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The reproducibility of the autonomic and cardiovascular responses to lower body negative pressure (LBNP) is surprisingly limited. We postulated that LBNP tolerance, autonomic, and cardiovascular responses to LBNP would be reproducible. To test this, 14 active college age men underwent an orientation exposure and 2 trials of graded LBNP to presyncope or -100 mmHg, separated by 1 week. Heart rate, stroke volume, cardiac output, blood pressure and forearm blood flow were assessed, as was heart rate variability in both time and frequency domains. LBNP tolerance was lower in trial 1 than trial 2 (Cumulative stress index [CSI]: 1698 ± 171 vs 1938 ± 139 mmHg-min; P<0.05) but the CSI rehability estimate was high (0.87). The trial-to-trial responses to LBNP common to all subjects (LBNP < -60 mmHg and at test termination) did not differ between trials although systolic blood pressure was lower in trial 2 at test termination (105 + 3 vs 112 + 3 mmHg, P<0.05). Relatively high reliability estimates were found for the resting data (lowest: 0.60 for low frequency/high frequency ratio; highest: 0.94 for standard deviation of R-to-R intervals). Reliability estimates varied by parameter measured and LBNP stage but were generally similar to that seen at rest although they were lower at test termination [lowest: 0.23 for high frequency (abs) power, highest: 0.87 for cardiac output]. These data suggest that mean autonomic and cardiovascular responses to graded LBNP are consistent across trials.

Utility of spectrum and wavelet analysis of heart rate variability as a tool to assess orthostatic intolerance