Relation between isoflurane concentration, body temperature and EEG variables during hypothermic cardiopulmonary bypass in juvenile sheep

Isolation of lungs from the circulation during cardiopulmonary bypass (CPB) results in the loss of end-tidal anaesthetic measurement as a monitor of anaesthetic depth. The objective of our study was to test whether the isoflurane concentration measured at the oxygenator exhaust (ISOPO-OX) could be used as an alternative. Five juvenile ewes were anaesthetised for experimental aortic valve replacement during hypothermic CPB. One sheep recovered from anaesthesia with signs of CNS excitement and was excluded from electroencephalogram (EEG) data analysis. ISOPO-OX was similar to the end-tidal anaesthetic concentration measured before start of CPB (ISOET(PRE-CPB)) and was positively correlated (P < 0.001; R = 0.431) to the bypass temperature (TCPB). EEG burst suppression ratio was negatively correlated with the oesophageal temperature (TESO) (P = 0.002; R = 0.433) whilst data analysis revealed only a weak association between EEG variables and ISOPO-OX. Therefore, EEG monitoring may be the only valuable quantitative measure of adequate anaesthetic depth during hypothermic CPB.