Onset of mild lower body negative pressure induces transient change in mean arterial pressure in humans.

Mild (0 to -20 mmHg) lower body negative pressure (LBNP) has traditionally been considered to elicit reflex responses mediated by cardiopulmonary baroreceptors only, without any arterial baroreflex involvement. Mild LBNP has therefore frequently been used to study the influence of cardiopulmonary baroreceptors on the human circulatory system. In a previous study we found that mean arterial pressure (MAP) was transiently but strongly affected by rapid (0.3 s) onset and release of -20 mmHg LBNP. In the present study we tested whether MAP is also transiently affected by slow onset and release of -20 mmHg LBNP. A group of 12 subjects participated in this study, which was approved by the local Ethics Committee. Heart rate, stroke volume, cardiac output, MAP, total peripheral resistance, acral and non-acral skin blood flow, and blood flow velocity in the brachial artery were continuously recorded during the pre-LBNP period, during LBNP and during the post-LBNP period. The LBNP was gradually applied and released over a 15 s period. The main finding was that MAP was transiently but strongly affected by the gradual onset of LBNP as mild as -20 mmHg. During onset of LBNP MAP was significantly ( P=0.003) lower than MAP in the pre-LBNP period. This shows that not only the cardiopulmonary baroreceptors but also the arterial baroreceptors must be activated during mild LBNP.