Variability of effect of positive end expiratory pressure

Mechanical ventilation with positive end expiratory pressure (PEEP) has been reported to produce prompt improvement in oxygeneration when used to treat acute respiratory failure. Reports of the effect of PEEP on cardiac output have been conflicting. We studied 14 patients and found that, although mean values of arterial oxygen tension (PaO2) increased as PEEP increased, on eight occasions in the acute studies and three in the elective studies, it fell as PEEP was increased. On five occasions the fall in PaO2 was associated with an increased shunt. Arterial pressure did not, on the average, change at any level of PEEP, but in 13 of 30 measurements it fell by more than 20% and necessitated curtailment of the study at blood pressure levels of less than 50 torr. Since these effects of PEEP cannot be predicted, careful patient monitoring is essential when ventilation with PEEP is utilized.