Lightfoot, JT, S Febles, and SM Fortney. Adaptation to repeated presyncopal lower body negative pressure exposures . Aviation, Space, and Environmental Medicine, 60: 17-22, 1989.
Adaptation to chronic stressors such as exercise and thermal challenges are well-documented. However, it is not known whether the body can adapt to repeated central hypovolemia. The purpose of this study was to determine if presyncopal symptom limited lower body negative pressure (PSL-LBNP; a central hypovolemic stressor) tolerance, as measured by a cumulative stress index (CSI), was altered by daily PSL-LBNP exposures. On each of nine consecutive days, with a two day break between days five and six, six subjects underwent a PSL-LBNP exposure. By the fifth PSL-LBNP exposure, LBNP tolerance had increased 47%. No further significant improvement was seen after the fifth exposure. While, there was no alteration in mean arterial pressure response during the repeated PSL-LBNP exposures, maximum heart rates were increased significantly over day one after the third daily PSL-LBNP exposure. Rate- pressure product was also significantly increased over day one on days seven and eight. These findings suggest that adaptation to a simulated hypovolemic stress does occur. Presumably, either the body's compensatory mechanisms become more effective, or there is a resetting of the threshold needed to elicit the presyncopal reactions.
KEY WORDS: LBNP TOLERANCE; HYPOVOLEMIA; HEART RATE; BAROREFLEXES; ORTHOSTATIC TOLERANCE.