AUGMENTED SPONTANEOUS BREATHING

Impaired pulmonary gas exchange can result from lung parenchymal failu re inducing oxygenation deficiency and fatigue of the respiratory musc les, which is characterized by hyercapnia or a combination of both mec hanisms. Contractility of and coordination between the diaphragm and t he thoracoabdominal respiratory muscles predominantly determine the ef ficiency of spontaneous breathing. Sepsis, cardiac failure, malnutriti on or acute changes of the load conditions may induce fatigue of the r espiratory muscles. Augmentation of spontaneous breathing is not only achieved by the application of different technical principles or devic es. it also has to improve perfusion, metabolism, load conditions and contractility of the respiratory muscles. Intermittent mandatory venti lation (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 advan tages include lower mean airway pressure (P-AW), as compared with cont rolled mechanical ventilation, and improved haemodynamics. Suboptimal IMV systems may impose increased work and oxygen cost of breathing, fa tigue of the respiratory muscles and CO2 retention. During pressure su pport 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 inspiratio n. PSV may be combined with other modalities of respiratory therapy su ch as IMV or CPAP. Claimed advantages of PSV include decreased effort of breathing, reduced systemic and respiratory muscle consumption of o xygen, prophylaxis of diaphragmatic fatigue and an improved extubation rate after prolonged periods of mechanical ventilation. Minimum alveo lar 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 brea thing. CPAP has been conceptually designed for the augmentation of spo ntaneous breathing and requires the intact central and peripheral regu lation 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 decreas ed P-AW and lung volume, which increases rapidly to pre-exhalation val ues after closure of the valve due to the high gas flow within the cir cuit (90-100 l/min). APRV may improve haemodynamics and V-A/Q distribu tion as compared with conventional mechnical ventilation. Biphasic pos itive airway pressure (BIPAP) is characterized by the combination of s pontaneous breathing and time-regulated, pressure-controlled mechanica l ventilation. During the respiratory cycle the ventilator generates t wo 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 improve s the safety of both modes of respiratory therapy. The contribution of spontaneous breathing to total minute ventilation may be Important, s ince a decreased shunt and improved V-A/Q relationship have been obser ved in experimental non-cardiogenic lung oedema. These data give suppo rt to the concept that spontaneous breathing should be maintained and augmented in the setting of acute respiratory failure.