Mechanisms and Management of Exercise Intolerance in Older Heart Failure Patients

Heart Failure, Diastolic

muscle sympathetic nerve activity, exercise tolerance, exercise training, peak oxygen uptake

Heart failure with preserved ejection fraction (HFpEF) is the fastest growing form of heart failure with a high morbidity and mortality rate, and is associated with severe exercise intolerance. The mechanisms responsible for the reduced exercise tolerance remain poorly understood. The investigators propose a novel paradigm shift, focusing on peripheral limitations to exercise. In particular, the investigators will test the hypothesis that muscle sympathetic nerve activity (MSNA) is elevated in older HFpEF patients compared to healthy controls, and is associated with reduced exercise tolerance. The investigators will also test whether 16-weeks of exercise training will lower MSNA compared to attention control, and correlate with improved exercise tolerance in older HFpEF patients.

Heart failure with preserved ejection fraction is the fastest growing form of heart failure, is almost exclusively found in older persons, particularly older women, and is associated with a high morbidity and mortality rate. The primary chronic symptom in HFpEF patients is severe exercise intolerance measured objectively as decreased peak exercise oxygen uptake (peak VO2). A consequence of the reduced exercise tolerance is that activities of daily living require near maximal effort, resulting in further deconditioning and reduced quality of life. The majority of work to date has focused on cardiac limitations, showing impaired cardiac output and marked diastolic dysfunction. Although these findings have provided important insight into the pathophysiology of HFpEF, drug therapies targeting cardiac function do not improve peak VO2, quality of life, or survival in HFpEF patients. Older HFpEF patients have multiple skeletal muscle abnormalities including reduced skeletal muscle oxidative capacity and capillary-to-fiber ratio resulting in increased anaerobic metabolism during low-level exercise. Importantly, accumulation of anaerobic metabolites within the exercising muscles are known to activate skeletal muscle afferent fibers (called metaboreceptors), that elicit a reflex-mediated increase in efferent muscle sympathetic (vasoconstrictor) nerve activity (MSNA). The investigators here propose a novel paradigm of exercise intolerance in older HFpEF patients whereby skeletal muscle abnormalities lead to overactivation of the muscle metaboreflex and MSNA mediated vasoconstriction that limits delivery of oxygenated blood to the active muscles. Further, exercise training mediated improvements in skeletal muscle function will alleviate the metaboreflex, thereby reducing MSNA and improve oxygen delivery to the contracting muscles. To test this novel paradigm, the investigators will first perform an initial cross-sectional comparison of older (≥60 years) HFpEF patients (N=24) with age and sex-matched healthy controls (N=24), and then enter the HFpEF patients into a randomized, controlled, single blind, trial of exercise training to test the following hypothesis: (i) that MSNA is elevated in older HFpEF patients compared to healthy controls, and is associated with reduced peak VO2, physical functional performance, aerobic endurance, muscle blood flow, and quality of life; and (ii) Exercise training will attenuate MSNA compared to attention control, and will correlate with improved peak VO2, physical functional performance, aerobic endurance, muscle blood flow, and quality of life in older HFpEF patients.

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