Bioheat Transfer - Therapeutic Hyperthermia & Hypothermia

Therapeutic Hyperthermia & Hypothermia

Heat can be used not only to destroy cells, but also to aid in the recovery of cells and tissues. Such use of heat is sometimes called therapeutic hyperthermia, perhaps to distinguish it from malignant hyperthermia.

An example of therapeutic hyperthermia is the ThermaCare HeatWrap by Procter & Gamble. The application of heat to injured tissues works to heal target tissues by a temperature dependent vasodilation. This vasodilation increases the mass transport of wastes and nutrients from and to the site of injury. Because damaged tissue is more metabolically active the enhanced mass transport can facilitate more rapid healing.

Hypothermia
Heat transfer is not only study of raising temperatures, but also reducing them. It is commonly understood that lower temperatures help to preserve living or "once living" tissue. For example, foods stored in refrigerators last longer because the metabolic processes of cellular decay and bacterial growth are slowed due to the lower free energy (e.g. heat) in the refrigerator. This principle is often applied in surgical wards to individuals who experience head trauma.

In cases of head trauma, it is common procedure to reduce the body temperature to about 32°C (90°F) which is about 5°C (8.5°F) lower than normal core temperature of 37°C (98.5°F). This presents a good example of how bioheat transfer engineers can contribute to medical treatments. Such a situation would be broken down first into a system, which in this case would include the entire surface of the body. Assuming the individual is submerged in cold water up to the neck and ears, or otherwise surrounded by an environment that can be modeled as a temperature sink, the engineering would model the body as a composite system composed of skin, fast, muscle bone and possibly organs depending on how complex the model must be. The head could be considered a separate system connected to the body via a third major system, the vasculature. The vasculature would bring cooled blood from the submerged body to the brain where there would be some amount of conductive and convection heat transfer based on the passage of blood through the vessels cooling the vessel via forced convection and the subsequent cooling of the brain via conduction between blood vessel and brain tissue. Each of these heat transfer processes i.e. water to body, body to vasculature, and vasculature to brain would require knowledge of the thermal conductivity, specific heat, density, blood perfusion rate, and diameter of blood vessel at the least to predict the temperature history at any point within the body e.g. to know when the temperature at the center of the head trauma reaches 32°C.

Such application of hypothermia is also employed during open heart surgery where perfusion to the body and brain must stop while cardiac output is rerouted through a heart lung machine. Furthermore, biomedical engineers who specialize in bioheat transfer are able to design such medical devices to perform within a specified range of temperatures and rates of cooling.