Intraindividual validation of heart rate variability indexes to measure vagal effects on hearts

Heart rate variability ( HRV) has been widely used as a measure of vagal activation in physiological, psychological, and clinical examinations. We studied the within-subject quantitative relationship between HRV and vagal effects on the heart in different body postures during a gradually decreasing vagal blockade. Electrocardiogram and respiratory frequency were measured in subjects ( 8 endurance athletes and 10 participants of nonendurance sports) in supine, sitting, and standing postures before the blockade, under vagal blockade ( atropine sulfate, 0.04 mg/kg), and four times during a 150-min recovery from the blockade. Fast Fourier transform was used to calculate low-frequency power ( LFP, 0.04 - 0.15 Hz), high-frequency power ( HFP, 0.15 - 0.40 Hz), and total power ( TP, 0.04 - 0.40 Hz). A within-subject linear regression analysis of recovery time on each HRV index was conducted. Complete vagal blockade decreased all HRV significantly, particularly HFP ( P < 0.001). A linear fit explained a large portion of the within-subject variance between recovery time and natural log-transformed ( ln) HRV indexes in every posture, with coefficients of determination ( R-2) in the supine posture [ means ( SD)]: 98 ( SD 2)% for mean R-R interval, 87 ( SD 10)% for lnLFP, 87 ( SD 13)% for lnHFP, and 91 ( SD 10)% for lnTP. Neither body posture nor endurance-training background had an impact on R-2 values. There was marked between-subject variation in the R-2 values, slopes, and intercepts. In conclusion, all HRV, particularly HFP, is predominantly under vagal control. Within subjects, lnLFP, lnHFP, and lnTP increased linearly with the gradually decreasing vagal blockade in all postures.