Differences in heart rate responses to upright posture are associated with variations in the high-frequency power of heart rate variability

Abstract

High resting heart rate is a cardiovascular risk factor, but limited data exists on the underlying hemodynamics and reproducibility of supine-to-upright increase in heart rate. We recorded non-invasive hemodynamics in 574 volunteers (age 44.9 years, BMI 26.4 kg/m2, 49% male) during passive head-up tilt (HUT) using whole-body impedance cardiography and radial artery tonometry. Heart rate regulation was evaluated using heart rate variability (HRV) analyses. Comparisons were made between quartiles of supine-to-upright heart rate changes, in which heart rate at rest ranged 62.6-64.8 beats/min (p=0.285). The average upright increases in heart rate in the quartiles 1‑4 were 4.7, 9.9, 13.5, and 21.0 beats/min, respectively (p<0.0001). No differences were observed in low frequency power of HRV whether supine or upright, or supine high frequency power of HRV. Upright high frequency power of HRV was highest in quartile 1 with lowest upright heart rate, and lowest in quartile 4 with highest upright heart rate. Mean systolic blood pressure before and during HUT (126 vs. 108 mmHg), and the increase in systemic vascular resistance during HUT (650 vs. 173 dyn×s/cm5×m2), were highest in quartile 1 and lowest in quartile 4. The increases in heart rate during HUT on three separate occasions several weeks apart were highly reproducible (r=0.682) among 215 participants. To conclude, supine-to-upright increase in heart rate is a reproducible phenotype with underlying differences in the modulation of cardiac parasympathetic tone and systemic vascular resistance. As heart rate at rest influences prognosis, future research should elucidate the prognostic significance of these phenotypic differences.