An improved rebreathing method for measuring mixed venous carbon dioxide tension and its clinical application

The mixed venous carbon dioxide tension (PVCO2) can be measured at the bedside by a rebreathing equilibrium technique that is quick, simple and noninvasive. Only one brief period of rebreathing is required. The technique is accurate even when the lungs are not normal, and gives a graphic record that allows verification of the accuracy of the estimate. The PVCO2 is affected mainly by changes in alveolar ventilation and cardiac output. It can be measured instead of the arterial carbon dioxide tension (PACO2) to follow changes in alveolar ventilation when the cardiac output is normal (PaCO2 = 0.8 PVCO2). When the cardiac output is abnormal, measurement of both PaCO2 and PvCO2 is useful in determining how much the cardiac output is reduced. Consideration of the relation between oxygen consumption and carbon dioxide production suggests that the equation PaCO2 = 0.8 PVCO2 - 12 indicates a reduction in cardiac output that may impair the oxygen supply to tissues. Simple corrections can be applied to allow for variations in arterial oxygen saturation and hemoglobin concentration that will affect this relationship.