. A higher level of set pressure during inspiration (20-40 cm of water column) represents a full-fledged mode of mechanical ventilation, requiring undisturbed central regulation of breathing and stability of the mechanical properties of the lungs. The main advantage of pressure-assisted ventilation is its ability to increase spontaneous tidal volume and reduce the work of breathing for the patient. This mode is used when transferring from mechanical ventilation to spontaneous breathing.

E. Pressure Control Ventilation: In this mode, as with volume-switching ventilation, inspiratory flow decreases as airway pressure increases and stops when a preset maximum is reached. The main disadvantage of pressure-controlled ventilation: tidal volume is not constant, it depends on the compliance of the chest and lungs, the set respiratory rate and the initial pressure in the airways. Moreover, when airway resistance is increased, inspiratory flow ceases even before alveolar pressure rises to airway pressure.

G. Ventilation with an inverse inhalation/exhalation ratio (Inverse I:E Ratio Ventilation): In this mode of ventilation, the inhalation/exhalation duration ratio exceeds 1:1, most often amounting to 2:1. This is achieved in various ways: setting a pause at the end of the inhalation; reduction in maximum inspiratory flow during volume-switched ventilation; The most common method is to limit inspiratory pressure in combination with adjusting the frequency of instrumental inhalations and the duration of inspiration so that the duration of inspiration exceeds the duration of exhalation (Ventilation with pressure control and reverse inhalation/exhalation ratio).

During mechanical ventilation with a reverse inhalation/exhalation ratio, spontaneous PEEP, since each new inhalation begins before the previous exhalation is completely completed; The air retained in the lungs increases the FRC until a new equilibrium state occurs. This regimen does not allow the patient to breathe on his own and requires the administration of high doses of sedatives and muscle relaxants. The effectiveness of reverse inspiratory/expiratory ratio ventilation in improving oxygenation in patients with reduced FRC is the same as that of PEEP. As with PEEP, oxygenation is usually directly proportional to mean airway pressure. The main advantage of reverse inhalation/exhalation ratio ventilation is the lower peak inspiratory pressure. Proponents of mechanical ventilation with a reverse inhalation/exhalation ratio believe that, compared with PEEP, it more effectively involves the alveoli in gas exchange and ensures a more uniform distribution of the respiratory mixture in the lungs.

3. Ventilation with periodic reduction of pressure in the respiratory tract (Airway Pressure Release Ventilation): This mode facilitates spontaneous breathing under constant positive airway pressure. Periodically reducing the pressure in the airways makes it easier to exhale, which stimulates spontaneous breathing. Thus, the pressure in the airways decreases with spontaneous inhalation and mechanical exhalation. Parameters that determine the minute volume of breathing: the duration of inhalation, exhalation, as well as the period of decrease in pressure in the respiratory tract; depth and frequency of spontaneous breaths. Initial settings: positive airway pressure 10-12 cmH2O. Art.; inhalation duration 3-5 s; exhalation duration is 1.5-2 s. The duration of inspiration determines the frequency of instrumental breaths. The main advantage of mechanical ventilation with periodic reduction of airway pressure: a significant reduction in the risk of circulatory depression and pulmonary barotrauma. This mode is a good alternative to pressure-controlled ventilation with an inverse inhalation/exhalation ratio in solving problems caused by high peak inspiratory pressure in patients with reduced lung compliance.

