Arterial isoflurane concentration and EEG burst suppression during cardiopulmonary bypass

Isoflurane (1.5 to 3.0 vol% in oxygen) was used to control intraoperative hypertension in 10 patients undergoing hypothermic cardiopulmonary bypass surgery. Isoflurane was administered through the membrane oxygenator of the bypass pump and yielded plateau concentrations in arterial blood ranging from 36.6 to 84.4 micrograms/ml (0.5 and 1.16 vol%, respectively). Isoflurane dosing resulted in prolonged periods (21 to 63 minutes) of EEG burst suppression and isoelectric activity in nine patients. Burst suppression was not a result of hypothermia. There was a close temporal relationship between isoflurane concentration and the onset of burst suppression (mean onset time: 27.3 +/- 4.56 minutes after isoflurane begun). The mean arterial isoflurane concentration at the onset of burst suppression was 46.5 +/- 10.7 micrograms/ml; the nasopharyngeal temperature was 26.0 degrees +/- 0.61 degrees C. Isoflurane was eliminated rapidly from blood with a mean apparent t1/2 of 18.8 +/- 5.46 minutes.