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Reliability of Heart Rate Variability as an Assessment of Cardiac Sympathetic Activity in Humans

Spectral analysis of heart rate variability (HRV) has been widely used as an indirect, non‐invasive method for assessing cardiac autonomic function in numerous psychophysiological and clinical research studies. However, there is debate whether or not the low‐frequency (LF) component of the analysis provides accurate insight into cardiac sympathetic activity in humans. The purpose of this study was to evaluate the reliability of the LF component of HRV compared to that of peripheral muscle sympathetic nerve activity (MSNA), which remains the only direct method to assess sympathetic neural outflow in humans and has been consistently shown to have high intra‐individual reproducibility. Sixteen adults (9 men and 7 women, 21 ± 1 years, body mass index of 23 ± 1 kg/m2) were examined, and completed three experimental sessions across two laboratory visits. Experimental sessions one and two were conducted during the first laboratory visit and separated by 45 minutes of supine rest, while the third session was completed approximately one month after the first laboratory visit. Each session included a five‐minute supine baseline that included recordings of MSNA via microneurography and R‐R intervals using a three‐lead electrocardiogram. Intraclass correlation analyses (Cronbach's α coefficient) were performed to determine the consistency of HRV and MSNA across the three sessions. The spectral analysis revealed that the high‐frequency (HF) component, which is primarily associated with cardiac parasympathetic activity, was highly reliable (Cronbach's α = 0.90, P < 0.001). In contrast, the LF component, which is often utilized as a noninvasive estimate of cardiac sympathetic activity, was not significantly consistent across the three sessions (Cronbach's α = 0.45, P = 0.078). As previously reported (Fonkoue & Carter, Am J Physiol Regul Integr Comp Physiol, 2015), MSNA was highly consistent across the three sessions (Cronbach's α = 0.84, P < 0.001). In summary, our findings demonstrate that unlike MSNA, which has been consistently shown to be a stable and reliable measurement of peripheral sympathetic activity, the LF component of HRV is not a stable and reliable measurement of cardiac sympathetic activity in humans.