I. High-Frequency Ventilation: There are three types of HF ventilation. With HF positive pressure ventilation, the device delivers a small tidal volume into the airways at a rate of 60-120/min. HF injection ventilation (HFIV) is carried out using a small cannula through which a respiratory mixture is supplied at a frequency of 80-300/min; the air flow sucked in by the gas jet (Ber-nulley effect) can increase tidal volume. With HF oscillatory ventilation, a special piston creates oscillatory movements of the gas mixture in the respiratory tract with a frequency of 600-3000/min. The tidal volume during HF ventilation is below the anatomical dead space, and the mechanism of gas exchange is not precisely known; it is believed that it may occur as a result of enhanced diffusion. High frequency ventilation is most often used in the operating room for interventions on the larynx, trachea and bronchi; in addition, it can save lives in emergency situations when tracheal intubation and standard mechanical ventilation are impossible (Chapter 5). For thoracotomy and lithotrpsy, HFIV ventilation has no advantages over standard ventilation modes. In the intensive care unit, high-frequency ventilation is indicated for bronchopleural and tracheoesophageal fistulas if other modes of ventilation are ineffective. The inability to warm and humidify the respiratory mixture during HF mechanical ventilation is associated with the risk of certain complications. Initial settings for high-frequency ventilation: frequency of mechanical inspirations: 100-200/min, inspiratory phase 33%, operating pressure 1-2 atm. To avoid errors, mean airway pressure should be measured in the trachea at a point at least 5 cm distal to the injector. CO 2 elimination is directly proportional to the working pressure, while oxygenation is directly proportional to the average pressure in the respiratory tract. With high-frequency ventilation with high operating pressure and an inspiratory phase >40%, spontaneous PEEP may occur.

K. Differential Lung Ventilation: This regimen is used for severe damage to one lung that is resistant to PEEP. In this case, standard modes of ventilation with PEEP can aggravate disturbances in the ventilation/perfusion relationship. Uneven ventilation and overdistension of the healthy lung aggravate hypoxemia and barotrauma. After installing a double-lumen endobronchial tube, separate ventilation of each lung is performed using one or two ventilators. When using two devices, carry out temporary synchronization of hardware breaths.

PCV (pressure control ventilation) - pressure controlled ventilation is similar to CMV mode, and when the trigger is set, it is similar to ACMV. The only difference is that the doctor needs to set the inspiratory pressure, not the DO.

BiPAP (biphasic positive airway pressure) - ventilation with two phases of positive pressure in the respiratory tract. In its technical implementation, this mode of ventilation is similar to PCV.

A distinctive feature is the possibility of independent breathing attempts at the height of inspiration (segment 2-3 in Fig. 3.5). Thus, the mode provides the patient with greater freedom of breathing. BiPAP is used when transitioning from PCV to more assisted ventilation modes.

With an increase in the level of wakefulness in patients with intracranial hemorrhage, the aggressiveness of respiratory support is gradually reduced and they switch to auxiliary modes of ventilation.

Basic modes of auxiliary ventilation, Used when transferring a patient to spontaneous breathing

Rice. 3.6. Airway pressure (Paw) curve as the patient breathes in SIMV mode. Alternation of breaths with a given tidal volume (1) (the frequency of these breaths is set by the doctor) and the patient’s spontaneous breathing (2).

Rice. 3.7. Airway pressure curve (Paw) when the patient breathes in the “Pressure Support” mode. Spontaneous breathing of the patient with slight support from the pressure of each breath (Psup); CPAP - see text.

Rice. 3.8. Airway pressure (Paw) curve as the patient breathes in CPAP mode. Breathing spontaneously, without any support (1).

The patient will breathe spontaneously at a lower volume (eg 350 ml). Thus, the patient’s ventilation MO will be 700 ml x 5 + 350 ml x 10 = 7 l. The mode is used to train patients to breathe independently. Alternating the patient’s own breathing attempts with a small number of triggered breaths allows one to inflate the lungs with a large DO and prevent atelectasis.

PS (pressure support) - pressure breathing support. The principle of inhalation in this mode is similar to PCV, but fundamentally differs from it in the complete absence of specified hardware inhalations. When switching to PS mode, the doctor gives the patient the opportunity to breathe on his own and sets only slight pressure support for the patient’s own breathing attempts (Fig. 3.7). For example, the doctor sets pressure support to 10 cm of water. Art. above the PEEP level. If the patient breathes at a rate of 15 breaths per minute, then all his attempts will be triggered and supported by an inspiratory pressure of 10 cm of water. Art.

