Cardiovascular dynamics associated with tolerance to lower body negative pressure.

The purpose of this investigation was to identify cardiovascular responses associated with tolerance to lower body negative pressure (LBNP). In this study, 18 men, ages 29-51 years, were categorized as high (HT) or low (LT) LBNP tolerant based on a graded presyncopal-limited LBNP exposure criterion of -60 mm Hg relative to ambient pressure. Groups were matched for physical characteristics and preLBNP cardiovascular measurements, with the exceptions of greater (p less than 0.05) end-diastolic volume and cardiac output in the HT group. During peak LBNP, cardiac output was similar (NS) in both groups, although the HT group displayed a greater heart rate (p less than 0.05). In both groups, venous return appeared to limit cardiac output resulting in decreased arterial pressure. Tolerance to LBNP did not appear solely dependent on the absolute amount of blood pooled in the legs since the HT group demonstrated a greater (p less than 0.05) peak LBNP-induced increase in midthigh-leg volume. Greater tolerance to LBNP was associated with a larger preLBNP cardiac output reserve and higher compensatory increases in heart rate and peripheral resistance.