Respiratory muscle work influences locomotor convective and diffusive oxygen transport in human heart failure during exercise

Introduction: It remains unclear if naturally occurring respiratory muscle (RM) work influences leg diffusive O₂ transport during exercise in heart failure patients with reduced ejection fraction (HFrEF). In this retrospective study, we hypothesized that RM unloading during submaximal exercise will lead to increases in locomotor muscle O₂ diffusion capacity (D_MO₂) contributing to the greater leg VO₂. Methods: Ten HFrEF patients and 10 healthy control matched participants performed two submaximal exercise bouts (i.e., with and without RM unloading). During exercise, leg blood flow was measured via constant infusion thermodilution. Intrathoracic pressure was measured via esophageal balloon. Radial arterial and femoral venous blood gases were measured and used to calculate leg arterial and venous content (CaO₂ and CvO₂, respectively), VO₂, O₂ delivery, and D_MO₂. Results: From CTL to RM unloading, leg VO₂, O₂ delivery, and D_MO₂ were not different in healthy participants during submaximal exercise (all, p >.15). In HFrEF, leg VO₂ (CTL: 0.7 ± 0.3 vs. RM unloading: 1.0 ± 0.4 L/min, p <.01), leg O₂ delivery (CTL: 0.9 ± 0.4 vs. RM unloading: 1.4 ± 0.5 L/min, p <.01), and leg D_MO₂ (CTL: 31.5 ± 11.4 vs. RM unloading: 49.7 ± 18.6 ml min⁻¹ mmHg⁻¹) increased from CTL to RM unloading during submaximal exercise (all, p <.01), whereas CaO₂-CvO₂ was not different (p =.51). The degree of RM unloading (i.e., % decrease in esophageal pressure-time integral during inspiration) was related to the % increase in leg D_MO₂ with RM unloading (r = −.76, p =.01). Conclusion: Our data suggest RM unloading leads to increased leg VO₂ due to greater convective and diffusive O₂ transport during submaximal exercise in HFrEF patients.