A sensitive orthostatic test on tilt table, useful in the detection of diabetic autonomic neuropathy

Gravity, the hydrostatic indifference concept and the cardiovascular system

Gravity, like any acceleration, causes a hydrostatic pressure gradient in fluid-filled bodily compartments. At a force of 1G, this pressure gradient amounts to 10 kPa/m. Postural changes alter the distribution of hydrostatic pressure patterns according to the body's alignment to the acceleration field. At a certain location--referred to as hydrostatically indifferent--within any given fluid compartment, pressure remains constant during a given change of position relative to the acceleration force acting upon the body. At this specific location, there is probably little change in vessel volume, wall tension, and the balance of Starling forces after a positional manoeuvre. In terms of cardiac function, this is important because arterial and venous hydrostatic indifference locations determine postural cardiac preload and afterload changes. Baroreceptors pick up pressure signals that depend on their respective distance to hydrostatic indifference locations with any change of body position. Vascular shape, filling volume, and compliance, as well as temperature, nervous and endocrine factors, drugs, and time all influence hydrostatic indifference locations. This paper reviews the physiology of pressure gradients in the cardiovascular system that are operational in a gravitational/acceleration field, offers a broadened hydrostatic indifference concept, and discusses implications that are relevant in physiological and clinical terms.

Are heart rate responses reproducible in the tilt-table test?

Measurement of blood pressure and heart rate in active orthostasis has proven to be effective in the diagnosis of autonomic regulatory dysfunctions. The following study was carried out in order to clarify to what extent reproducible heart rate reactions also occur in passive orthostasis. 61 people with normal results in five standardized tests were examined. All 61 test persons had individually differing courses of heart rate. After an initial increase of frequency an almost straight line results from a superimposed projection of the graphs. In 20 of these cases the tilt-table test was repeated in order to detect intra-individual regularities. Even when the test was repeated, no reproducible intraindividual heart rate reactions occurred. And here as well, projection of the graphs produced an almost straight line. We could not find any quotient, such as the 30/15 ratio in active orthostasis, during our tests with the tilt-table.