CARDIOVASCULAR ENGINEERING Journal for Extracorporeal Circulation, Assist Devices,Transplantation and Artificial Organs
	Volume 1, 1996, No 1

Cerebral Blood Flow and Metabolic Rate During Hypothermic Cardiopulmonary Bypass

D. J. Cook

Abstract:
In itself, bypass of the heart and lungs with a mechanical pump and oxygenator has surprisingly little effect on cerebral blood flow (CBF) and metabolism (CMRO2). If the perfusate is normothermic, the prime whole blood, mean arterial pressure and PaCO2 in a normal range, global cerebral perfusion and oxygen consumption are essentially unchanged from the nonbypass state. While the determinants of cerebral physiology are unchanged during cardiopulmonary bypass (CPB), CPB typically does not mimic the nonbypass state. Virtually all perfusion practice is associated with hemodilution and temperature change, and the temperature changes which occur are rapid and impact on virtually all determinants of CBF and CMRO2. Temperature is typically the most unstable of physiologic variables during CPB, and in a two-hour bypass run, body temperature may be stable for only 30 min. As temperature changes, so does whole body and brain oxygen demand, the rheologic characteristics of blood, the pH of neutrality, CO2 solubility, hemoglobin oxygen affinity, vascular resistance, and probably cerebral vascular responsiveness. These variables are changing simultaneously, may be competitive, and have differing rates of change. This interaction makes definition of an optimal perfusion condition for hypothermic CPB a difficult proposition. This review will provide a structural foundation for discussion of the variables determining CBF and CMRO2 during hypothermic CPB and will introduce some of their complex interactions. A brief discussion of the potential role of CBF manipulation during periods of temperature change will be provided as an overview of techniques for assessing cerebral perfusion and oxygenation during hypothermic CPB.

Keywords:
cerebral blood flow, cerebral metabolic rate, cardiopulmonary bypass, hypothermia

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