LIMITATIONS ON ARTERIOVENOUS COOLING OF THE BLOOD-SUPPLY TO THE HUMANBRAIN

Arteriovenous heat transfer (AVHT) is a thermoregulatory phenomenon wh ich enhances tolerance to thermal stress in a variety of animals. Seve ral authors have speculated that human responses to thermal stress ref lect AVHT in the head and neck, even though primates lack the speciali zed vascular arrangements which characterize AVHT in other animals. We modeled heat transfer based on the anatomical relationships and blood flows for the carotid artery and associated venous channels in the hu man neck and cavernous sinus. Heat transfer rate was predicted using t he ''effectiveness-number of transfer units'' method for heat exchange r analysis. Modeling showed that AVHT is critically dependent upon (1) heat exchanger effectiveness and (2) arteriovenous inlet temperature difference. Predicted heat exchanger effectiveness is less than 5.5% f or the neck and 0.3% for the cavernous sinus. These very low values re flect both the small arteriovenous interface for heat exchange and the high flow rate in the carotid artery. In addition, humans lack the st rong venous temperature depression required to drive heat exchange; bo th the cavernous sinus and the internal jugular vein carry a large pro portion of venous blood warmed by its passage through the brain as wel l as a small contribution from the face and scalp, whose temperature v aries with environmental conditions. Under the most optimistic set of assumptions, carotid artery temperature would be lowered by less than 0.1 degrees C during its passage from the aorta to the base of the bra in. Physiologically significant cooling of the blood supply to the bra in cannot occur in the absence of a suitably scaled site specialized f or heat exchange.