CPAP (continuous positive airway pressure) - spontaneous breathing with constantly positive pressure in the airways. This is the most auxiliary mode of ventilation. The doctor does not establish either forced breaths or pressure support (Fig. 3.8). Positive pressure is created using the PEEP knob. The usual CPAP level is 8 -10 cmH2O. Art. The presence of constant positive pressure in the respiratory tract facilitates spontaneous breathing of the patient and helps prevent atelectasis.

Due to the fact that in auxiliary modes of mechanical ventilation the frequency of forced breaths is minimized or absent, in case of severe bradypnea or apnea in the patient, the so-called apnea mode of ventilation is installed on the ventilator. If there are no independent breathing attempts by the patient for a certain period of time (set by the doctor), the device begins ventilation in CMV mode with the specified RR and DO.

Pressure Control Ventilation(PCV)

In Pressure Controlled Ventilation (PCV) mode, set the following parameters:
airway pressure (P),
time to maintain this pressure (t INSP),
number of machine breaths per minute (f)
PEEP.

In many modern respirators, it is also possible to regulate the rate of increase in pressure in the airways by changing the slope of the pressure curve.
Typical values ​​are P = 18-20 cm water column, t INSP = 0.7-0.8 sec, f = 10-12 per minute, PEEP = 5 cm water column. Art., the slope of the pressure curve is from (-2) to (+2).

Mode algorithm. When you inhale, an oxygen-air mixture is supplied to the respiratory tract until the desired pressure is established there. Then this pressure is maintained for a specified time, after which the flow of the respiratory mixture stops, the exhalation valve opens, and exhalation occurs.

The magnitude of the tidal volume depends on the compliance of the lungs: the more flexible they are, the greater the volume of the respiratory mixture will enter them at the pressure created by the respirator (Fig. 6.11). Depending on the patient's needs, the slope of the pressure curve is changed. A smaller angle of inclination of the curve allows for a slower flow of the oxygen-air mixture into the respiratory tract, a larger angle allows for a faster flow. Although the choice of this indicator is individualized each time, faster flows are most often required by patients with chronic pulmonary problems and increased resistance in the airways.

Considering the importance of the tidal volume for ensuring ventilation and oxygenation, the main alarms are set in order to control it: the value of the minimum MOR, the maximum respiratory rate. Classic PCV mode is similar to CMV in that all breaths are untriggered. However, most often a modified PCV is used, in which the sensitivity is set, and it becomes an analogue of the usual Assist Control mode, in contrast to which machine breaths are focused not on supplying volume, but on creating pressure in the airways.

Additional parameter of modified PCV:
trigger sensitivity (usually (-3) - (-4) cm H2O or (-2) - (-3) l/min).

In some respirator models, machine pressure breaths can be set in SIMV mode.
It is generally accepted that all pressure-based ventilation modes lead to a more rational distribution of the respiratory mixture in the lungs than volume-based modes. It is believed that this may have a more beneficial effect on damaged lungs. It seems to us that this assumption does not have such serious grounds. There is no significant difference in what the respirator focuses on - the pressure under which a certain volume of the respiratory mixture enters the lungs, or the volume that creates a certain pressure in the lungs. It is important how this volume is supplied (at what speed, what form of flow), what pressure is created, and how much oxygen-air mixture ultimately enters the lungs.

Pressure Support (PS)
Pressure Support (in some models called Assisted Spontanious Breathing, ASB) can be used as a separate mode (Fig. 6.12) or to support spontaneous breaths together with the SIMV mode (Fig. 6.13). In this mode, set the following parameters:

Airway pressure (P),
trigger sensitivity
PEEP.

Typical values: P = 18-20 cm water column, PEEP = 5 cm water column. Art.

