Respiratory muscle oxygen demand and cardiac output distribution during exercise: revisiting an underweighted mechanism of exercise intolerance in heart failure

Exercise intolerance remains a defining feature of heart failure (HF), yet its physiological determinants remain incompletely understood.Traditionally, limitations to exercise performance have been interpreted through a dichotomous framework involving "central" mechanisms, including impaired cardiac output and pulmonary hemodynamics, and "peripheral" mechanisms, including skeletal muscle dysfunction and impaired oxygen extraction.However, this framework may overlook a critical intermediary process: the distribution of cardiac output among competing metabolic territories during exercise.Indeed, although regional blood flow distribution and peripheral vascular, endothelial, muscular, and reflex abnormalities are recognized contributors to exercise intolerance in HF, the interaction among these mechanisms has received comparatively less attention (1).Nearly three decades ago, Harms et al. (2-5) demonstrated that respiratory muscles represent a substantial metabolic burden during intense exercise in healthy trained subjects.By experimentally manipulating respiratory muscle work, they showed that increasing the work of breathing altered the distribution of cardiac output.This shift directed a larger fraction of flow toward the respiratory muscles while reducing locomotor muscle perfusion.Respiratory muscles were estimated to receive up to 14%-16% of total cardiac output during maximal exercise, and increased respiratory muscle work was associated with reductions in leg blood flow, increases in leg vascular resistance, and impaired exercise performance.Together, these studies established an important physiological principle: exercise performance depends not only on the magnitude of cardiac output but also on how that cardiac output is distributed among competing tissues.Importantly, Harms et al. proposed that these hemodynamic effects were mediated by reflex sympathetic vasoconstriction originating from metabolically stressed respiratory muscles.Their physiological model incorporated activation of respiratory muscle afferents and reflex increases in sympathetic vasoconstrictor outflow, providing a mechanistic explanation for the observed reduction in locomotor muscle blood flow during heavy exercise.Thus, the essential physiological framework linking respiratory muscle work to exercise blood flow redistribution was already present in these