A Vaginal Photoplethysmographic Transducer

The purpose of the transducer is to monitor blood-volume changes in the vascular bed of the vagina during a menstrual cycle by monitoring the resistance of a photoconductive sensor exposed to light backscattered from the vaginal wail. The intensity of the backscattered light and therefore the resistance of the photosensor, are functions of the blood volume in the illuminated tissue. Measurements have to be performed on the same subject at repeated sessions during a menstrual cycle. The construction and calibration of the transducer ensures that at each session monitoring is performed on the same segment of the vaginal wall, illuminated by the same light intensity. The measurement proper consists in comparing the resistance of the photosensor with a known resistance by balancing a potentiometric circuit to zero and observing the zero balance on a recorder. Zero balancing is interfered with by a constantly ongoing smaller short-term vasomotor activity due to heart beat, respiration, and metabolic processes. A specially designed circuit resetting the baseline automatically to zero and measuring the degree of resetting helps to overcome this difficulty.