A new miniature plethysmograph to measure volume changes in small circumscribed tissue areas

With an ultrasonic method tissue layer thickness was measured in man in circumscribed superficial tissue areas where the underlaying bone provided good backwall echos. In a 5 mm tissue layer changes of +/- 0.2% could be reliably detected. Knowing the height of the tissue cylinder between the surface of the skin and the bone allowed to calculate the tissue volume. The ultrasonic probes could therefore serve as miniature plethysmograph. Several probes were attached in the frontal region, sternum, along the vertebral column and along the tibia simultaneously. Changes of the volume content of the superficial shell tissues were induced by orthostasis, water immersion and heat exposure. It was possible to assess quantitatively the volume shifts into or out of the superficial tissues. During orthostasis 166 ml of fluid left the superficial tissues of the cephalad parts of the body and 164 ml could be traced in the dependent parts. Heat exposure was followed by a pooling 140 ml in the tissues studied. The most pronounced tissue volume changes were observed in the forehead region during heat exposure.