60 Applied Biophysics 



likely that therapeutic effects will be obtained only if the amount 

 of heat administered to the body, or to part of it, approaches the 

 total metabolic heat, or the proportion of this normally allocated 

 to the part of the body in question. It is thus clear that, in 

 contrast to X-ray or ultraviolet therapy which relies on a selec- 

 tive action of the radiation, in the case of heat application 

 therapeutic effects wnll require the application of considerable 

 energy. As in every other kind of therapy, the chief danger to be 

 guarded against is overdosage. From what has been said, two 

 different kinds of overdosage can be foreseen. In the first place 

 the tolerable concentration of heat input over a restricted area 

 may be exceeded. When heat is applied to one square centimeter 

 of the skin its temperature is raised, and the degree to which 

 this happens depends on the strength of the energy flow pro- 

 vided by the heat source, and on the capacity of the tissues to 

 remove the local heating. With increasing heat flow, removal 

 processes are stimulated, but they will break down eventually 

 and a serious local over-heating of the tissues will be the result ; 

 in other words a burn will be produced. The limiting tempera- 

 ture above which the skin tissues must not be heated has been 

 determined by Mendelssohn and Rossiter,^ and has been found 

 to be 45-50° C. 



The other danger lies in the general application of heat. If 

 the amount of heat applied becomes of the same order as the 

 total metabolic heat, and especially if, in addition, normal 

 methods of heat excretion (radiation and perspiration) are 

 restricted, then the total heat balance of the body may be upset, 

 and the patient may develop heatstroke. 



Methods of Heat Transfer 



The physical distinction between methods of heat transfer is 

 usually made as between convection, conduction, and radiation. 

 However, in the methods employed by the clinician, this clear 

 distinction can rarely be drawn, for usually several modes of 

 heat transfer are operative simultaneously. Pure convection is 

 met with, for example, only in the case of a hot-air cabinet, 



