Cardiorespiratory response to lower body negative pressure.

The cardiovascular effects of supine lower body negative pressure (LBNP, 0 mm Hg, -8 mm Hg, -15 mm Hg, -21 mm Hg, -31 mm Hg, and -41 mm Hg) were studied in humans (n=10). The LBNP's wore applied in a random order (three per session) for 20 min, with 15 min between each LBNP. Lag blood flow, cardiac output (Q), stroke volume (SV) and estimated lung blood volume were significantly reduced at -15 mm Hg. Increasing LBNP to -31 mm Hg did not result in further changes. When the LBNP was increased to -41 mm Hg, Q and SV were lower than comparable values at -15 mm Hg. Heart rate was unchanged up to -21 mm Hg, after which it increased proportionally to the LBNP. Systolic blood pressure was maintained throughout