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 exist on the underlying hemodynamics and reproducibility of supine-to-upright increase in heart rate. We recorded noninvasive hemodynamics in 574 volunteers [age, 44.9 yr; body mass index (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 the low-frequency power of HRV, whether in the supine or upright position, or in the high-frequency power of HRV in the supine position. 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.NEW & NOTEWORTHY Subjects with similar supine heart rates are characterized by variable increases in heart rate during upright posture. Individual heart rate increases in response to upright posture are highly reproducible as hemodynamic phenotypes and present underlying differences in the modulation of cardiac parasympathetic tone and systemic vascular resistance. These results indicate that resting heart rate obtained in the supine position alone is not an optimal means of classifying people into groups with differences in cardiovascular function.

Keywords: head-up tilt; heart rate; phenotype.

Figure 1.

Heart rate in all participants (A), in women (B), and in men (C) during supine position and passive head-up tilt. Division to sex-stratified quartiles (Q1–Q4) was performed according to supine-to-upright change in heart rate; adjusted for age, body mass index, and fasting plasma glucose. The inserts show the supine-to-upright increases in heart rate in individual quartiles. Statistics are from the 5-min supine posture and last 3-min upright posture when the variables had stabilized, means (SE); *P < 0.05.

Figure 2.

Power of heart rate variability (HRV) in low-frequency (A) and high-frequency (B) range, and low- to high-frequency ratio of HRV (C) during supine position and passive head-up tilt; analyses adjusted for age, body mass index, and fasting plasma glucose. Both sexes were divided separately to quartiles (Q1–Q4) according to supine-to-upright change in heart rate, lg₁₀-transformed values are presented. White background (supine), gray background (upright); means (SE); n = 121 in Q1, n = 130 in Q2, n = 136 in Q3, and n = 120 in Q4 (all participants did not have available HRV values); *P < 0.05.

Figure 3.

Aortic systolic blood pressure (A) and diastolic blood pressure (B) during supine position and passive head-up tilt. Both sexes were divided separately to quartiles (Q1–Q4) according to supine-to-upright change in heart rate; adjusted for age, body mass index, and fasting plasma glucose. Statistics are from the 5-min supine posture and last 3-min upright posture when the variables had stabilized, means (SE); *P < 0.05.

Figure 4.

Stroke volume index (A), cardiac index (B), systemic vascular resistance index (SVRI) (C), and left cardiac work index (D) during supine position and passive head-up tilt, analyses are adjusted for age and fasting plasma glucose. Both sexes were divided separately to quartiles (Q1–Q4) according to supine-to-upright change in heart rate. Statistics are from the 5-min supine posture and last 3-min upright posture when the variables had stabilized, means (SE); *P < 0.05.

Figure 5.

Scatter plot and Pearson correlation of the heart rate changes during the first and the third passive head-up tilt from supine to upright position performed 90 days (median) apart in 215 subjects; women depicted with pink symbols and men depicted with light green symbols.