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 (RASUP 60 ± 13 vs. RAHUT 79 ± 16; 5%CO2SUP 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%CO2SUP 22.97 ± 11.54 vs. 5%CO2HUT 40.45 ± 21.41). Positive linear relations between the tilt-induced changes (D) in HFNU and R–R interval were recorded for RA (DHFNU = 0.0787(DR-R) - 11.3, R 2 = 0.79, P\0.05), and for 5%CO2 (DHFNU = 0.0334(DR-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 DHFNU and DR-R suggested that the greater the increase in heart rate with HUT, the greater the vagal withdrawal. However, a reduced range of DHF during HUT with respiratory acidosis suggested vagal withdrawal was lower with a respiratory acidosis.