Exercise training in chronic heart failure: improving skeletal muscle O2transport and utilization

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Abstract
Chronic heart failure (CHF) impairs critical structural and functional components of the O2transport pathway resulting in exercise intolerance and, consequently, reduced quality of life. In contrast, exercise training is capable of combating many of the CHF-induced impairments and enhancing the matching between skeletal muscle O2delivery and utilization ( Q̇mO2and V̇mO2, respectively). The Q̇mO2/ V̇mO2ratio determines the microvascular O2partial pressure (PmvO2), which represents the ultimate force driving blood-myocyte O2flux (see Fig. 1). Improvements in perfusive and diffusive O2conductances are essential to support faster rates of oxidative phosphorylation (reflected as faster V̇mO2kinetics during transitions in metabolic demand) and reduce the reliance on anaerobic glycolysis and utilization of finite energy sources (thus lowering the magnitude of the O2deficit) in trained CHF muscle. These adaptations contribute to attenuated muscle metabolic perturbations (e.g., changes in [PCr], [Cr], [ADP], and pH) and improved physical capacity (i.e., elevated critical power and maximal V̇mO2). Preservation of such plasticity in response to exercise training is crucial considering the dominant role of skeletal muscle dysfunction in the pathophysiology and increased morbidity/mortality of the CHF patient. This brief review focuses on the mechanistic bases for improved Q̇mO2/ V̇mO2matching (and enhanced PmvO2) with exercise training in CHF with both preserved and reduced ejection fraction (HFpEF and HFrEF, respectively). Specifically, O2convection within the skeletal muscle microcirculation, O2diffusion from the red blood cell to the mitochondria, and muscle metabolic control are particularly susceptive to exercise training adaptations in CHF. Alternatives to traditional whole body endurance exercise training programs such as small muscle mass and inspiratory muscle training, pharmacological treatment (e.g., sildenafil and pentoxifylline), and dietary nitrate supplementation are also presented in light of their therapeutic potential. Adaptations within the skeletal muscle O2transport and utilization system underlie improvements in physical capacity and quality of life in CHF and thus take center stage in the therapeutic management of these patients.

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