PERIOPERATIVE MEDICINE Relationship between Anesthetic Depth and Venous Oxygen Saturation during Cardiopulmonary Bypass

ABSTRACT Background: During cardiopulmonary bypass, mixed venous oxygen saturation (SvO 2 ) is frequently measured to assess circulatory adequacy. Fluctuations in SvO 2 not related to patient movement or inadequate oxygen delivery have been attributed clinically to increased cerebral oxygen consumption due to "light" anesthesia. To evaluate the relationship between anesthetic depth and SvO 2 , we prospectively measured bispectral index (BIS) and SvO 2 values in patients undergoing cardiac surgery with cardiopulmonary bypass. Methods: Adults scheduled for cardiac surgery with cardiopulmonary bypass were recruited for this prospective observational study. During bypass, BIS and SvO 2 values were recorded every 5 min. To control for confounding effects of changes in other variables known to affect SvO 2 , temperature, hematocrit, bypass pump flow, muscle relaxant use, and intravenous and inhaled anesthetic doses were also recorded. Only periods with limited variation in other variables affecting SvO 2 were analyzed. The relationship between BIS and SvO 2 was evaluated using mixed linear regression. Results: One thousand thirty-four data points were obtained in 41 patients. No overall association between BIS and SvO 2 was observed, either in unadjusted analysis or adjusted for covariates. In data points with temperatures less than the median (T Ͻ 34.1°C), a significant association between BIS and SvO 2 was observed both in unadjusted (␤ ϭ Ϫ0.32, P ϭ 0.01) and adjusted (␤ ϭ Ϫ0.27, P ϭ 0.04) analyses. Conclusions: In patients undergoing cardiopulmonary bypass, we found no overall association between BIS and SvO 2 . A weak but statistically significant association between BIS and SvO 2 was observed in patients with temperatures less than 34.1°C. These data suggest that low SvO 2 values on bypass are unlikely to be due to light or inadequate anesthesia. The relationship among temperature, BIS and SvO 2 deserves further study. D URING cardiopulmonary bypass, mixed venous oxygen saturation (SvO 2 ) is helpful for measuring circulatory adequacy. A decreased SvO 2 indicates inadequate oxygen delivery relative to oxygen consumption and suggests either that oxygen consumption has increased or that oxygen delivery has decreased. 1 In principle, during periods of cardiopulmonary bypass when oxygen delivery is constant and the anesthetized patient is at a constant temperature, SvO 2 should not change. However, SvO 2 values can vary in the absence of changes in temperature or oxygen delivery. One explanation cited in cardiac anesthesia textbooks for changes in SvO 2 is a change in anesthetic depth. 2,3 Existing data demonstrate that decreased anesthetic depth increases cerebral metabolic rate and cerebral oxygen consumption. 4 -6 Conversely, deeper levels of anesthesia decrease oxygen consumption. 7 Against a background of constant temperature, unchanged systemic oxygen delivery, and no patient movement, changes in SvO 2 may thus be potentially What We Already Know about This Topic ❖ Cardiac surgery caries a higher than normal risk for awareness ❖ During cardiopulmonary bypass, some clinicians consider lower than expected mixed venous oxygen saturation (SvO 2 ) to indicate possible awareness What This Article Tells Us That Is New ❖ In more than 1,000 data points in 41 patients, there was no relationship between the bispectral index as a measure of anesthetic depth and SvO 2 during cardiopulmonary bypass, suggesting that SvO 2 is not a good measure of anesthetic dept