Augmentierte Spontanatmung

Zusammenfassung

Eine akute Störung des pulmonalen Gasaustauschs kann pathophysiologisch unterteilt werden in eine Insuffizienz des Lungenparenchyms mit Einschränkung der Oxygenierung und in eine Schwäche der respiratorischen Muskulatur mit alveolärer Hypoventilation und CO ₂ -Retention. Die Spontanatmung kann durch verschiedene Verfahren unterstützt werden. Intermittent Mandatory Ventilation (IMV) verbessert die alveoläre Ventilation durch periodische Überdruckatemhübe. Bei der Pressure Support Ventilation (PSV) wird der Atemwegsdruck (P _AW ) durch eine maschinelle inspiratorische Gasströmung auf ein vorgewähltes Niveau angehoben. Mögliche Vorteile von PSV sind die große Variationsmöglichkeit der Atemhilfe und Verminderung der inspiratorischen Atemarbeit. Bei Continuous Positive Airway Pressure Beathing (CPAP) liegt der P _AW während des gesamten Atemzyklus über dem Atmosphärendruck, wodurch die funktionelle Residualkapazität (FRK) zunimmt. Biphasic Positive Airway Pressure (BIPAP) – Atemhilfe ist charakterisiert durch eine simultane Mischung einer Spontanatmung mit einer zeitgesteuerten, druckbegrenzten Beatmung. Der Spontanatmungsanteil unter BIPAP kann die V _A /Q-Verhältnisse beim experimentellen Lungenödem verbessern. Diese Daten unterstützen das Konzept, auch bei respiratorischer Insuffizienz eine Spontanatmung zu erhalten und durch geeignete Formen der Atemhilfe zu unterstützen.

Abstract

Impaired pulmonary gas exchange can result from lung parenchymal failure inducing oxygenation deficiency and fatigue of the respiratory muscles, which is characterized by hyercapnia or a combination of both mechanisms. Contractility of and coordination between the diaphragm and the thoracoabdominal respiratory muscles predominantly determine the efficiency of spontaneous breathing. Sepsis, cardiac failure, malnutrition or acute changes of the load conditions may induce fatigue of the respiratory muscles. Augmentation of spontaneous breathing is not only achieved by the application of different technical principles or devices; it also has to improve perfusion, metabolism, load conditions and contractility of the respiratory muscles. Intermittent mandatory ventilation (IMV) allows spontaneous breathing of the patient and augments alveolar ventilation by periodically applying positive airway pressure tidal volumes, which are generated by the respirator. Potential advantages include lower mean airway pressure (P_AW), as compared with controlled mechanical ventilation, and improved haemodynamics. Suboptimal IMV systems may impose increased work and oxygen cost of breathing, fatigue of the respiratory muscles and CO₂ retention. During pressure support ventilation (PSV), inspiratory alterations of P_AW or gas flow (trigger) are detected by the respirator, which delivers a gas flow to maintain P_AW at a fixed value (usually 5–20 cm H₂O) during inspiration. PSV may be combined with other modalities of respiratory therapy such as IMV or CPAP. Claimed advantages of PSV include decreased effort of breathing, reduced systemic and respiratory muscle consumption of oxygen, prophylaxis of diaphragmatic fatigue and an improved extubation rate after prolonged periods of mechanical ventilation. Minimum alveolar ventilation is not guaranteed during PSV; thus, close observation of the patient is mandatory to avoid serious respiratory complications. Continuous positive airway pressure breathing (CPAP) maintains P_AW above atmospheric pressure throughout the respiratory cycle, which may increase functional residual capacity and decrease the effort of breathing. CPAP has been conceptually designed for the augmentation of spontaneous breathing and requires the intact central and peripheral regulation of the respiratory system. Airway pressure release ventilation (APRV) improves alveolar ventilation by intermittent release of P_AW, which is kept above atmospheric pressure by means of a high-flow CPAP system. The opening of an expiratory valve for 1–2 s induces a decreased P_AW and lung volume, which increases rapidly to pre-exhalation values after closure of the valve due to the high gas flow within the circuit (90–100 l/min). APRV may improve haemodynamics and V_A/Q distribution as compared with conventional mechnical ventilation. Biphasic positive airway pressure (BIPAP) is characterized by the combination of spontaneous breathing and time-regulated, pressure-controlled mechanical ventilation. During the respiratory cycle the ventilator generates two alternating CPAP levels, which can be modified with regard to time and pressure. As with APRV, alveolar ventilation is maintained even if the spontaneous breathing efforts of the patient cease, which improves the safety of both modes of respiratory therapy. The contribution of spontaneous breathing to total minute ventilation may be important, since a decreased shunt and improved V_A/Q relationship have been observed in experimental non-cardiogenic lung oedema. These data give support to the concept that spontaneous breathing should be maintained and augmented in the setting of acute respiratory failure.