Quantification of tolerance to lower body negative pressure in a healthy population.

Subtolerance and presyncopal lower body negative pressure (LBNP) has been used extensively to investigate the effect of various forms of exercise training on central hypovolemic tolerance. However, there are no population data concerning LBNP tolerance, and the validity of common LBNP tolerance indices has not been investigated. LBNP tolerance data from 86 males and 33 females were analyzed. All LBNP exposures were terminated with the onset of presyncopal signs and/or symptoms. LBNP tolerance was quantified using duration of exposure (DNP), maximal negative pressure tolerated (MNP, cumulative stress index (CSI), and the LBNP tolerance index (LTI). Average (+/- SD) LBNP tolerance was 22.12 +/- 5.01 min (DNP), -77 +/- 16 mm Hg (MNP), 975 +/- 402 mm Hg.min-1 (CSI), or 222 +/- 50 mm Hg.min-1 (LTI). All cardiovascular parameters exhibited the same responses as have been documented in the literature. While DNP, MNP, and LTI were normally distributed, CSI in this population was not normally distributed. Age, weight, height, gender, and VO2peak were not associated with LBNP tolerance. Compared to this population, it appears that the subjects that have participated in previous LBNP tolerance studies have had abnormally high LBNP tolerances and that the LTI is the most valid index of LBNP tolerance. Furthermore, in this population LBNP tolerance could not be predicted from physical characteristics.