Beat-to-beat cardiovascular responses to rapid, low-level LBNP in humans

The hypothesis tested was that there are significant transient changes in t he cardiovascular variables after rapid onset and release of mild lower bod y negative pressure (LBNP, -20 mmHg), even in experimental situations where there is no detectable change in steady-state values. Twelve subjects part icipated in the study. Heart rate, stroke volume (SV), cardiac output, mean arterial pressure (MAP), total peripheral resistance (TPR), acral and nona cral skin blood flow, and blood flow velocity in the brachial artery were c ontinuously recorded during the pre-LBNP period (0-120 s), during LBNP (120 -420 s), and during the post-LBNP period (420-600 s). The main finding was that MAP is transiently but strongly affected by rapid changes in LBNP as s mall as -20 mmHg. There was also a characteristic asymmetry in cardiovascul ar responses to the onset and release of LBNP, particularly in the response s in SV. The transient changes in MAP indicate that the neural responses th at affect TPR are not fast enough to compensate for the rapid changes in LB NP. In this case, the arterial baroreceptors will be activated as well as t he low-pressure baroreceptors that sense central venous pressure. This must be taken into consideration in future discussions of the results of LBNP p rotocols.