Indirect measures of human vagal withdrawal during head-up tilt with and without a respiratory acidosis

Abstract

Human ECG records were analyzed during supine (SUP) rest and whole body 80° head-up tilt (HUT), with a respiratory acidosis (5%CO2) and breathing room air (RA). HUT increased heart rate in both conditions (RA SUP 60 ± 13 vs. RAHUT 79 ± 16; 5%CO 2SUP 63 ± 12 vs. 5%CO2HUT 79 ± 14 beats min-1) and decreased mean R-R interval, with no changes in the R-R interval standard deviation. When corrected for changes in frequency spectrum total power (NU), the high frequency (0.15-0.4 Hz) component (HFNU) of heart rate variability decreased (RASUP 44.01 ± 21.57 vs. RAHUT 24.05 ± 13.09; 5%CO2SUP 69.23 ± 15.37 vs. 5%CO2HUT 47.64 ± 21.11) without accompanying changes in the low frequency (0.04-0.15 Hz) component (LFNU) (RASUP 52.36 ± 21.93 vs. RAHUT 66.58 ± 19.49; 5%CO 2SUP 22.97 ± 11.54 vs. 5%CO2HUT 40.45 ± 21.41). Positive linear relations between the tilt-induced changes (Δ) in HFNU and R-R interval were recorded for RA (ΔHFNU = 0.0787(ΔR-R) - 11.3, R 2 = 0.79, P < 0.05), and for 5%CO2 (ΔHFNU = 0.0334(ΔR-R) + 1.1, R 2 = 0.82, P < 0.05). The decreased HF component suggested withdrawal of vagal activity during HUT. For both RA and 5%CO2, the positive linear relations between ΔHFNU and ΔR-R suggested that the greater the increase in heart rate with HUT, the greater the vagal withdrawal. However, a reduced range of ΔHF during HUT with respiratory acidosis suggested vagal withdrawal was lower with a respiratory acidosis. © 2008 The Physiological Society of Japan and Springer.