To the Editor,
Awareness and recall during general anesthesia is a rare yet serious complication that has been reported to occur in an estimated one in 8,600 cases of cardiothoracic anesthesia.1 There are numerous potential causes for awareness during cardiovascular operations. During cardiopulmonary bypass (CPB), general anesthesia is often a collaborative effort between the anesthesiologist and the perfusionist. Standard monitoring of the depth of anesthesia with end-tidal volatile concentration from the respiratory circuit is not feasible during CPB due to cessation of ventilation and lack of pulmonary blood flow. Various techniques are used to monitor the state of anesthesia during CPB, including serum volatile anesthetic concentration, end-tidal volatile anesthetic concentration at the CPB pump oxygenator exhaust line, and the bispectral index (BIS™; Minneapolis, MN, USA).2 Each of the above techniques has limitations that make universal application challenging. Analysis of serum volatile anesthetic concentration is not available for point-of-care testing, making real-time utilization impractical. Sampling end-tidal volatile anesthetic concentration at the CPB oxygenator exhaust line does not correlate well with serum plasma volatile anesthetic concentration due to the “leakiness” engineered into the membrane oxygenator. Furthermore, it can also vary with the utilization of vacuum scavenger gas systems that can further entrain room air into the exhaust line.3,4 The use of BIS during cardiac surgery is common; however, displayed numerical values cannot always reliably predict the depth of anesthesia and thus far have failed to prevent awareness in large studies.