Response to Letter Regarding Article, “Physical Activity and Heart Rate Variability in Older Adults: The Cardiovascular Health Study”

We are pleased to address the suggestion of Monfredi and colleagues that further consideration of resting heart rate (HR) might provide insight into the relationship between physical activity and heart rate variability (HRV) reported in our article.1 As we have previously discussed,2 influences on HRV are complex and multifactorial. Although HR influences HRV, HRV indices are not merely linear nor nonlinear surrogates of HR, but independent measures influenced by numerous other physiological pathways. Although we agree that HRV indices such as 24-hour standard-deviation-of-all-normal-to-normal-intervals (SDNN) cannot directly characterize vagal activity, HRV indices can provide insights into autonomic influences on and responsiveness of the heart. We also note that we made no statements about the physiological determinants of resting bradycardia, which was not an outcome in our analysis.Previous studies have also demonstrated that associations of HRV with outcomes such as mortality …