Mode algorithm. When a patient attempts to breathe, the respirator creates a preset pressure in the airways, “supporting” the patient’s inhalation. It is important to immediately note the difference between Pressure Support and Pressure Control Ventilation. The first occurs only in response to breathing attempts, the second - without them. But the main thing is not this, but the principle of interrupting inhalation and switching the ventilator from inhalation to exhalation. In PCV, this is a specified time during which pressure is maintained in the patient’s airways; in Pressure Support, it is a reduction in the peak inspiratory flow to 25-30% of the initial flow. This feature of Pressure Support is one of its disadvantages. If the patient does not have a completely sealed airway, for example, if the tracheostomy tube cuff is not fully inflated, the pressure in the airway will never reach the set level due to air leakage. As a result, the desired reduction in peak flow will not occur and exhalation will not begin. To prevent such a situation, a maximum inhalation time is usually set, for example, no more than 3 seconds. If the inhalation exceeds 3 seconds, then exhalation must occur. In modern respirator models, the peak flow reduction value, which switches inhalation to exhalation, can be set not only at 25-30%, but also at several different levels, which helps prevent problems with leakage of the oxygen-air mixture.

Another problem is the patient’s mandatory breathing efforts. If the patient breathes in Pressure Support mode, then there is a theoretical possibility of apnea due to the cessation of his breathing attempts. In this case, an emergency ventilation mode is provided, which is usually represented by CMV. When breathing attempts are restored, this mode is disabled. It must be remembered that not all respirators provide inspiratory duration limitation and emergency ventilation.

Biphasic Positive Airway Pressure (BiPAP)
This mode in some respirators is called Spontaneous Positive Airway Pressure (SPAP) and represents a biphasic alternating airway pressure. Despite the similarity in name, SPAP should not be confused with CPAP.

In BiPAP mode, set the following parameters:

Upper airway pressure (P max),
lower pressure in the respiratory tract (P min),
inspiratory time (t INSP),
number of machine breaths per minute (f).

Typical values: P max = 18-20 cm water column, P min = 5 cm water column. art., t INSP = 0.8 sec, f = 10 in 1 min.

Mode algorithm. Two different levels of continuous positive pressure are alternately created in the airway. The upper level is maintained for a certain time, regulated by the doctor. The duration of maintaining the lower pressure level is determined by the specified frequency of breaths. The upper pressure level actually creates a Pressure Control-type breath, the lower one is similar to CPAP. At each level, the patient is allowed to breathe independently (Fig. 6.14). Due to spontaneous breaths, ventilation-perfusion ratios and arterial oxygenation are improved.

BiPAP is one of the most interesting modes of ventilation. It does not require synchronization between the patient and the respirator at all. In this case, the patient does not struggle with the ventilator and intrathoracic pressure does not increase. However, there are no universal regimens for all patients. There is a category of patients who, when using the BiPAP regimen, develop severe tachypnea, accompanied by hypocapnia.

Usually in such cases, switching the respirator to Assist Control helps. In this case, it is possible to use the BiPAP Assist modification. Unlike conventional BiPAP, this mode does not always maintain a constant exhalation time. If the patient makes a breathing attempt during exhalation, the respirator immediately creates upper pressure in the respiratory tract (P max), i.e. inhalation comes.

Airway Pressure Release Ventilation (APRV)
Airway Pressure Relieving Ventilation (ARPV) is similar to BiPAP in that it also creates two levels of airway pressure. At the upper level of pressure, the patient can breathe on his own. Unlike BiPAP, the lower pressure level is created only for a short period of time, the duration of which is not adjustable. The patient exhales, “pressure in the airways is released” and the upper level of pressure is again created (Fig. 6.15).

Automatic Tube Compensation (ATC)
The automatic tracheal tube resistance compensation (ATC) mode is also called “electronic extubation”. It is based on the following principles. The endotracheal tube has resistance that limits air flow and increases the work of breathing. These problems are compensated to a certain extent by the use of Pressure Support. But PS creates a constant pressure in the airways during inspiration, while the flow of blown air changes during inspiration from 1.5-2 l/min to zero. Accordingly, at the beginning of inspiration, pressure support will not be enough to compensate for the resistance of the endotracheal tube, and at the end of inspiration, support will be excessive. Unnecessary overinflation of the lungs occurs, and the increased work of breathing is not fully compensated. The ATC mode is based on the amount of gas flow taking into account the size of the tube and creates a higher pressure of the air mixture at the beginning of inspiration, and less at the end.

