RESUMO
Prompt active postural manoeuvres induce an immediate arterial pressure variation followed by a period of regulation. For the squatting manoeuvre, initial hypertension was explained by a rise of cardiac filling pressure due to "squeezing blood out of the veins of the legs", leading to an increase in stroke output by Frank-Starling mechanism. For a minor part, it was also explained by "kinking" of the femoral arteries. O'Donnel and Mc Ilroy observed an increase in central blood volume and accepted the idea of a rise of cardiac filling. However, they did not observe so consistent circulatory variations when postural changes were realized in a water tank. Therefore, they concluded that kinking of the arteries and veins of the legs could not be very important. Moreover, the immediate pressure variations, most often appearing in the first beat succeeding the postural manoeuvres cannot be easily explained by the previously invoked modifications of cardiac filling pressures. When Hoffman et al lifted dogs until they stood erect, the right ventricular stroke volume usually fell in the first beat after the postural change, but the left ventricular stroke volume did not fall for another 1-3 beats. When the dogs were rapidly lowered to standing on four legs again, a delay of 2-3 beats was also observed. Thus, another interpretation of immediate hypotension must be added. It should especially take into account the natural gravitational fluid mechanics phenomena imposed to the arterial blood. Besides, to allow the investigation of the orthostatic regulation of arterial blood pressure, it would be necessary to separate the cardiovascular regulation component of the arterial pressure time course from the pressure evolution that would naturally appear in the network without physiological contribution. It is the aim of this study.
