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35АППАРАТ ИСКУССТВЕННОЙ ВЕНТИЛЯЦИИ ЛЕГКИХ
Абдунабиева Хакима Умарова Мухаббат Абдурахимов Абдухалим Нугманов Озодбек
Андижанский государственный медицинский институт
Андижан, Узбекистан
Аппарат искусственного дыхания включает высокочастотный вентилятор и вентилятор промежуточного положительного давления. Высокочастотный вентилятор соединен с источником дыхательного газа для подачи кислорода пациенту с заданной высокой частотой. Кроме того, вентилятор с промежуточным положительным давлением подключен к источнику дыхательного газа для подачи дыхательного газа в контур пациента с заданной частотой. В настоящее время спрос на эти устройства растет во всем мире.
Ключевые слова: вентиляционный аппарат, источник дыхательного газа, подача кислорода, высокочастотный вентилятор
SU'NIY NAFAS OLISH APPARATI
Sun'iy nafas olish uchun ventilyatsion apparati yuqori chastotali ventilyator va oraliq musbat bosimli shamollatgichni o'z ichiga oladi. Yuqori chastotali ventilyator, oldindan belgilangan yuqori chastotada bemorga kislorod berish uchun nafas olish gaz manbaiga ulangan. Shuningdek, oraliq musbat bosimli shamollatish moslamasi oldindan belgilangan chastotada bemorga nafas olish gazini berish uchun nafas olish gaz manbaiga ulanadi. Ayni vaqtda ushbu apparatlarga dunyo miqiyosida talab juda kuchli bo'lmoqda.
Kalit so'zlar: Ventilyatsion apparat, nafas oluvchi gaz manbai, kislorod ta'minoti, yuqori chastotali ventilyator
ARTIFICIAL VENTILATION APPARATUS
Ventilation apparatus for artificial respiration includes a high frequency ventilator and an intermediate positive pressure ventilator. The high frequency oscillation ventilator is connected to a breathing gas source to supply oxygen to the patient at a predetermined high frequency. In addition, an intermediate positive pressure ventilator is connected to the respiratory gas source to supply respiratory gas to the patient circuit at a predetermined frequency. Currently, the demand for these devices is growing worldwide.
Key words: ventilation apparatus, breathing gas source, supply oxygen, high frequency oscillation ventilator.
DOI: 10.24411/2181-0443/2020-10077
Introduction: Artificial ventilation, (also called artificial respiration) is means of assisting or stimulating respiration, a metabolic process referring to the overall exchange of gases in the body by pulmonary ventilation, external respiration, and internal respiration. It may take the form of manually providing air for a person who is not breathing or is not making sufficient respiratory effort, or it may be mechanical ventilation involving the use of a mechanical ventilator to move air in and out of the lungs when an individual is unable to breathe on their own, for example during surgery with general anesthesia or when an individual is in a coma or trauma [1].
Classification of artificial ventilators. According to research [2] the lungs can be artificially ventilated either by reducing the ambient pressure around the thorax (negative pressure ventilation) or by increasing the pressure within the airways (positive pressure ventilation). Negative pressure ventilators use a rigid chamber that encloses either the thorax (cuirass) or the whole body below the neck (tank respirator or "iron lung"). The pressure in the chamber is reduced cyclically by means of a large volume displacement pump, thus causing the lungs to expand and contract. Negative pressure ventilation is fully discussed in article 5 of this series. These ventilators were used extensively for poliomyelitis victims, and are still in use for long term respiratory support or overnight support for patients with respiratory muscle weakness, Tank ventilators occupy much space, access to the patient is poor, and the neck seal can create problems. They are not suitable for use in general intensive care units. There has been a recent revival in interest in negative pressure ventilators in paediatric intensive care units to avoid the need for endotracheal intubation. According to research [3] the basic classification of positive pressure artificial ventilators was first proposed by Mapleson Artificial ventilators are devices that control inspiration; expiration is usually passive, and so the classification is based on the mechanism of gas delivery during inspiration. There are two types of machine. A flow generator produces a known pattern of gas flow during inspiration, and the lungs fill at a rate entirely controlled by the ventilator and independent of any effect of lung mechanics. A pressure generator produces a preset pressure in the airway and the rate of lung inflation depends not only on the pressure generated by the ventilator but also on the respiratory resistance and compliance, which determine the time constant of the lungs. With a square wave pattern of pressure the lungs fill in an exponential fashion. The effects of the two types of ventilator on inspiratory flow and lung volume. In general, the flow generator ventilator is used for adults and the pressure generator ventilator for children, or adults when control of peak airway pressure is important. The pressure generator is particularly useful in children where uncuffed endotracheal tubes are used and there is a leak of gas around the endotracheal tube during inspiration. A pressure generator tends to compensate for this leak by increasing the flow into the airway, whereas with a flow generator a proportion of the tidal volume is lost. A ventilator must also have a mechanism to cause it to cycle between inspiration and expiration. Ventilators are usually subclassified as time cycled, pressure cycled, or volume cycled machines. Time cycled ventilators switch between inspiration and expiration after a preset time interval, pressure cycled ventilators switch when a preset airway pressure threshold has been reached, and volume cycled ventilators cycle when a preset tidal volume has been delivered. Cycling from expiration to inspiration is usually effected by a timing mechanism or by a patient triggering device that senses the subatmospheric pressure or the flow generated in the inspiratory tube by the patient's inspiratory effort. This traditional classification was devised during a period when ventilators were totally mechanical, and driven either by compressed gas or by an electrically powered piston or bellows. By 1954, however, Donald had used an electronic trigger that initiated inspiration and in 1958 an electronic timing device was incorporated in the Barnet ventilator [4]. In 1971 the Siemens-Elema company introduced the Servo 900 ventilator, which combined a simple pneumatic system with a sophisticated electronic measuring and control unit. Gas flow to and from the patient's lungs was controlled by a pair of scissor valves and monitored with pressure and flow sensors. The control unit adjusted the scissor valves to ensure that the flow patterns measured by the sensors corresponded to those selected by the operator. This method of control is termed a servo or feedback system and is very flexible. Potentially one machine could mimic all the previously described categories of ventilator [5].
