A patient that “needs to be cooled” is not an infrequent occurrence in the intensive care unit (ICU). In order to effectively lower a patient’s core body temperature, an understanding of temperature physiology is required.

The human body can be roughly divided into two thermal compartments: a core compartment, consisting of the trunk and head, excluding the skin, and a peripheral compartment, consisting of the skin and extremities. The core temperature is regulated by limiting or increasing heat transfer to the periphery through vasoconstriction and vasodilatation, respectively. Heat loss from the peripheral compartment is regulated through changes in skin perfusion (again through vasodilatation or vasoconstriction) and by increasing or decreasing the production of sweat.

When cooling blankets are placed over or under a patient, they increase sympathetic tone (with concomitant increase in systemic vascular resistance index) and induce vasoconstriction in the skin. This actually prevents heat dissipation and causes an increase in core body temperature, not a decrease. This physiological response complicates attempts to induce therapeutic hypothermia or normothermia by other means of external cooling. In addition, heat production will be increased through the shivering response and, in later phases, through the increased metabolism of fats, carbohydrates, and proteins. As a result there can be an increase (not a decrease) in body temperature.