Regulation of arterial blood pressure in humans during isometric muscle contraction and lower body negative pressure
Previous studies have shown that the blood pressure response to isometric handgrip remains unchanged during reductions in preload induced by lower body negative pressure (LBNP). The purpose of the present study was to assess the beat-by-beat haemodynamic mechanisms allowing for precise control of mean arterial pressure (MAP). We have followed the cardiovascular variables involved in the regulation of MAP during isometric handgrip contraction with and without additional application of LBNP during defined protocols. Ten subjects participated. MAP, heart rate, stroke volume, cardiac output, blood flow velocity in the brachial artery, acral skin blood flow, as well as total - and local peripheral resistance were continuously recorded before, during and after handgrip, both with and without concomitant LBNP.
The main finding was that MAP had a uniform increase under isometric handgrip contraction, but at the onset of LBNP at -20 mmHg and -40 mmHg, we recorded a further increase in MAP. This may be a result of exhaustion after repeated protocols. Anyway this finding was unexpected, because previously reported studies suggest a uniform increase in MAP by LBNP-induced alteration in preload. Future investigations should be directed to know relationship between the pressor response and the reduction in preload induced by LBNP. Cardiac output and stroke volume were constant or declining gradually upon LBNP application. At the same time, both total and local peripheral resistance increased markedly and continuously; and explains the pressor response to isometric exercise.