This chapter presents information regarding those modalities that rely on a gradient-based heat exchange that alters the body's physiology. The chapter consists of two sections, cold and heat. Each section describes the physics, physiological effects, and evidence that support or question their use. Use of specific thermal modalities and their unique effects are described in the next chapter.
• Based on a difference in temperatures, thermal modalities transfer energy (heat) to or from the tissues (Appendix A). Compared with the extreme range of temperatures found throughout the universe, there is a scant difference between the upper and lower temperature limits of thermal treatments. Within our tissues, the 65°F (18.3°C) that span the therapeutic upper limits of heat modalities and the lower limits of cold modalities elicit a wide range of cellular and vascular events (Box 5-1).
Box 5-1. HEAT AS A PHYSICAL ENTITY
Temperatures and their effects are relative. If the temperature outside is 60° today, but was 80° yesterday you would think, "It's cold." If it was 40° yesterday, you would think, "It's warming up." This concept holds true for the application of thermal modalities.
The classifications of "heat" and "cold" are based on the physiological response elicited by the temperature. Temperature is a measurement of the speed of molecular motion that describes the amount of kinetic energy, heat, in an object. Infrared energy is emitted from any object having a temperature greater than absolute zero •. An increased rate of motion is identified as an increase in temperature.
The basic principle of thermal modalities is to transfer heat across a temperature gradient (i.e., one object is hotter than the other). Heat is lost from the warmer object and moved into the cooler object. The greater the temperature gradient, the more quickly energy is transferred. When a moist heat pack is placed on a patient, energy is transferred away from the pack and absorbed by the tissues. When an ice pack is applied heat is drawn away from the tissues and delivered to the pack, melting the ice.
Heat is measured in calories. The common definition of a calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1°C (note that this calorie is different from the k-calories used to describe food energy). Scientists, however, changed this definition to be 1.0 calorie equals 4.1860 joules of energy.1
Different materials require different amounts of energy to increase their temperature. Specific heat capacity (often simply shortened to "specific heat") is the amount of energy needed to increase the temperature of a unit of mass by 1°C. A substance's specific heat varies with its temperature. Thermal conductivity is the quantity of heat (in calories per second) passing though a substance 1 cm thick by 1 cm wide separating a temperature gradient of 1°C. As it relates to therapeutic modalities, thermal conductivity is used to describe ...