Hemodynamic and myocardial metabolic consequences of PEEP

The cardiac effects of positive end expiratory pressure (PEEP) were examined in 50 patients six hours after elective coronary bypass surgery. Increasing the level of PEEP from 5 to 10 to 15 cm H2O decreased cardiac index (evaluated by thermodilution), stroke index and left ventricular end diastolic volume index without a change in left ventricular ejection fraction (evaluated by nuclear ventriculography). Right ventricular end diastolic volume index remained unchanged. Coronary sinus blood flow (measured by the continuous thermodilution technique) and myocardial oxygen and lactate consumption were unchanged with the application of 15 cm H2O PEEP. In 21 patients, volume loading (250 ml [mL] of plasma) was performed at 5 cm, and again at 15 cm H2O PEEP. Volume loading produced a similar increase in cardiac volumes and cardiac index at 5 and 15 cm H2O PEEP. Right and left ventricular performance and left ventricular systolic function were not altered by PEEP (by analyses of covariance). Coronary sinus blood flow and myocardial oxygen consumption increased with volume loading at 5 and 15 cm H2O of PEEP, but myocardial lactate utilization tended to increase at 5 cm, and decrease at 15 cm H2O PEEP (p = 0.08). Of the 33 patients who underwent complete hemodynamic and metabolic measurements, 16 increased cardiac lactate utilization at 15 cm H2O PEEP and 17 decreased cardiac lactate utilization at 15 cm H2O PEEP. PEEP decreased cardiac index, perhaps by reducing left but not right ventricular volumes. Volume loading during PEEP restored cardiac index and revealed no depression in myocardial performance or systolic function. With the application of PEEP, myocardial metabolism was maintained in half the patients, but ischemic metabolism was observed in the other half.