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

Human ECG records were analyzed during supine (SUP) rest and whole body 80A degrees 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 +/- A 13 vs. RA(HUT) 79 +/- A 16; 5%CO2SUP 63 +/- A 12 vs. 5%CO2HUT 79 +/- A 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 (RA(SUP) 44.01 +/- A 21.57 vs. RA(HUT) 24.05 +/- A 13.09; 5%CO2SUP 69.23 +/- A 15.37 vs. 5%CO2HUT 47.64 +/- A 21.11) without accompanying changes in the low frequency (0.04-0.15 Hz) component (LFNU) (RA(SUP) 52.36 +/- A 21.93 vs. RA(HUT) 66.58 +/- A 19.49; 5%CO2SUP 22.97 +/- A 11.54 vs. 5%CO2HUT 40.45 +/- A 21.41). Positive linear relations between the tilt-induced changes (Delta) in HFNU and R-R interval were recorded for RA (Delta HFNU = 0.0787(Delta R-R) - 11.3, R (2) = 0.79, P < 0.05), and for 5%CO2 (Delta HFNU = 0.0334(Delta R-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 Delta HFNU and Delta R-R suggested that the greater the increase in heart rate with HUT, the greater the vagal withdrawal. However, a reduced range of Delta HF during HUT with respiratory acidosis suggested vagal withdrawal was lower with a respiratory acidosis.