Mixed venous oxygenation in fatal gram-negative septicemia

Changes in mixed venous and arterial PO2 values were examined during the early, middle and late stages of fatal gram-negative septicemia in seven patients. The early stage of septicemia was characterized by tachycardia (mean rate 108/min), mild hypotension (mean arterial pressure (MAP) 83 mmHg), arterial hypoxemia requiring supplemental oxygen therapy, and high, low or normal mixed venous PO2 (mean 37 mmHg). During the middle stage, hypotension was severe (MAP 67 mmHg) and mixed venous PO2 invariably rose (mean 50 mmHg). Arterial PO2 increased in association with the rise in mixed venous PO2, despite constant FIO2 and expired ventilation. Severe metabolic acidosis developed in six of the seven patients. In the final stage, mixed venous PO2 was normal (mean 42 mmHg) in the presence of continuing metabolic acidosis. We postulate that the elevations of mixed venous PO2 seen result from systemic arterial-venous shunting, and that during septicemia, the mixed venous PO2 is an unreliable index of tissue oxygenation. A theoretical model of the relationship of tissue oxygenation and mixed venous oxygenation in septicemia, based upon the above study, is presented.