Alterations in Sympathetic Nervous System Activity with Intraoperative Hypothermia during Coronary Artery Bypass Surgery: Discussion

Human plasma NE levels have been used as an index of SNS activity. On the other hand, plasma EPI concentrations indicate adrenal medullary activity because EPI is almost exclusively formed in this tissue.
The present observation regarding hypothermia and catecholamines is consistent with previous work in hypothermic baboons in which we described a “switch off” of the SNS below a core temperature of 31°C. The SNS, which is important during hypothermia, has been studied by Oyer et al, who showed that rewarming hamsters from hypothermia is delayed if beta-adrenergic blockade is present. The release of NE has also been well documented only during mild surface cooling. Some authors believe that this response may in part be due to cold’s local effect upon blood vessels. In addition, oxygen consumption and core temperature are lower during cold exposure in rats who have received ganglionic blockade with hexamethonium when compared to untreated animals. The NE administered to rats in this setting reverses the effects of hexamethonium. Catecholamines (NE»EPI) are necessary for mammalian thermogenesis. A fall in their plasma concentrations, paralleling a fall in core temperature, may reflect a critical defect in the body’s homeostatic mechanisms to regulate core temperature as well as peripheral vascular resistance and cardiac output. During cooling, intense SNS activation is expected due to reflex peripheral vasoconstriction in a homeostatic attempt to decrease heat loss from skin. During this stage of cooling, hypotension may not respond to the administration of exogenous alpha stimulants. Arrhythmias may respond to beta-blockers. The subsequent fall in plasma NE and EPI concentrations with continued cooling may be due to dilutional factors or SNS exhaustion of releasable NE. At this stage, we speculate that exogenous alpha agonists may reverse hypotension.

Category: Coronary Artery

Tags: blood pressure, Coronary Artery Bypass Surgery, heart, hypothermia