Relationship between mixed venous oxygen saturation and markers of tissue oxygenation in progressive hypoxic hypoxia and in isovolemic anemic hypoxiain 8-to 12-day-old piglets

Objective: To examine the hypothesis that mixed venous oxygen saturation (S (v) over bar o(2)) values, which reflect the residual oxygen after tissue oxygen extraction, would be similar during hypoxic and anemic hypoxia. Design: S (v) over bar o(2) values, oxygen delivery, arterial oxygen conten t, and fractional oxygen extraction were compared, and critical values were determined based on lactate, the lactate/pyruvate ratio, and oxygen consum ption during hypoxic and anemic hypoxia. Setting. Laboratory of physiology at a university hospital. Subjects: Two groups of eight piglets, 8 to 12 days old. Interventions: Piglets were anesthetized, tracheotomized, intubated, and ve ntilated. A thoracotomy was performed and a fiberoptic catheter was placed in the pulmonary artery to monitor S (v) over bar o(2). A transit time ultr asound flow probe was positioned around the ascending aorta to measure aort a flow. Progressive hypoxic hypoxia was induced by decreasing F10(2) from b aseline (0.30-0.75) to 0.21, 0.15, and 0.10. Progressive anemic hypoxia was induced by a repeated isovolemic exchange transfusion with 50 mL of pasteu rized plasma. Measurements and Main Results: Fifteen or 30 mins after each intervention, samples were taken from the carotid artery for blood gases, hemoglobin, lac tate, and pyruvate and from the pulmonary artery for blood gases and hemogl obin. Hemodynamic, arterial oxygen saturation, and S (v) over bar o(2) meas urements were made. The calculated oxygen delivery and oxygen consumption d ecreased in both hypoxic and anemic hypoxia. At the lowest oxygen delivery level of anemic hypoxia, the decrease in S (v) over bar o(2) was less than that in hypoxic hypoxia (-26% vs. -55%). The range of critical values for S (v) over bar o(2) calculated for each individual piglet below which lactat e, the lactate/pyruvate ratio, and oxygen consumption rapidly changed from baseline value was significantly lower in hypoxic hypoxia (11% to 24%) than in anemic hypoxia (26% to 48%). Fractional oxygen extraction increased sig nificantly but not with a change as high as in hypoxic hypoxia 0.31 (range, 0.20-0.41) vs, 0.49 (range, 0.41-0.54). Conclusions: ln comparison with hypoxic hypoxia, critical values of S (v) o ver bar o(2) are higher in anemic hypoxia, indicating that oxygen unloading from blood to tissues is impaired in anemic hypoxia. These characteristics in oxygen transport and capillary hemodynamics should be taken into consid eration when S (v) over bar o(2) is used in clinical critical care.