Rapid progress in electronics and computer technology has made it possible to implement more complex algorithms for controlling the flow of the gas mixture and ventilation modes based on them. Two main directions can be distinguished:

1. The use of two levels of positive pressure, which is referred to as "BiPAP".
2. Dynamic change of ventilation parameters based on feedback.

There are at least five situations where this term is used:

a) as a synonym for the combination of CPAP and PS (Respironics). At the same time, the level of expiratory “E-PAP” and inspiratory “I-PAP” pressure in the breathing circuit is set. In addition, it becomes possible to periodically, with a frequency of several times per minute, reduce expiratory pressure (IMPRV - Intermittent Mandatory Pressure Release Ventilation, “Cesar”);

b) as a synonym for pressure-controlled ventilation, when the CPAP level acts as expiratory pressure - “E-PAP”, and the set value of inspiratory pressure - “I-PAP”.

c) with spontaneous breathing at two different levels of positive pressure in the ventilation circuit, which change every 5-10 s (Drager Evita).

d) as a variant of the case described above (c), when the duration of high pressure is relatively short, and the patient breathes most of the time at lower pressure, similar to the SIMV mode with pressure control.

e) another option for this case (c) is ventilation with pressure reduction in the respiratory tract, or APRV - Airway re Release Ventilation, when the patient breathes at high pressure in the circuit most of the time. The attitude towards the APRV regime is ambiguous. A number of experimental studies using the ARDS model have demonstrated worse results compared to CPAP. At the same time, there is evidence of an improvement in the ventilation-perfusion ratio during unobstructed spontaneous breathing in the APRV mode compared with pressure support ventilation. There are isolated reports of the positive effect of the APRV regimen in various lung pathologies.

Feedback-based ventilation modes are becoming increasingly common. The outdated term "servo", which actually means feedback, is often used in those devices where ventilation parameters change automatically depending on the condition of the lungs. In each case, it is necessary to highlight the controlled parameter and those changes in the characteristics of the respiratory cycle that are the result of feedback.

PRVC (Pressure-regulated volume control) - a mode that provides for a change in tidal volume depending on the value of inspiratory pressure. Similar to pressure-controlled ventilation: the limited parameter is inspiratory pressure; switching is carried out according to time. It differs in that the operator sets the tidal volume, and the device selects the inspiratory pressure necessary to achieve this volume based on the results of several previous respiratory cycles (Siemens Servo 300).

Auto flow - similar to PRVC, but combined with BiPAP - 3rd type BiPAP, see above (Drager Evita Dura). Volume Support is another modification of PRVC, characterized in that switching is carried out by stream.

Minimum Minute Ventilation - a mode that guarantees the provision of the specified minimum minute ventilation. This uses feedback mechanisms similar to Volume Support (Hamilton Weolar).

Mandatory Rate Ventilation - ventilation with a given frequency, on the contrary, controls the respiratory rate, increasing the level of inspiratory pressure if the patient breathes faster.

Mandatory Minute Ventilation - ventilation mode with a specified minute ventilation (not to be confused with Minimum Minute Ventilation), regulates the breathing rate. When the patient's spontaneous breathing provides an adequate amount of minute ventilation, the device does not add mandatory breaths - unlike ot SIMV, where the set number of mandatory breaths remains constant Erica Engstrom).

Proportion Assist Ventilation - proportional auxiliary ventilation is a rather complex mode in which the device, with each inhalation attempt, based on determining the flow and tidal volume, evaluates the patient's effort and sets the corresponding inspiratory pressure value. This mode showed greater comfort compared to PCV in healthy volunteers with artificially reduced compliance of the respiratory system.

The wide range of different ventilation modes in itself reflects the fact that to date there is no convincing evidence of significant advantages of any particular technique. Differences in treatment results can be associated to a greater extent with the design features of the devices used rather than with the control algorithm.

