Exertional fatigue due to skeletal muscle dysfunction in patients with heart failure

Exertional fatigue in patients with chronic heart failure is usually attributed to skeletal muscle underperfusion. Recently, skeletal muscle atrophy, abnormal muscle metabolic responses, and reduced muscle enzyme levels have been noted in such patients, raising the possibility that some patients may develop muscle fatigue due to intrinsic muscle abnormalities. The present study was undertaken to determine if a subpopulation of patients with heart failure develops exertional fatigue due to skeletal muscle dysfunction rather than to reduced muscle flow.

All exercise hemodynamic studies performed in our laboratory on patients with heart failure were reviewed to identify those who exhibited peak exercise VO2 levels < or = 18 ml.min-1 x kg-1 due to leg fatigue and who underwent insertion of a Swan-Ganz catheter and leg blood flow catheter. Thirty-four patients were identified. Six normal subjects were also studied to define normal leg flow and femoral venous lactate responses to exercise. Patients with peak exercise leg flow levels within the normal mean flow level +/- 2 SEM were considered to have normal skeletal muscle flow during exercise. Nine of the 34 patients with heart failure were found to have normal leg blood flow during exercise. All of these patients terminated exercise due to leg fatigue, and all exhibited abnormal increases in femoral venous lactate concentrations (slope of work load versus femoral venous lactate: normal, 0.33 +/- 0.07 mg/W; heart failure with normal flow, 0.81 +/- 0.08 mg/W; p < 0.002). There was no significant difference between patients with normal leg flows and those with reduced flow in age, ejection fraction, and resting hemodynamic measurements. However, patients with normal flows exhibited more normal cardiac output responses to exercise and tended to have higher peak exercise VO2 (14.1 +/- 0.9 versus 11.5 +/- 0.7 ml.min-1 x kg-1, p < 0.05).

A substantial percentage of patients with chronic heart failure develop exertional fatigue due to skeletal muscle dysfunction rather than to reduced skeletal muscle blood flow. In such patients, therapeutic interventions probably should be directed at improving the skeletal muscle abnormalities rather than at improving skeletal muscle flow.