Wavelet transform assessment of heart rate variability during simulated orthostatic stress after antecedent hypoglycemia

MARCELO RISK; ISTVAN BONYHAY; GAIL ADLER; ELIZABETH WARING; VALERIE CURREN; ROY FREEMAN

St. Thomas, U.S. Virgin Islands

Simposio; 20th International Symposium on the Autonomic Nervous System; 2009

American Autonomic Society

Objective: To investigate heart rate (HR) oscillations by time-frequency analysis during lower body negative pressure simulated orthostatic stress (LBNP) after prior exposure to hypoglycemia. Background: Recent evidence suggests that rigorous glucose control may be associated with increased cardiovascular mortality. Previously we found that prior hypoglycemia impairs cardiovagal baroreflex sensitivity and the sympathetic response to hypotensive stress and the simulated orthostatic stress of LBNP. It is not clear if prior hypoglycemia alters HR modulation during LBNP. HR oscillations during LBNP are non-stationary necessitating time-frequency analysis. Methods: Twenty healthy subjects participated in two 3-day admissions, separated by 1–3 months. On day 1 and day 3 of each admission, graded LBNP was used to induce orthostatic stress without the confounding effects of muscle contraction. On day 2, a 2-h hyperinsulinemic [hypoglycemic (50 mg/dL) or euglycemic (90 mg/ dL)] clamp was performed in the morning and repeated in the afternoon. The LBNP was performed on supine subjects sealed at the waist in a metal tank. Following a 5-min baseline period, 4 min of negative pressures of -10, -20, and -30 mmHg were generated. The last 60 s of RR interval time series was used to estimate the low-frequency (LF) and high-frequency (HF) power spectrum using the Daubechies 4 wavelet function. Results: The mean RR and the LF-RR at -30 mmHg LBNP was similar on Day 1 (835 ± 156 vs. 817 ± 183 ms, P = 0.9) (15.0 ± 1.5 vs. 14.6 ± 1.4 log ms2, P = 0.8) whereas on Day 3, RR was longer (778 ± 157 vs. 737 ± 138 ms, P = 0.04) and LF-RR was lower (11.1 ± 1.5 vs. 12.4 ± 1.5 log ms2, P = 0.004) after the hypoglycemic clamp than after the euglycemic clamp. There was no difference in HF-RR on Day 3. Conclusion: The decreased LF power with increased RR during simulated orthostatic stress following hypoglycemia suggests decreased autonomic control of HR and is consistent with the previously observed impaired sympathetic response to orthostatic stress. These changes may contribute to the increased mortality observed in association with rigorous glucose control.