Reliability of mixed venous oxygen saturation as an indicator of the oxygen extraction ratio demonstrated by a large patient data set.
BACKGROUND Mixed venous saturation (SVO2) represents the oxygen extraction ratio (O2ER) through the following relationship: SVO2 approximately equal 1 - O2ER, assuming a negligible contribution of dissolved oxygen to blood oxygen content. We sought to determine whether altered FiO2 levels would affect the correlation between SVO2 and O2ER.


METHODS
A theoretical analysis was performed to determine the extent to which SVO2 should be affected by different Fio2 levels at various ranges of O2ER using several linked spreadsheets. The relationships obtained were compared with actual patient data stored in a large patient data set. RESULTS Theoretical analysis indicated that increases in Fio2 should produce a small stepwise increase in the observed SVO2 that is independent of O2ER. However, analysis of 8,324 simultaneous data points for SVO2 and O2ER failed to demonstrate this pattern, primarily because patients with higher Fio2 concentrations tend to have higher O2ERs, and hence lower SVO2. CONCLUSION SVO2 measurements are not significantly affected by Fio2 levels in critically ill patients.
Keech
John
J
Stritch School of Medicine, Loyola University Medical Center, Chicago, Illinois 60153, USA
Reed
R Lawrence
RL
D000328:Adult; D006801:Humans; D008962:Models, Theoretical; D010100:Oxygen / Q000097:blood*; D010101:Oxygen Consumption / Q000502:physiology*
D010100:Oxygen