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A study of intracardiac and intravascular temperatures was undertaken with the thought that such measurements might provide further information concerning the mechanisms which normally maintain body temperature within its narrow limits and also that they might help to explain the deviations from normal temperature in certain disease states. Whereas the temperature of the peripheral tissues may vary widely, homeothermic man maintains the temperature of the deep tissues, or of a critical deep tissue, fairly constant under widely varying conditions of heat loss and heat gain (2). The identity of the critical deep tissue is not completely understood but a considerable body of evidence places it in the diencephalon and in normal man small variations in its temperature are thought to initiate compensating mechanisms which return the temperature of the body to the homeothermic norm. Generally, rectal temperature has been considered to indicate deep tissue, and critical tissue, temperatures but often rectal temperature appears to be a lagging and unsatisfactory index; witness the onset, when the body is heated, of such compensating mechanisms as peripheral vasodilatation and sweating before a rise in rectal temperature has occurred; and conversely, in subjects who are cooled, the presence of vasoconstriction and pilomotor activity before a fall in rectal temperature has taken place. It seemed desirable, therefore, to look for some other index of critical deep tissue temperature. Considerations of the mixing of blood in the left ventricle, probably in proportion to the volume flow and temperature of the blood from all organs,
Eichna et al. (Sun,) studied this question.