An important recent achievement, which has greatly facilitated the selection of parameters and made mechanical ventilation more convenient, is the monitoring and graphical display of ventilation indicators (flow, pressure and tidal volume). This can be clearly demonstrated in the following examples:

Rice. 2. Graphic display of ventilation parameters in a patient with ARDS

Due to a sharp decrease in lung compliance, a high inspiratory pressure is observed with a low tidal volume. A kink in the inspiratory portion of the flow curve (marked by an arrow) indicates that inspiration ceases before maximum tidal volume is reached. Increasing the inspiratory duration (next cycle) allows you to use this reserve and increase the efficiency of ventilation without reaching the critical inspiratory pressure.

In Fig. Figure 2 shows curves reflecting the dynamics of ventilation parameters in a patient with ARDS. In this case, a serious problem is a sharp decrease in the compliance of the lung tissue, high inspiratory pressure with a low tidal volume. However, the kink (indicated by an arrow) in the flow curve, which is the most informative during pressure-limited ventilation, shows that by the beginning of the next respiratory cycle, expansion of the lungs is still ongoing and there are certain reserves of tidal volume. To use them, it is necessary to increase the duration of inspiration, which is accompanied by an increase in tidal volume and ventilation efficiency.

Rice. 3. Graphic display of ventilation parameters in a patient with bronchospastic syndrome

Due to the high resistance of the respiratory tract, a “gas trap phenomenon” develops, which is reflected in the expiratory part of the flow curve in the form of a kink (marked with an arrow). Increasing the duration of exhalation by reducing the respiratory rate avoids this, reduces the residual pressure in the airways and increases the effective tidal volume.

During mechanical ventilation in a patient with exacerbation of bronchial asthma and severe bronchospasm (Fig. 3), high airway resistance leads to the so-called gas trap phenomenon, when a significant part of the tidal volume remains in the lungs at the beginning of the next inspiration. This is evidenced by the kink in the expiratory part of the flow curve (marked with an arrow). In such a situation, the residual pressure in the airways (auto-PEEP) can reach critical values, causing a decrease in the effectiveness of ventilation and circulatory decompensation.

The only way out is to increase the duration of exhalation. This is achieved by reducing the respiratory rate and the inspiratory to expiratory ratio (I/E).

Rice. 4. Indicators of ventilation during mechanical ventilation in a patient with normal lung condition

A tidal volume of 12-15 ml/kg is achieved at an inspiratory pressure not exceeding 15 cm of water. Art.

For comparison, in Fig. Table 4 shows the corresponding indicators for mechanical ventilation in a patient with normal lung condition. A tidal volume of 12-15 ml/kg is achieved at an inspiratory pressure within 15 cm of water. Art. without significant changes in respiratory rate and I/E ratio.

Significant progress in the pathophysiology of artificial ventilation allows us to identify the main ways to reduce the incidence of complications. The Acute Respiratory Distress Syndrome Network (ARDSNET) study is probably the most important work on mechanical ventilation in the last decade. It is well organized and clearly demonstrates that reducing tidal volume to 6 ml per 1 kg of ideal weight, compared with the “usual” 12 ml/kg, is associated with reduced mortality and improved treatment outcomes. Even more interesting is the observation that this occurred against the background of moderate hypoxemia. Another significant aspect concerns breathing rate. Contrary to the opinion of some researchers that in ARDS it should be low, the ARDSNET group showed improved treatment outcomes with an average respiratory rate of 29 per minute (compared to 1/2 of this value in the control). Attention should be paid to the introduction of the specific term “volume trauma”. This is unnecessary because pressure and volume are closely related. This neologism appears to result from a failure to understand that the relationship between transalveolar and transthoracic pressure is nonlinear. However, measurement of intrapleural pressure (or intraesophageal pressure as its equivalent) is generally not available in intensive care settings. Therefore, the tidal volume reflects the degree of lung damage to a greater extent than the pressure in the ventilation circuit. Regardless of the terminology, it is obvious that overstretching of the alveoli leads to the destruction of alveolar-capillary membranes and the rapid development of inflammation in the lung tissue.

=================
You are reading the topic:
Choosing a mode of artificial ventilation for intensive care of acute respiratory failure

1. Modern modes of ventilation.