Determination of the Conductive Heat Exchange of the Skin in Relation to Environmental Temperature

For more than 2,000 years, local tissue heating has been used to treat rheumatism, arthritis, muscle strains and sprains, and a variety of other afflictions. Heat can be applied with hot packs, ultrasound, diathermy, microwave, lasers, or a whirlpool bath. The overall effect is the same. Heat is applied to the surface of the skin in an effort to transfer the heat deep into the tissue. However, there is always a possibility of harmful as well as beneficial effects from the heating of the skin. Numerous models have been developed, including the Pennes equation, to understand how blood flow and skin conductivity are related. However, this model does not take into consideration the effect of central sympathetic outflow on skin thermal conductivity. Therefore, the present study examined heat dissipation in the skin on 4 areas on the body in 10 subjects in thermally neutral and warm environments to alter skin sympathetic activity. The results of the experiments showed that if skin thermal conductivity is measured with subjects exposed to a warm environment, heat loss is more pronounced in the skin. Thus in a cold room, hot packs cause a greater change in skin temperature and provide greater stress on the skin. INTRODUCTION Unlike the core tissues of the body where tissue is maintained at a fairly constant temperature of approximately 37oC, the shell tissues of the body, that is the arms, legs, and superficial areas of the skin, are maintained at considerably lower temperatures.1,2 For more than 2,000 years, heating of tissue has been used to increase vasodilatation of the Determination of the Conductive Heat Exchange of the Skin in Relation to Environmental Temperature Jerrold S. Petrofsky, PhD, JD*† Everett Lohman III, DPT Sc* Hye Jin Suh, DPT* Jason Garcia, BS† Alexa Anders, BS† Cassandra Sutterfield, BS† Joseph Grabicki, MPT* Chetan Khandge, BS* *Deparment of Physical Therapy, School of Allied Health Professions, Loma Linda University, Loma Linda, California †Deptartment of Physical Therapy, Azusa Pacific University, Azusa, California skin and the tissues below.3 Numerous studies point to the therapeutic benefits of warming tissue.4-6 Irrespective of the means of warming the tissue, be it diathermy, microwave, or ultrasound, 4-6 there is an increase in temperature of both the skin and the tissue below that allegedly increases tendon extensibility,7 reduces joint stiffness,8 and increases local tissue blood flow.9 Although numerous studies argue the advantages and disadvantages of one therapeutic modality versus the other in heating tissue,10 or using combined modalities such as hot packs and ultrasound together,7 the fact remains that heat must be transferred from the skin to deep tissue in order to be effective. By warming the skin, a thermal gradient is established between the surface of the skin and the deep arteries of the body, thereby warming the tissue in between. For example, typical forearm temperature at the surface of the skin is approximately 31oC, whereas temperature of the bone is approximately 33oC.1 To increase deep tissue temperature, considerably higher temperatures are necessary at the surface of the skin. For example, increasing skin temperature to 42oC causes an increase in circulation, which then dissipates some of the heat but allows other heat to transfer through conductive heat loss to deep tissues below.1,11 Body fat has a strong influence on skin and deep tissue temperature in men at rest.12 Subjects with a high body fat content have poor conductive heat loss to the deep tissues, and therefore require higher surface temperatures for heat transfer.13 There is also a tendency to use higher temperatures in some populations where local tissue temperatures are sometimes elevated to 42oC or 43oC using microwave,14,15 ultrasound,16 or lasers.17 When thermal conduction of the skin is overloaded, damage can result to the skin.18 To understand the factors that influence the conduction of heat across the skin, Pennes heat equation is commonly used.19,20 Recently, Gowrishankar showed that a transport lattice matrix is also a good approach to understanding heat dissipation in the skin.18 However, neither model takes into consideration changes in skin blood flow associated with altered sympathetic outflow. Table 1. Skin Blood flow, skin temperature and skin thermal index after exposure to a thermally neutral and warm environment Thermally Neutral Room Skin Blood Blood Skin Skin Thermal Flow Flow Temp Temperature Index Rest Change Rest oC Change oC Back 14.4 84.0 41.6 31.8 3.4 Hand 22.6 118.1 68.7 31.8 3.1 Quadriceps 22.8 57.6 60.4 30.6 3.3 Toe 20.8 173.9 70.4 24.4 4.7 Mean 20.2 108.4 60.3 29.7 3.6 Hand occluded 16.4 n/a n/a 31.8 3.3 Skin thermal index = number of calories to increase skin temperature 1oC; blood flow change = the increase in blood flow during local heating over the 30-second period; skin temp rest oC = the skin temperature at each area examined at rest; skin temperature change oC = the skin temperature at each area examined after 30-second exposure to the brass disc Vol. 6, No. 2, 2006 • The Journal of Applied Research 158 The Journal of Applied Research • Vol. 6, No. 2, 2006 159 ly screened prior to the investigation and signed a form acknowledging their consent to participate in the study. All protocols and procedures were approved by the Committee on Human Experimentation at Loma Linda University. METHODS Measurement of Heat Dissipation The ability of the skin to dissipate heat was measured by applying a 48-gram brass disc to the surface of the skin (diameter 25 mm). The disc was coated with 3 coats of polyurethane rubber to avoid heat loss except on the bottom surface. A hole was drilled (5 mm) through the center of the disc to allow simultaneous blood flow measurements under the area where the disc was applied by allowing the laser (Moor Instruments, Oxford England) to scan through the brass disc. Furthermore, a small hole was drilled (1 mm) through the side of the disc so that a thermistor could be inserted into the center to measure the temperature of the brass disc (Biopac Systems, Goletta, CA). The disc was then held by a thin wire and Because sympathetic vasoconstriction can override all other stimuli to the skin and reduce skin blood flow even in the face of noxious stimuli,2 alteration in sympathetic outflow due to global heating, for example, may alter the thermal conductivity of the skin. Therefore, in the present investigation, conductivity of the skin was assessed in 10 subjects at 2 environmental temperatures, a thermally neutral temperature and a warm temperature. The ability of the skin to dissipate heat was then assessed under both environmental conditions by applying a heated brass disc to the skin and monitoring the change in skin temperature and skin blood flow as the brass disc cooled. By knowing the number of calories added to the skin and the change in skin temperature, the thermal conductivity of the skin could be calculated. SUBJECTS A group of 10 subjects was examined. The average age of the subjects was 25.9 ± 3.4 years. The average height was 165.3 ± 3.3 cm and the average weight was 61.8 ± 7.6 kg. All subjects were medicalWarm Room Skin Blood Blood Skin Skin Thermal Flow Flow Temp Temperature Index Rest Change Rest oC Change oC 41.1 94.3 46.8 35.8 1.1 33.8 184.8 71.5 36.3 1.3 27.0 128.2 107.5 34.8 1.9 50.8 198.2 100.9 35.5 1.2 38.2 151.4 81.7 35.6 1.4 26.3 n/a n/a 36.5 1.4