Respiratory controversies in the critical care setting. Does airway pressure release ventilation offer important new advantages in mechanical ventilator support?
Airway pressure-release ventilation (APRV) is a mechanical ventilation strategy that is usually time-triggered but can be patient-triggered, pressure-limited, and time-cycled. APRV provides 2 levels of airway pressure (P(high) and P(low)) during 2 time periods (T(high) and T(low)), both set by the clinician. APRV usually involves a long T(high) and a short T(low). APRV uses an active exhalation valve that allows spontaneous breathing during both T(high) and T(low). APRV typically generates a higher mean airway pressure with a lower tidal volume (V(T)) and lower positive end-expiratory pressure than comparable levels of other ventilation strategies, so APRV may provide better alveolar recruitment at a lower end-inflation pressure and therefore (1) decrease the risk of barotrauma and alveolar damage in patients with acute lung injury or acute respiratory distress syndrome (ALI/ARDS), and (2) provide better ventilation-perfusion matching, cardiac filling, and patient comfort than modes that do not allow spontaneous breaths. However, if the patient makes a spontaneous breath during T(high), the V(T) generated could be much larger than the clinician-set target V(T), which could cause the end-inflation transpulmonary pressure and alveolar stretch to be much larger than intended or produced in other ventilation strategies. It is unknown whether a patient's inspiratory effort (and consequent larger V(T)) can damage alveoli in the way that mechanically delivered, positive-pressure breaths can damage alveoli in ALI/ARDS. Other ventilation modes also promote spontaneous breaths, but at overall lower end-inflation transpulmonary pressure. There is a dearth of data on what would be the optimal APRV inspiratory-expiratory ratio, positive end-expiratory pressure, or weaning strategy. The few clinical trials to date indicate that APRV provides adequate gas exchange, but none of the data indicate that APRV confers better clinical outcomes than other ventilation strategies.
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