It is known to science that [6] what numerous classifications have been suggested and include classifications by Ward and Mapleson. Classifications refer to the following
elements. Control describes how the ventilator delivers flow to the patient. In volume-controlled ventilators the rate of flow delivered is constant, the tidal volume is targeted, with a variable pressure delivered relative to compliance of the lung. In pressurecontrolled ventilators the rate of flow delivered decelerates through the breath to maintain the targeted pressure at the peak inspired pressure. The tidal volume delivered is determined by the compliance of the ventilated lung. Cycling determines how the ventilator switches from inspiration to expiration. Time cycling is used in pressure-controlled ventilation. Flow-cycling is used in pressure-support ventilation, where a reduction of the peak inspiratory flow cycles the ventilator into expiration. Volume cycling is used in volume-controlled ventilation. The ventilator cycles to expiration when a set tidal volume has been delivered. If an inspiratory pause is added the breath will be classified as both volume and time cycled. Triggering is how inspiration is initiated in association with patient breaths. Ventilators may be triggered by changes in pressure, flow or by a preset time interval having elapsed. Pressure: the ventilator delivers a breath when the baseline pressure decreases during the patient's inspiratory effort. Flow: some ICU ventilators deliver a constant background flow throughout the respiratory cycle (flow by). A change in this constant flow, caused by patient inspiration is detected at the flow sensor in the expiratory limb. This triggers the ventilator to increase the flow and a breath is delivered to the patient. Flow triggering reduces the work of breathing when compared with pressure triggering because there is always some background gas flow from the patient and no delay in inspiratory valve opening. Time: the ventilator cycles at a frequency determined by the respiratory rate or the ratio of inspiration to expiration (I:E).
Ventilatory strategies for special conditions. We know [6,7] what ARDS ARDS is characterized by generalized pulmonary infiltration secondary to increased permeability, and results in interstitial and alveolar oedema. The ARDS is defined by: a PaO2/FiO2 of less than 200 mm Hg; bilateral interstitial infiltrates on a chest radiograph; no evidence of left ventricular failure with a pulmonary artery wedge pressure of less than 18 mm Hg; and a recognized cause of ARDS. Hypoxia is secondary to decreased compliance, increased pulmonary shunt and pulmonary hypertension. It is this pulmonary hypertension that causes an increase in microvascular pressure, resulting in increased capillary leak and interstitial oedema. It may also precipitate right ventricular failure. Poor lung compliance and attempts to ventilate with normal tidal volumes may lead to high airway pressures and possible volutrauma. Ventilatory strategies in ARDS: the ventilator should have graphical, real-time displays of flow and pressure waveforms. Protective ventilation is key to a safe and effective management plan of a patient with ARDS. Ventilation strategies are aimed at preventing the detrimental effects of volutrauma. Strategies include pressure-controlled forms of ventilation with small tidal volumes (5-6 ml/kg) and high respiratory rates (>25 breaths/min) to prevent over-distension. When volumecontrolled ventilation is used, gas is preferentially delivered to more compliant lung units, with a risk of over-distension. The application of pressure-controlled ventilation results in better gas distribution and less distension of non-compliant lung units.
