In up to six different sides along the body axis during tilting manoeuvres, volume shifts into or out off superficial tissues were followed with a newly developed miniature plethysmograph. It was possible to localize a region where no or only minor volume changes during the tilt table experiments occurred. This region is identical with the Hydrostatic Indifferent Point (HIP) being localized below the apex of the heart in the upper third of abdominal vena cava. Above the HIP fluid is drained out off the tissues during assumption of upright posture whereas below the HIP fluid volume is pooled. The volume changes occurred in two phases. Within the first 5 s in the cephalad parts of the body a rapid decrease occurred, thereafter the volume remained unchanged or even increased; below the HIP within the first 5 s a large volume increase was followed by a slow continuous volume increment. The functional peculiarities of the low pressure system as a whole were visible studying only superficial shell tissues of the body with the non invasive miniature plethysmographic technique.
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.
A previously described miniature plethysmograph which allowed the measurement of tissue volumes in superficial tissues was enclosed in a small plexiglass chamber and attached to the frontal area, sternum, dorsum and the tibia. The tissues interposed between bone and skin underneath the chamber were exposed to pressures between +/- 3 and +/- 15 mmHg in order to test tissue deformability. The pressure application induced within the first 5 s a fast component of tissue deformation comprising between 75-90% of the total deformation followed by a slow component which lasted till the end of the pressure application. The highest deformability was found in the tissues of the sternum and dorsum whereas the stiffest tissues were in the pretibial area. Assuming the tissue deformation is due to a translocation of fluid into or out of the pressurized tissue, the tissue compliance was calculated. This calculated tissue compliance was 19.2 ml . 1,000 ml-1 . mmHg-1 in the sternum and 6.4 ml . 1,000 ml-1 . mmHg-1 (P < 0.01) in the pretibial area applying a pressure of +/- 3 mmHg. The differences observed are due to the morphological arrangement of the tissue fibres which in turn have to counteract the gravity forces to which the tissues are usually exposed during upright standing.
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