Cerebral metabolic suppression during hypothermic circulatory arrest in humans

Background. Hypothermic circulatory arrest (HCA) is used in surgery for aor tic and congenital cardiac diseases. Although studies of the safety of HCA in animals have been carried out, the degree to which metabolism is suppres sed in patients during hypothermia has been difficult to determine because of problems with serial measurements of cerebral blood flow in the clinical setting. Methods. To quantify the degree of metabolic suppression achieved by hypoth ermia, we studied 37 adults undergoing operations employing HCA. Cerebral b lood flow was estimated using an ultrasonic flow probe on the left common c arotid artery, and cerebral arteriovenous oxygen content differences were c alculated from jugular venous bulb and arterial oxygen saturations. Cerebra l metabolic rates while cooling were then ascertained. The temperature coef ficient, Q(10), which is the ratio of metabolic rates at temperatures 10 de grees C apart, was determined. Results. The human cerebral Q(10) was found to be 2.3. The cerebral metabol ic rate is still 17% of baseline at 15 degrees C. If one assumes that cereb ral blood flow can safely be interrupted for 5 min at 37 degrees C, and tha t cerebral metabolic suppression accounts for the protective effects of hyp othermia, the predicted safe duration of HCA at 15 degrees C is only 29 min . Conclusions. The safe intervals calculated from measured cerebral oxygen co nsumption suggest that shorter intervals and lower temperatures than those currently used may be necessary to assure adequate cerebral protection duri ng hypothermic circulatory arrest. (C) 1999 by The Society of Thoracic Surg eons.