The induction of deep, systemic hypothermia is a commonly used technique during cardiac surgery. Although operative hypothermia has been used since 1949, the hormonal responses to this stress have been incompletely studied. A “switch off” of sympathetic nervous system (SNS) activity (as determined by changes in plasma norepinephrine concentration) in a hypothermic non-human primate model has been described with core body temperatures below 31°C.U We and others have described rapid changes in serum thyroid hormone values and angiotensin converting enzyme (ACE) activity in human patients after hypothermic coronary artery bypass surgery (CABS) and during critical illness. We, therefore, hypothesized that some surgery-induced hormonal changes may be, in part, due to profound intraoperative hypothermia. To test this theory, we performed serial determinations of plasma norepinephrine (NE), epinephrine (EPI), serum total and free triiodothyronine (T3), thyroxine (TJ, and ACE activity levels in cardiac surgical patients during progressive intraoperative hypothermia and again during rewarming.
TWenty-one patients (19 men, two women) were enrolled in this study which was approved by our hospitals institutional review board. All patients signed written informed consents preoperatively. Ten of the patients were studied before and during the extracorporeal cooling used in their cardiac bypass surgery. The other 11 patients were studied during rewarming and again three and four hours after rewarming from similar heart surgery. Since our protocol called for multiple blood sampling (see below), we purposely divided our subjects into these two groups so as to avoid excessive blood letting in any one patient. Cardiopulmonary bypass was performed using a bubble oxygenation system and pump for extracorporeal circulation.