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The principle of the photoelectric plethysmograph has been applied to the comparison of the richness of the blood supply in several skin areas. Vertical illumination of the skin is provided by an ophthalmoscope bulb. Enough light is scattered through the subcutaneum to provide a sufficiently satisfactory transillumination to permit the detection of changes in the circulation in the skin by a photoelectric cell placed over the transilluminated area. The variations in the photoelectric current with changes in the blood content of the skin are recorded on the electrocardiograph after amplification.The volume pulse of the skin so recorded probably is an index of the arterial inflow with each heart beat and so can be used as an index of the blood supply of that skin area. (Special hemodynamic conditions will, of course, limit the specific application of the statement). Arterial constriction in the skin decreases the volume pulse; vasodilatation increases it. The amplitude and form of the recorded wave will also vary with the size and number of arterial vessels in the transilluminated area.Quantitation of the recorded wave in terms of arbitrary units is effected by comparing the size of the wave with the deflection obtained from inserting a glass filter (a thin glass sheet) between the photoelectric cell and the transilluminated skin area. Although the question as to what is the actual blood equivalent of the filter in terms of cc of blood per 100 cc of skin tissue is an involved one, a tentative approach to the problem is provided in the data of Turner, et al. Their average volume pulse in the finger tip at heart level is 2.8 mm3 or .06% of the soft tissue volume.
Alrick B. Hertzman (Sun,) studied this question.