Sympathoexcitation is attenuated during low level lower body negative pressure in subjects who develop pre-syncope.

The purpose of this study was to compare the hemodynamic and sympathetic nerve activity (SNA) responses to graded lower body negative pressure (LBNP) in healthy subjects with either a positive (n = 24, SNA in 8) or a negative (n = 18, SNA in 6) LBNP response. A positive LBNP response was defined as an abrupt drop in systolic blood pressure associated with a decrease in heart rate and/or a decrease in SNA. All positive responses were accompanied by symptoms common to pre-syncope, defined as lightheadedness, diaphoresis, tunnel vision and/or nausea. If subjects tolerated 30 minutes of LBNP, this was considered a negative response. Comparisons were made between baseline, -10 mmHg (low-level LBNP) and -60 mmHg (high-level LBNP). Baseline SNA and arterial baroreflex sensitivity were not different between the 2 groups. However, subjects with pre-syncope had a significantly attenuated SNA response during low-level LBNP (p < 0.05) compared to subjects who did not experience pre-syncope. The hemodynamic data during high-level LBNP were similar until the occurrence of pre-syncope. Pre-syncope was preceded by a significant decrease in heart rate and SNA. Our findings suggest that subjects with LBNP induced pre-syncope might have an impairment in the cardiopulmonary baroreflex gain function in the setting of a preserved arterial baroreflex gain.