Comparative study of haemodynamic responses to active and passive posture inducing head-ward pooling of blood in man

To study the relative effects of passive head-down tilt and squatting on the cardiovascular dynamics in man.

A pre-test/post-test study.

Physiology Research Laboratories in the Schools of Medicine at Addis Ababa University and the University of Dundee, UK.

Ten normal healthy subjects aged nineteen to twenty two years.

Cardiovascular variables measured in Passive body tilt and squatting.

Similar responses in forearm blood flow, arterial blood pressure, peripheral vascular resistance, and pulse rate observed in both active and passive postural changes.

A change from the sitting to the squatting position resulted in marked changes of forearm blood flow (FBF=1.63ml/100 ml/min, p<0.05), mean arterial blood pressure (BP=9mmHg, p<0.05), mean forearm vascular resistance (PVR=-10 PRU, p<0.05) and mean pulse rate (PR =-5 beats/min, p>0.05). The corresponding changes in the head-down position were: FBF=2.37ml/100 ml/min, p<0.05; BP=11.0 mmHg, p<0.05; VR=-12.85 PRU, p<0.05; PR=-10 beats/min, p>0.05).

These results suggest that the movement of blood towards the head is only slightly more marked in the head-down tilt than in the squatting position. The changes in flow and arterial blood pressure are similarly correlated (r=0.6291 in squatting and r=0.553 in 30 degrees head-down tilt). In both positions, blood moves to the thorax, neck and the head thus traversing the same baroreflex regions. This means that changes in cardiovascular variables in the two positions are detected by the same receptors signalling information to integrating centres via the same afferent pathways. Squatting and head-down tilt can, therefore, be used alternately to counteract orthostatic dizziness and to assess the significance of cardiovascular autonomic neuropathy in patients with disease conditions like diabetes mellitus.