Session 4
Cerebral metabolic suppression during hypothermic circulatory arrest in humans

Presented at the Aortic Surgery Symposium VI, April 30–May 1, 1998, New York, NY.
https://doi.org/10.1016/S0003-4975(99)00441-5Get rights and content

Abstract

Background. Hypothermic circulatory arrest (HCA) is used in surgery for aortic and congenital cardiac diseases. Although studies of the safety of HCA in animals have been carried out, the degree to which metabolism is suppressed 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 hypothermia, we studied 37 adults undergoing operations employing HCA. Cerebral blood flow was estimated using an ultrasonic flow probe on the left common carotid artery, and cerebral arteriovenous oxygen content differences were calculated from jugular venous bulb and arterial oxygen saturations. Cerebral metabolic rates while cooling were then ascertained. The temperature coefficient, Q10, which is the ratio of metabolic rates at temperatures 10°C apart, was determined.

Results. The human cerebral Q10 was found to be 2.3. The cerebral metabolic rate is still 17% of baseline at 15°C. If one assumes that cerebral blood flow can safely be interrupted for 5 min at 37°C, and that cerebral metabolic suppression accounts for the protective effects of hypothermia, the predicted safe duration of HCA at 15°C is only 29 min.

Conclusions. The safe intervals calculated from measured cerebral oxygen consumption suggest that shorter intervals and lower temperatures than those currently used may be necessary to assure adequate cerebral protection during hypothermic circulatory arrest.

Section snippets

Clinical protocol

Thirty-seven patients undergoing nonemergent operations on the ascending aorta or arch utilizing an interval of DHCA between January 31, 1996 and April 16, 1997 were included in the analysis. Anesthesia was induced and maintained with high-dose opioids, 100% oxygen, and muscle relaxant technique, supplemented with isoflurane and midazolam as needed to maintain hemodynamic stability. Intraoperative monitoring included an intraarterial catheter, a pulmonary artery catheter placed via an internal

Results

The demographic characteristics of the population are reported in Table 1. An interval of selective cerebral perfusion was used in 5 (14%) patients for an average of 37 minutes. A brief interval of retrograde perfusion to flush air and debris from the open aorta was used in 6 (16%) patients for an average interval of 7 ± 3 min. A more prolonged period of retrograde cerebral perfusion was used in 13 (35%) of patients, with an average duration of 25 ± 7 min.

The physiologic variables and cerebral

Comment

We found Q10, the metabolic coefficient for the adult human brain during cooling to profoundly hypothermic temperatures, to be 2.3. The relationship between CMRO2 and temperature best fit a log-linear model over the temperature range investigated. The theoretical “safe” duration for hypothermic circulatory arrest derived from our data fit closely with observed clinical experience 1, 8, implying that hypothermic metabolic suppression is a major protective mechanism for the brain during intervals

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