Reliability of the heart rate variability threshold during treadmill exercise

Abstract

The heart rate variability threshold (HRVT) is a clinical parameter used to gain insight into autonomic balance. Prior validation of the HRVT has been with cycle ergometry, with no studies examining the viability of treadmill exercise. The purpose of this study was to examine the reliability of the HRVT during treadmill exercise, and to compare the HRVT to the ventilatory threshold (VT). Ten healthy, college-aged males completed two maximal graded exercise tests on a treadmill. A Polar RS800CX watch was used for heart rate and HRVT data. The HRVT was determined from three HRV variables including the root mean square of successive differences of continuous R-R intervals (RMSSD), the standard deviation of normal R-R intervals (SDNN) and the standard deviation of instantaneous beat intervals (SD1). A metabolic cart was utilized to determine the VT. Results showed no difference between the HRVT (2.4 ± 0.6 and 2.2 ± 0.3 for RMSSD, 2.8 ± 0.5 and 2.7 ± 0.5 for SDNN and 2.4 ± 0.6 and 2.3 ± 0.6 for SD1) or the VT (3.0 ± 0.3 and 3.1 ± 0.3) between trials. When compared to the VT, averaged HRVT values for RMSSD (2.3 ± 0.3) and SD1 (2.3 ± 0.5) were lower than averaged VT (2.8 ± 0.4, p < 0.05). The averaged HRVT from SDNN (2.8 ± 0.5) did not differ from the VT. These results suggest that treadmill is a viable mode for HRVT determination, and that HRVT determined by SDNN may be a better comparison to the VT.

Keywords: V-slope method; autonomic activity; graded exercise test; root mean square of successive differences of continuous R−R intervals; standard deviation of instantaneous beat intervals; ventilatory threshold.

Figure 1

Bland−Altman plots assessing agreement between exercise trials for (a) HRVT_RMSSD; (b) HRVT_SDNN; (c) HRVT_SD1; (d) VT. HRVT_RMSSD, heart rate variability threshold root mean square of successive differences of continuous R−R intervals; HRVT_SD1, heart rate variability threshold standard deviation of instantaneous beat intervals; HRVT_SDNN, heart rate variability threshold standard deviation of normal R−R intervals; VT, ventilatory threshold.

Figure 2

Comparison of the oxygen consumption (L min⁻¹) at which the HRVT was identified for three heart rate variability indexes. *p = 0.002. ^# p = 0.02. HSRT, heart rate variability threshold; RMSSD, root mean square of successive differences of continuous R−R intervals; SD1, standard deviation of instantaneous beat intervals; SDNN, standard deviation of normal R−R intervals.

Figure 3

Bland−Altman plots assessing agreement between HRVT and VT. (a) RMSSD; (b) SD1; (c) SDNN. HRVT, heart rate variability threshold; RMSSD, root mean square of successive differences of continuous R−R intervals; SD1, standard deviation of instantaneous beat intervals; SDNN, standard deviation of normal R−R intervals; VT, ventilatory threshold.