Effect of PaCO2 on O2 consumption during cardiopulmonary bypass in man

Under conditions of anesthesia with controlled respiration, the changes in the state of the circulation due to altered PaCO2 are often overlooked or attributed to the anesthetic agent. During cardiopulmonary bypass when cardiac output is kept constant, change in fluid level of the extracorporeal reservoir reflects overall change in the circulatory beds. Following an hour on total bypass for stabilization of all parameters, a change in the oxygenator ventilating-gas mixture was induced to increase or decrease the PaCO2 an average of 11 torr in 40 patients. A reduction in PaCO2 from 37 to 25 torr increased mean arterial blood pressure (MAP), central venous pressure (CVP), and the volume of blood stored in the extracorporeal circuit, signifying an overall reduction in the intravascular capacity. This was associated with a decrease in total O2 consumption (VO2). An abrupt increase of PaCO2 from 26 to 37 torr produced a decrease in MAP,in CVP, and in the reservoir blood level, signifying blood "take up" by the patient. There was also an associated increase in VO2.