Adaptation to repeated presyncopal lower body negative pressure exposures.

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 tolerance to presyncopal symptom limited lower body negative pressure (PSL-LBNP; a central hypovolemic stressor), as measured by a cumulative stress index (CSI), was altered by daily PSL-LBNP exposures. On each of nine consecutive days, with a 2-d break between Days 5 and 6, 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 1 after the third daily PSL-LBNP exposure. Rate-pressure product was also significantly increased over Day 1 on Days 7 and 8. 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.