Contributor: Steven Stroud

Exercise intolerance is a hallmark of HF. We tend to blame the weak heart for not delivering enough oxygen to exercising muscles, but this is wrong.

Muscles without oxygen are hypoxic. This could happen because of poor cardiac output or because the skeletal muscle cells are not handling oxygen well. In other words, does low cardiac output cause fatigability or are there differences in skeletal muscle energetics in HF?

All patients had similar energetics at rest. When experiencing fatigue the energetic threshold was no different among groups. However, the rate at which patients with heart failure reached this threshold was much faster; having rapid decline in high energy phosphates can now be the answer when a patient asks, “Why am I so tired all of the time?”

Exercise intolerant patients, those with NYHA Class II-III HFrEF or HFpEF, had rapid decline in high energy phosphate concentration, while patients with NYHA Class I HFrEF were indistinguishable from normal patients. The rapid decline was most pronounced in HFpEF patients.

Training has known benefit in HF, especially HFpEF. Energetic improvements could be the underlying mechanism.

In a heartbeat… Exercise intolerance in HF may be the result of rapid decline in skeletal muscle high energy phosphate, particularly in HFpEF. Future therapies targeting skeletal muscle energetics may improve exercise tolerance in HF.

Study Link: Fatigability, Exercise Intolerance, and Abnormal Skeletal Muscle Energetics in Heart Failure