Heart Rate Variability in Healthy and Heart Failure Populations: Associations and Responses to Exercise and Specific Interventions

Autonomic function is severely disrupted in heart failure. Depressed heart rate variability (HRV) in these patients is associated with an increased risk of cardiac events and death. Studies from 24 h HRV assessments demonstrate an improvement following exercise training as part of cardiac rehabilitation. At the most severe level, a need to transplant has been offset following structural recovery as a result of mechanical support to the failing heart yet the effect on autonomic dysfunction is unknown. The aim of this thesis was to examine the effects of cardiac rehabilitation (CR) and mechanical support on short-term measures of HRV in heart failure patients of varying disease severity. The first few chapters and subequent papers assessed the reliability and agreement of newly developed wireless technologies and measurement software in healthy participants. The findings revealed agreement was poor between systems but the new technologies demonstrated similarly fair reliability compared to each other and criterion measures. A potential role for resting HRV underlying the physiology and prediction of higher risk heart rate (HR) responses to graded exercise testing was then explored. The consequent chapter found that resting vagally mediated HRV measures were able to predict a low risk but not a high risk HR recovery accurately. Lower HRV also underlined an increased risk profile based on known prognostic HR measures in healthy populations. An observation was made for a lack of normative data with which comparisons could be made. A review of all papers publishing short-term HRV data in healthy adults revealed poor methodological standards in many of the studies, limiting the final outcomes. For all measures of HRV, data from the literature were lower than previously published norms but known age and gender differences remained. These data provide a new source for identification of so called normal and abnormal HRV. Reviewing the literature concerning the diagnostic and prognostic use of HRV in heart failure identified gaps in the literature. There were no data available relating to the effect of differing exercise training modalities on autonomic function. A randomised trial of 12 weeks aerobic or resistance CR training wsa successful in increasing functional and aerobic capacities but did not significantly alter resting absolute HRV values. However, the harmony between HR and HRV was favourably altered and better matched that of healthy participants. Prior to this thesis, there were no data relating to the autonomic profile of patients receiving mechanical support via left ventricular assist device (LVAD) therapy. The study of patients recovered and currently undergoing LVAD treatment revealed signigicantly higher HRV in the former and latter compared with heart failure patients receiving standard care. Patients recovered from LVAD therapy demonstrated a decreased risk for known HRV markers and a normalisation of autonomic modulations. In conclusion, a depressed HRV remains a significant risk factor in heart failure patients. Exercise training may afford a beneficial effect in mild-to-moderate patients. In more severe patients, HRV risk factors are favourably altered by mechanical support and should be considered in the assessment of these patients