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

Three studies were performed to test the hypotheses: (1) lower body negative pressure (LBNP) tolerance, autonomic, and cardiovascular responses to the LBNP are reproducible, (2) women have a lower LBNP tolerance and less ability to maintain homeostasis in cardiovascular system against orthostatic stress, and (3) wavelet analysis technique is a unique tool to study the autonomic nervous activities during LBNP test. College-age men and women underwent graded LBNP to either presyncope or -100 mmHg in increments of 10 mmHg negative pressure every 6 minutes.;The LBNP tolerance were highly reproducible (cumulative stress index: 1698 +/- 171 vs 1938 +/- 139, trial 1 vs trial 2, respectively; Cronbach alpha coefficient = 0.87). Changes of the autonomic and cardiovascular responses in the two trials were similar. Autonomic and cardiovascular responses to LBNP at most level were reproducible (alpha coefficient ≥ 0.41).;The females showed 44% less tolerance to the orthostatic stress induced by LBNP than the male. Heart rate increased above rest at ≥-40 mmHg (P < 0.05) in both groups and mean arterial pressure was maintained in males while it fell below rest at ≥-30 mmHg (P < 0.05) in females. Based on spectral analysis, sympathetic nervous system activity as quantified by the LF/HF ratio increased above rest at ≥-40 mmHg in males but it increased only at presyncope in females (P < 0.05).;The wavelet analysis technique is a valuable-unique tool to study autonomic responses to LBNP over spectral analysis since: (1) this method provides a dynamic autonomic response to LBNP by showing both time- and frequency-description simultaneously and (2) this method yield the additional valuable information from transient and nonstationary part of heart rate signal which normally discarded to satisfy the underlying condition of spectral analysis.;These results suggest that (1) LBNP tolerance, autonomic, and cardiovascular responses to LBNP are reproducible, (2) women have a lower orthostatic tolerance than men and this gender difference in orthostatic tolerance may be due to differences in the autonomic activity responses to this stress, and (3) wavelet analysis is a unique tool over spectral analysis to access autonomic responses to LBNP.