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The optimal time for measuring the cardiorespiratory effects of positive end-expiratory pressure.

No study has elucidated the minimum time taken for oxygen delivery to reach steady-state levels after an increase in positive end-expiratory pressure (PEEP). We therefore investigated ten ventilated patients who received a total of 27 increments in PEEP of 5 cm H2O. Cardiorespiratory parameters were measured at baseline, at 1 min, and thereafter at 5-min intervals until 2 consecutive measurements of oxygen delivery were constant. Cardiac output was measured continuously using an esophageal Doppler transducer. The PEEP was increased to a maximum of 20 cm H2O or until oxygen delivery fell by 15 percent. Increases in PEEP were always associated with increases in oxygen saturation. Cardiac output decreased in the majority of cases, the greatest falls occurring in the first minute after an increase in PEEP, and with higher levels of PEEP. No significant changes in oxygen delivery occurred after 15 min. We therefore recommend that 15 min be allowed to elapse after an increase in PEEP before oxygen delivery is reassessed.