Cerebral Oxygenation During A Passive Heating And Cooling Protocol

Abstract

Near Infra-red Spectroscopy (NIRS) is a technology which allows for non-invasive measurement of cerebral oxygenation. While study of muscular and heart rate responses to heating have been examined, the effect of passive heating and cooling on cerebral oxygenation is not well documented. PURPOSE The current study represents pilot research into the effect of passive heating and cooling on cerebral oxygenation as measured by NIRS. METHODS Five healthy male subjects (age 22 ± 3 years: mean ± SD) were passively heated (∼2 hr) in an environmental chamber while simultaneously wearing a circulating water-liquid thermoregulation suit until their rectal core body temperature reached 39.0 degrees Celsius, or until they voluntarily terminated the heating protocol. This was immediately followed by passive cooling (∼2hr) until core body temperature reached its initial baseline value (∼37 degrees Celsius). During this procedure, a maximal voluntary contraction (MVC) of the quadriceps was performed at each 0.5 degree change in core temperature. Cerebral NIRS measurements were continuously monitored for blood volume (tHb), tissue oxygenation index (TOI), oxygenated (HbO2) and deoxygenated (HHb) haemoglobin throughout the procedure. RESULTS An increase in tHb as indicated by an increased HbO2 and HHb was found during the heating component. The opposite trend was found during cooling, in which tHb decreased as did HbO2 and HHb. Specifically, tHb increased from 0.85 μm at 37 degrees Celsius to 10.57 μm at 39 degrees Celsius, with a recovery (cooling) baseline of −12.78 μm at 37 degrees Celsius. HbO2 followed a similar pattern, with values of 0.24, 7.45, and −14.87 respectively. CONCLUSIONS These results suggest that there was an active vasodilation in the brain during heating and decreased tHb with cooling. During cooling there was a plateau significantly lower than initial baseline values which did not recover, even when temperature reached baseline.