Circadian Variation of HRV Parameters during Rest and Exercise

Abstract

Many physiological parameters demonstrate a circadian pattern of variability, such as heart rate, blood pressure and autonomic nervous system (ANS) activity. The degree of cardiac ANS activity can be elucidated using non-invasive techniques such as analysis of heart rate variability (HRV). The periodic oscillations in cardiovascular activity can be detected in the frequency domain by power spectral analysis based on the beat-to-beat time differences between adjacent R-waves (R-R interval). Although previous studies have observed changes in sympathovagal balance between wake-sleep cycles, the temporal pattern of ANS activity during the daytime hours has not been investigated. PURPOSE: To determine whether a circadian pattern of variation exists for HRV parameters during rest and sub-maximal exercise. METHODS: Six sedentary but otherwise healthy male subjects (Age: 28 ± 4 yrs; VO2peak: 35 ± 7 ml·kg·min-1; BMI: 25.5 ± 1.9), undertook experimental trials at 0900, 1300 and 1700 on different days. Participants sat upright at rest on a cycle ergometer and then cycled at a self-selected cadence at a power output corresponding to 50% of their VO2peak. The R-R interval data was filtered so that any 'aberrant' beats could be discarded, then was transferred to a HRV analysis software package (v 1.1, Biosignal, University of Kuopio, Finland), where the first order linear trend was removed, and then re-sampled at 4 Hz with a cubic interpolation. An autoregressive time-series model (Order 16) was used for the spectrum estimation. Frequency domain measures used were high frequency power (HF) as an indicator of parasympathetic activity, and the ratio of low frequency (0.04–0.15 Hz) to high frequency (0.15–0.4 Hz) power (LF/HF) as an indicator of sympathetic activity. The time domain measures used in HRV analysis included the percentage of normal-normal intervals greater than 50 ms (pNN50), and the square root of the mean squared differences of successive R-R intervals (rMSSD). RESULTS: At rest, parasympathetic indicators (markers of heart relaxation) in both the time (rMSSD, pNN50) and frequency (HF) domains were suppressed at 0900 (P <0.05). During sub-maximal work, rMSSD and HF power increased throughout the day and peaked at 1700 (P >0.05). There were no circadian differences between the sympathetic indicator (LF/HF) at rest or during exercise (P >0.05). CONCLUSIONS: At rest and during sub-maximal exercise, cardiac parasympathetic control is lower during the early working hours (0900). This vagal withdrawal may explain the increased HR response to a given sub-maximal workload at this time of day.