Blood gas analysis of mixed venous blood during normoxic acute isovolemic hemodilution in pigs

Mixed venous oxygen saturation of hemoglobin (SvO2) and mixed venous oxygen tension (PvO2) may reflect the overall balance between oxygen consumption and delivery. Because of the potential value of monitoring SvO2 and PvO2 as indications of the state of tissue oxygenation, the aim of this study was to determine, during normoxic acute isovolemic hemodilution in pigs, the critical PvO2, critical SvO2, and critical oxygen extraction ratio (ER) at which oxygen uptake starts to decline during further induced hemodilution. During stepwise induced isovolemic hemodilution, a gradual decline in SvO2 and PvO2 was observed in all animals. The mean +/- SD of the critical PvO2 of six animals was 32.3 +/- 3.1 mm Hg. The mean +/- SD of the critical SvO2 was 44.2% +/- 7.9%. The ER increased gradually. At an ER of 0.57 +/- 0.08, oxygen uptake started to decline. A significant correlation was found between changes in SvO2 and changes in ER. These degrees of hemodilution were accompanied by an increase in cardiac index, pulmonary wedge pressure, heart rate, and left ventricular stroke work index. Only a slight decrease in systemic vascular resistance was observed. We conclude that measurements of PvO2 and SvO2 can be used as indicators of the critical point of hemodilution and that the SvO2 during hemodilution reflects the overall balance between oxygen uptake and oxygen delivery, confirmed by the strong correlation found between SvO2 and oxygen extraction ratio.