Blood/gas solubility coefficient and blood concentration of enflurane during normothermic and hypothermic cardiopulmonary bypass

Abstract

The blood/gas solubility coefficient and blood concentration of enflurane were measured at intervals in 10 patients undergoing coronary artery revascularization with cardiopulmonary bypass (CPB) and moderate hypothermia. A constant end-tidal concentration of enflurane was maintained throughout the study. Blood/gas solubility coefficient was determined at 37°C, which when combined with an initial single-step equilibration of the blood sample with air, permitted the accurate measurement of blood concentration. Blood/gas solubility coefficient and blood concentration both decreased significantly with the onset of CPB. During the period of hypothermia, blood/gas solubility as measured at 37°C showed little change; however, there was a progressive, marked increase in blood concentration with a mean increase of 80% prior to rewarming. Therefore, the level of anesthesia provided by enflurane may lighten with the onset of CPB, and a deeper level will accompany any decrease in blood temperature. On rewarming, blood concentration levels rapidly returned to levels similar to those measured before cooling. The increased uptake and accumulation of volatile anesthetic agent that occurred as a result of the period of hypothermic CPB was rapidly cleared. The rapidity with which blood concentration responded to the changes occurring during CPB make it unlikely that there was any significant increase in myocardial depression in response to the raised blood concentration secondary to the hypothermia.