Hysteresis in response to descending and ascending lower-body negative pressure.

We have investigated the pattern of fluid redistribution and cardiovascular responses during graduated orthostatic stress. Twelve men, age 30-39 yr, underwent a 25-min lower-body negative pressure (LBNP) test protocol that involved sequential stages of LBNP at -8 mmHg (1 min), -16 mmHg (1 min), -30 mmHg (3 min), -40 mmHg (5 min), -50 mmHg (5 min), -40 mmHg (5 min), -30 mmHg (3 min), -16 mmHg (1 min), and -8 mmHg (1 min). Data were recorded at the end of each stage. For many measured variables values during the descending phase of LBNP (-8 to -40 mmHg) were significantly different from values during the ascending phase of (-40 to -8 mmHg). These differences appear to be due to a component of fluid translocation that occurs during LBNP and cannot be reversed within the duration of the procedure. We hypothesize that this slowly reversed component is sequestration of fluid in the interstitial and lymphatic compartments. In contrast, venous pooling is a rapidly reversible component of fluid movement during LBNP. A scheme describing fluid and cardiovascular responses to LBNP based on these data and the data of other investigators is presented.