Airway pressure release ventilation : A systematic experimental approach

Abstract: Airway pressure release ventilation (APRV) is a ventilatory mode which allows unrestrictedspontaneous breathing (SB) throughout the ventilatory cycle. If SB is insufficient, mechanicalventilatory support is achieved by brief periodic decreases of the airway pressure below the CPAPlevel. Thus, APRV is a combination of pressure-controlled inverse ratio ventilation (PC-IRV) andCPAP-breathing. However, lung collapse may be provoked during the short expiratory time intervals,and the desirable proportion of SB and mechanical ventilation is not known. The aim of the study wastherefore to systematically investigate the different components of APRV which are PC-IRV and SBin regard to gas exchange, haemodynamics and lung collapse.Gas exchange, hemodynamics and the time course of lung collapse and recruitment were comparedin oleic acid-, lavage- and endotoxin-induced lung injury in pigs. Oleic acid injection caused a severelung damage with the most unstable lungs. Thus, we choose the oleic acid model to investigate APRVin a worst case scenario.During expiration, lung collapse could only be avoided by very short expiration times ≤ 0.6 seconds or by high PEEP levels ≥ 20 cm H2O or more. Recruitment occurred mainly within 1.4 seconds of inspiration, but airway pressures up to 40 cm H2O did not open up the lungs completely.Gas exchange was better when the lungs were stabilised by PEEP compared to the use of shortexpiration times, and aeration was more homogenous with PEEP. CPAP was superior to partialventilatory support using APRV. This may be due to beneficial effects of spontaneous breathing ongas exchange breathing per se, but also that APRV may allow rapid lung collapse during the phases ofairway pressure release below the CPAP level. Therefore, the amount of mechanical ventilatorysupport using the APRV mode should be kept at the lowest possible level, that is sufficient for thepatient to cope with the work of breathing and to avoid respiratory muscle fatigue.In conclusion, lung collapse and recruitment follow a rapid exponential time course. Therefore it isdifficult to stabilise the lungs by short expiration times only, and rapid lung collapse may occur whenairway pressure is decreased below the CPAP-level. In addition, PEEP combined with conventional I:E settings causes a more homogenous aeration and better oxygenation than PC-IRV. Therefore,mechanical ventilatory support during APRV should be provided using conventional I:E settings andthe highest possible PEEP, that enables enough ventilatory support for the patient to avoid respiratorymuscle fatigue.