Mixed venous oxygen tension as an estimate of cardiac output in anesthetized horses

The relationship between mixed venous O2 tension and cardiac output was studied in six anesthetized horses breathing 100% O2. Cardiac output, O2 consumption, mean arterial pressure, heart rate, and arterial and venous blood gases were measured after administration of xylazine or dobutamine to horses in lateral, sternal, and dorsal recumbencies. After approximately 3 hours, Escherichia coli endotoxin was administered while horses were in dorsal recumbency, and all measurements were repeated. Relationships between cardiac index (CI) and PVO2, heart rate, mean arterial pressure, jugular PVO2, and PVO2 of blood from a superficial limb vein were evaluated by linear regression analysis. Mean arterial pressure was significantly (P less than 0.05) correlated with CI in horses in all positions and after endotoxin administration. However, data points were poorly grouped. Heart rate and CI were significantly correlated in horses in all positions, but not after endotoxin administration. Correlations between jugular PVO2 and PVO2 of blood from a superficial limb vein were not significant in horses in sternal recumbency, and PVO2 of blood from a superficial limb vein was not significantly correlated with CI in horses in lateral recumbency. There was a significant and tight correlation between PVO2 and CI in horses in all positions and after endotoxin administration.