In addition [6,8] mean airway pressure (which corresponds with alveolar recruitment and improved PaO2) should be increased. The mean airway pressure correlates with the area under the pressure curve and can be increased by several manoeuvres. A higher mean airway pressure for a given peak airway pressure can be achieved with pressure-controlled ventilation. The normal alveoli are more susceptible to the effects of peak airway pressure. High airway pressures result in volutrauma and should be limited to less than 35 cm H2O. If volume-controlled ventilation is used, it can be difficult to keep the peak airway pressure below this value in patients with noncompliant lungs. With pressure-control modes it is possible to safely increase the mean airway pressure by prolonging inspiratory time, without a need to increase the peak pressure. A
reduction in expiratory time can induce auto-PEEP (PEEPi) where a new breath is delivered before expiratory flow is complete. The remedy for this is a reduction in the respiratory rate followed by a reduction in the I:E ratio, until gas trapping disappears on the flow waveform. This results in improved oxygenation within the pressure limit without overstretching the alveoli. If gas trapping develops with pressure-controlled ventilation, a reduction in tidal volume with each subsequent breath follows. When volume-controlled ventilation is used for inverse ratio ventilation, the tidal volume is sustained with each breath, and peak airway pressure rises. This progressively worsens gas trapping. Volume-controlled ventilation should not be used in inverse ratio mode. The optimal PEEP level should be set from the lower inflection point on a plot of the static pressure-volume curve. In ARDS secondary to diffuse, homogeneous lung injury, the PEEP level may be more than 12-15 cm H2O. The static curve can be constructed with pressure-controlled ventilation using the peak pressure value, provided the gas flow ceases before the end of expiration, or using volume-controlled ventilation and plotting the plateau pressure (no flow) against the respective tidal volumes. By randomly changing the size of the tidal volume every five breaths, a curve can be safely plotted in a compromised patient. Some commercially available ventilators can specifically construct a static compliance curve in addition to conventional dynamic loops. Higher levels of PEEP will ensure an FRC greater than the closing capacity, and reduce the tendency towards alveolar collapse, V/Q mismatch and hypoxaemia. In ARDS associated with focal consolidation (secondary to pneumonia) new evidence suggests that high PEEP levels may be less beneficial and lead to over-distension of healthy lung units. The principles of protective ventilation described above may lead to permissive hypercapnia (a higher than normal PaCO2), particularly as an increased Vd/Vt ratio is associated with ARDS. If renal function is preserved HCO3 concentration increases during 2-6 hours to buffer the rise in PaCO2 and normalize pH. The benefits of protective ventilation strategies may outweigh the detrimental effects of a higher PaCO2 in adult critical care. Prone ventilation can improve oxygenation in 50-75% of patients with ARDS. In the supine position ventilation is preferentially directed to the anterior part of the lung, but the optimal perfusion will be in the lung bases close to the diaphragm. In addition, the weight of the mediastinum and heart, together with heavy oedematous lung, collapses the lung posteriorly. When prone, the compliance of the anterior chest wall is increased, the posterior chest wall is already relatively non-compliant, and ventilation is preferentially directed to the bases where perfusion is optimal. The mediastinum and heart are also dependent, and lung previously compressed posterior to this can expand. The benefits are essentially from improved V/Q matching, decreased pulmonary dead-space fraction, and increased drainage of secretions. To date, prone ventilation has failed to improve outcome in adult patients with ARDS. However, a reduction in PaCO2 may translate into an improved outcome in patients with ARDS ventilated in the prone position. High-frequency oscillatory ventilation (HFOV) is a relatively new mode in adult practice. HFOV generates sub-dead-space tidal volumes at high rates (4-5/s) at a constant high mean airway pressure. Lung recruitment follows with reasonable clearance of CO2. It is commonly used in neonates but there are few good adult studies. Other treatment modalities include: inhaled nitric oxide; inhaled prostacyclin; continuous rotation; partial liquid ventilation; extracorporeal membrane; extracorporeal membrane oxygenation; and interventional lung-assist ventilation.
Conclusion: The distinction between a ventilated patient and a spontaneously breathing patient is becoming increasingly blurred as more sophisticated means of respiratory support are devised. In many cases "respiratory assistance" may be a more appropriate term than "artificial ventilation." The efficacy of many of these ventilatory strategies has not been properly assessed, in terms either of acceptability to the patient or of the effect on morbidity and mortality. It is still true, however, that "the type ofventilator
used, provided that its design is satisfactory, is of less importance than the experience ofthe person using it. It is essential for both the anaesthetist and the intensivist to have a good understanding of the pathophysiology of the lung and knowledge of ventilator/lung interactions. Some important basic principles of ventilation are listed below. The pathophysiology of the lung varies with time, so ventilator settings need to be regularly reviewed and adjusted. A healthy lung can be damaged with inappropriate ventilator settings (a diseased lung may be more susceptible). Therefore, peak airway pressure should be below 35 cm H2O, where possible. To minimize side effects, the physiological targets (PaO2 and PaCO2) do not need to be in the normal range. In the past PEEPi has gone unnoticed. When possible flow patterns of ventilation should be monitored to detect the abnormality.
References:
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