Exercise conditioning and heart rate variability: Evidence of a threshold effect

Abstract

Background: A protective effect of exercise in preventing sudden cardiac death is supported by studies in healthy populations as well as in patients with cardiac disease. The mechanisms involved in this protective effect are unknown. Hypothesis: We hypothesized that exercise conditioning would beneficially alter autonomic nervous system tone, measured by heart rate variability. Methods: We prospectively studied 20 cardiac patients enrolled in a Phase 2 12-week cardiac rehabilitation program following a recent cardiac event. The patients underwent 24 h Holter monitoring at program entry and 12 weeks later. Heart rate variability analysis was assessed for both time domain and spectral analysis. Results: The group demonstrated a modest mean conditioning effect, indicated by an average reduction in resting heart rate from 8 I k 16 to 75 12 beatshin (p = 0.03), and an increase in training METS from 2.1 rt 0.4 to 3.3 f 1.1 (p< 0.OOOl). Overall, 15 of 20 (75%) patients demonstrated increased total and high-frequency power, and mean high-frequency power was significantly increased (3.9 rt 1.4 vs. 4.4 k 1 .O In, p = 0.05). When stratified according to the magnitude of exercise conditioning, patients achieving an increase of > 1 .5 training METS demonstrated significant increases in SDNN, SDANN index, SDNN index, pNN50, total power,

Changes in short-term measures of heart rate variability after eight weeks of cardiac rehabilitation

“…Our findings agree with previous data reporting a 25% increase in short-term SDNN in patients undertaking 8 weeks of CR commencing 4 weeks after first uncomplicated MI [21] and from several studies using ambulatory ECG recordings [19,32] The data, however, differ from those of Duru et al [8] and Pardo et al [26] who found no such changes. RMSSD is predominantly mediated by vagal activity and over the course of CR there was a non-significant (P = 0.09) increase.…”

“…RMSSD is predominantly mediated by vagal activity and over the course of CR there was a non-significant (P = 0.09) increase. Previous studies [20,21] have shown very large (25-69%) changes in analogous measures (MSSD) whereas ambulatory data have shown no change [26,32] j Frequency domain measures…”

“…However, attempts to provide evidence for changes in autonomic function during CR using HRV remain mixed [8,11,19,20,22,24,26,32]. Reports of significant changes in power spectral measures are rare [8,19,20,22] and some studies [8,19] have failed to provide raw spectral values (ms 2 ) low frequency power (LF) or high frequency power (HF) as recommended [31].…”

Long-term estrogen deficiency lowers regional blood flow, resting systolic blood pressure, and heart rate in exercising premenopausal women

“…Attenuation of sympathetic activity and increased vagal tone, effecting decreased vascular resistance, is a recognized adaptation to chronic endurance training (35). In contrast to these findings, LT-E 2 -deficient women demonstrate increased regional vascular resistance despite exposure to chronic aerobic exercise training.…”

Heritability of Ambulatory Heart Rate Variability

“…Spouse correlation, however, may also reflect assortative mating for exercise behavior, a variable known to be associated with SDNN index. 26 It is of note that the age-corrected sibling correlations in the Framingham Study (0.23 to 0.26) correspond very closely to our agecorrected sibling correlations, suggesting that, when the studies can be compared directly, they are actually very consistent.…”