Influence of different release times on spontaneous breathing pattern during airway pressure release ventilation

Airway pressure release ventilation (APRV) is a ventilatory mode with a time cycled change between an upper (P(high)) and lower (P(low)) airway pressure level. APRV is unique because it allows unrestricted spontaneous breathing throughout the ventilatory cycle. We studied the influence of different release times (time of P(low)) on breathing pattern and gas exchange in patients during partial mechanical ventilation.

Mixed intensive care unit in a university hospital.

Twenty-eight patients were included in the study. Nine patients suffering from acute lung injury (ALI), 7 patients with a history of chronic obstructive pulmonary disease (COPD) and 12 patients with nearly normal lung function, ventilated for non-respiratory reasons (postoperatively), were studied prior to extubation.

At constant pressure levels and a pre-set airway pressure release rate of 12/min, P(low) was diminished and P(high) was prolonged in four steps of 0.5 s. Each respiratory setting was studied for 20 min after a steady state period had been achieved.

We measured gas exchange and respiratory mechanics. The different time intervals of P(high) and P(low) had only minor effects on the actual spontaneous inspiration and expiration times, but the proportion of spontaneous breathing on total ventilation increased when the duration of P(low) was decreased. Gas exchange was almost unaffected by the interventions despite a significant increase in mean airway pressure. However, when P(low) was set to only 0.5 s an increase in PaCO(2) occurred in patients with COPD and ALI, probably due to a decrease in mechanical ventilatory support.

Airway pressure release ventilation is an open system which allows patients to maintain the "time control" over the respiratory cycle independent of the chosen duration for P(high) and P(low).