CARDIORESPIRATORY RESPONSE TO LOWER-BODY NEGATIVE-PRESSURE

The cardiovascular effects of supine lower body negative pressure (LBN P, 0 mm Hg, -8 mm Hg, -15 mm Hg, -25 mm Hg, -35 mm Hg, and -45 mm Hg) were studied in humans (n = 10). The LBNP's were applied in a random o rder (three per session) for 20 min, with 15 min between each LBNP. Le g blood flow, cardiac output (Q), stroke volume (SV) and estimated lun g blood volume were significantly reduced at -15 mm Hg. Increasing LBN P to -35 mm Hg did not result in further changes. When the LBNP was in creased to -45 mm Hg, Q;l and SV were lower than com parable values at -15 mm Hg, Heart rate was unchanged up to -25 mm Hg, after which it i ncreased proportionally to the LBNP. Systolic blood pressure was maint ained throughout. Diastolic blood pressure was unchanged below -45 mm Hg, but was significantly elevated at -45 mm Hg. Mean arterial pressur e was maintained up to LBNP's of -35 mm Hg by increased vascular resis tance, in spite of reduced thoracic blood volume, as indicated by redu ced central venous pressure and Q. Greater levels of LBNP were outside the physiological adjustment range and blood pressure dropped progres sively.