Heat balance and thermoregulation

Body heat is unevenly distributed, with a typical core-to-peripheral temperature gradient of 2° C to 4° C. As with any other neurally mediated physiologic process, thermoregulation involves afferent thermal sensing, central processing, and efferent responses. Thermal receptors are distributed throughout the body (e.g., skin, abdominal and thoracic tissues, spinal cord, hypothalamus), with impulses in response to hypothermia and hyperthermia transmitted to the central nervous system via Aδ and C fibers, respectively. Central processing (primarily in the hypothalamus) results in voluntary (e.g., wearing appropriate attire, adjusting ambient temperature) and involuntary (autonomic) efferent responses. The slope of the efferent response intensity (e.g., magnitude of vasomotor changes) versus core temperature defines the gain.

In unanesthetized patients, cold-induced autonomic defenses follow a hierarchical pattern that progresses from vasoconstriction to nonshivering thermogenesis and, finally, shivering thermogenesis (Figure 161-1). Vasoconstriction decreases cutaneous blood flow and heat loss, primarily in the fingers and toes. Although its effects are minimal in adults, nonshivering thermogenesis can double metabolic heat production in the mitochondria-rich brown fat of neonates and infants. Shivering thermogenesis results from involuntary skeletal muscle activity that increases metabolic rate and heat production.

The threshold for warmth-induced autonomic responses, such as active vasodilation and sweating, is similar. Each gram of evaporated sweat dissipates approximately 540 calories of heat to the environment.

Core temperatures between the first cold-induced (i.e., vasoconstriction) and warmth-induced (i.e., vasodilation) responses define the interthreshold range (ITR). Temperatures within this 0.2° C range do not trigger thermoregulatory defense mechanisms.