Beneficial effect of continuous positive pressure ventilation on cardiac output
Continuous positive pressure ventilation (CPPV), when accompanied by hypovolemia, may be associated with a decrease in cardiac output (CO). The purpose of this study is to define the patient at risk of altering his CO due to CPPV. Left ventricular end diastolic pressure (LVEDP), functional residual capacity (FRC), total lung capacity (TLC) and blood volume (BV) were determined in 39 patients prior to cardiac surgery. Following surgery duplicate CO, arterial CO2 tension (PaCO2) and BV were obtained. Ten cm of water CPPV was instituted and CO and PaCO2 determined 30 min later. Five postcardiac surgery patients served as controls with serial CO's at 30 min intervals without CPPV. In 14 patients (Group I) CO decreased (mean -21%). In 6 patients (Group II) CO changed less than 5% (mean -1.3%). In 14 patients (Group III) CO increased (mean +17%). Change in CO over 30 min in the control group was mean 2.2%. Decrease in BV for Group I vs III vs control patients was 12% vs 15% vs 15%. The FRC to TLC ratio in Groups I and III patients was 58% and 54%. Mean resting LVEDP was 7.9 mm Hg (Group I) and 13.2 mm Hg (Group III). CPPV decreases CO in most but not all patients. Blood and pulmonary gas volumes of Group I and III patients were similar. LVEDP, however, in Groups I and III were found to be different by Student's 't' test at the 0.01 level. The increase in CO in Group III patients despite the decreased BV appears to be due to a decrease in left ventricular afterload with CPPV.