Cooling Study 4 of 4
Given the multitude of published reports on the effects of thermal manipulation of the hands or feet on core temperatures and performance and on the effects of thermal manipulations on performance and symptom exacerbation in individuals with MS, this is a reasonable inference. The effects of manipulating the internal thermal condition of MS patients by delivering a thermal load to a single appendage have been noted previously.
In 1950 Guthrie observed that MS patients became extremely weak while sitting in a hot water bath. A similar response could be elicited in those subjects by simply immersing a single arm or leg in hot water. An arterial tourniquet during arm immersion suppressed the symptom exacerbations. At the time it was theorized that the underlying mechanism for the heat response of the MS patients to limb immersion was due to a humoral factor, a reflex vasomotor response, or to a change in cerebral blood flow. An alternative explanation put forth at that time and discounted was that adequate amounts of heat were being delivered through one appendage to increase core temperature thereby inducing symptom exacerbation.
MS is a disease of the central nervous system (CNS). Presumably, it is increases in CNS temperature, rather than body temperature per se, that cause heat related symptom exacerbations. The brain and spinal cord are metabolically stable tissues that generate metabolic heat at a relatively constant rate. Heat is removed from CNS tissues through the vascular system. Thus, the temperature of the CNS is determined primarily by the temperature of the arterial blood. If the temperature of the arterial blood is elevated, CNS temperatures will rise.
Heat is produced by working skeletal muscles during physical activity. Heat generated by skeletal muscles is removed from the muscle by the circulation and the warmed venous blood returning from active skeletal muscles is mixed with other venous return blood as it passes through the heart and pulmonary circulation. Any thermal manipulation that delivers a cool thermal load to the venous return blood will counteract the effects of the heat produced by active skeletal muscles.
By accessing the circulating blood flowing through the retia venosa underlying the non-hairy skin surfaces it is possible to directly cool substantial volumes of venous blood. The results reported here agree with those of previous studies that examined the effects of cooling on ambulatory performance in heat-sensitive individuals with MS; cooling can improve performance.
In the previously reported studies the subjects were cooled for 30 min prior to fixed-duration exercise bouts followed by evaluation or for 60 min between evaluations. Evaluation of physical performance was time and number of steps required to complete a 25 foot walk. In those studies the thermal loads were delivered to the general skin surface via a circulating water garment or by cold water immersion. Applying a cool load to the general skin surface directly affects the thermal inertia of the skin and underlying tissues.
However, application of a thermal load to the general skin surface has little direct effect on the metabolically active core organs because the hairy skin surfaces are poorly perfused and therefore insulate the body core from the external thermal environment. Applying cooling to the general body surface takes advantage of the thermal inertia of poorly perfused superficial and peripheral tissues, however, to store a cold load. This cold load in the periphery then passively absorbs heat conducted into it by the blood flow and is the reason that precooling can improve endurance.
Conversely, with heat exchange involving the retia venosa, the thermal load is applied directly to the blood flowing through the palm during exercise. Cooling a volume of circulating blood directly affects the highly perfused core organs.
Precooling has been reported to result in lower heart rates during subsequent fixed load exercise in MS subjects. The difference in effect of cooling treatment on heart rate between the previous report and the results reported here is likely methodological. Precooling the peripheral tissues likely induced a vasoconstriction response that reduced blood flow to the skin and peripheral tissues.
Peripheral vasoconstriction increases peripheral resistance resulting in a rise of arterial pressure and a subsequent baroreceptor response decreasing heart rate. In the present study the temperature of the water perfusing the cooling device was maintained at 18-22° C to avoid triggering local vasoconstriction which would, in addition to increasing vascular resistance, reduce blood flow through the heat exchange vascular structures and, thereby, decrease heat transfer. The ability to effectively remove heat from the body could substantially improve the daily lives of heat-sensitive individuals with MS. Critical body surface regions for direct heat exchange are the non-hairy regions that contain the subcutaneous arteriovenous anastomoses and retia venosa.
Heat transfer devices incorporated into gloves or shoes could augment the use, or be used in lieu, of currently available cooling garments. The heat transfer devices used in this study used a pressure differential to maximize heat exchange. It has been demonstrated that, under certain conditions, substantial heat can be extracted from the body through the non-hairy skin surfaces without the application of a pressure differential. The application of a cool load alone to the non-hairy skin surfaces may provide a benefit to heat sensitive individuals with MS under some heat-stress conditions.
Conclusion
The results from this preliminary study demonstrate that, under a set of specific test conditions and in a small group of subjects who were selected in part because they engaged in regular exercise, utilization of the heat transfer capacity of the non-hairy skin surfaces can enable temperature-sensitive individuals with MS to extend participation in physical activities. Systematic longitudinal studies in larger cohorts of MS patients with specific deficits and levels of disability conducted under a variety of test conditions in independent research facilities are needed to confirm these preliminary findings. If these initial findings are replicated in future studies, the development of small wearable heat transfer equipment in the form of gloves or footwear could provide a benefit to individuals with MS.
NOTE:
I use cool packs in my hands when I walk. What do you use?