Changes in hemodynamic variables during hypothermic cardiopulmonary bypass. Effects of flow rate, flow character, and arterial pH.

During hypothermic cardiopulmonary bypass, the effects on hemodynamic variables of alternating pump flow rate between 1.5 and 2.0 L.min-1.m-2, flow character between nonpulsatile and pulsatile perfusion, and acid-base management between pH- and alpha-stat control were studied in a crossover factorial experiment. Twenty-four patients who were undergoing elective coronary artery bypass grafting were studied during stable hypothermic (27 degrees to 29 degrees C) cardiopulmonary bypass. A minimum of two (when time allowed, three) consecutive 10-minute periods (period 1, 2, or 3) were investigated. Only stage of the study period during cardiopulmonary bypass, flow rate, and interaction between stage and acid-base management were found to have significant effects on mean arterial pressure. In all patients, there were average increases in mean arterial pressure from period 1 to period 2 of 9.4 (95% confidence interval 5.8, 13.0) mm Hg, from period 2 to period 3 of 6.3 (95% confidence interval 1.2, 11.4) mm Hg, and from period 1 to period 3 of 15.7 (95% confidence interval 10.6, 20.9) mm Hg. At 2.0 L.min-1.m-2, mean arterial pressure was 7.2 (95% confidence interval 1.6, 12.9) mm Hg higher than at 1.5 L.min-1.m-2. Peripheral vascular resistance was significantly affected only by stage and flow rate. There were, in all patients, mean increases in peripheral vascular resistance from period 1 to period 2 of 239 (95% confidence interval 135, 343) dynes.sec.cm-5, from period 2 to period 3 of 85 (-64, 234) dynes.sec.cm-5, and from period 1 to period 3 of 324 (95% confidence interval 175, 473) dynes.sec.cm-5. At 1.5 L.min-1.m-2, the peripheral vascular resistance was 316 (95% confidence interval 152, 480) dynes.sec.cm-5 higher than at 2.0 L.min-1.m-2. Alteration in flow rate, but not flow character or arterial pH, had a significant effect on peripheral vascular resistance. It is hypothesized that the increase in peripheral vascular resistance during the course of cardiopulmonary bypass results from an active capillary mechanism, whereas the increase that is associated with reduction in flow rate reflects a passive mechanism. The increase in peripheral vascular resistance with decrease in flow rate indicates impaired tissue perfusion, unlike that occurring with stage.