Evaluation of skeletal muscle fatigue in patients with heart failure.

Exertional fatigue is a major limiting symptom in patients with heart failure. Nevertheless, the mechanism responsible for this fatigue remains unclear. Reduced skeletal muscle blood flow, altered muscle oxidative enzymes, decreased muscle mass and altered muscle contractility have all been implicated but the precise contribution of these factors remains to be determined. Clinically, the severity of muscle fatigue is usually determined from the patient history. This approach is unreliable since symptoms reported during normal daily activities correlate poorly with objective measures of exercise performance. The degree of perceived fatigue noted during formal exercise testing also correlates poorly with objective measures of muscle behavior, such as blood lactate levels. Cardiopulmonary exercise testing can be used to identify the onset of lactate release, or the anaerobic threshold, and therefore provides more objective information about muscle behavior. However, an increase in blood lactate does not necessarily indicate muscle fatigue. Surface recordings of muscle electromyograms during exercise permits detection of increasing EMG signal, an indicator of fatigue. This technique is potentially valuable but is technically challenging. Finally, muscle fatigue can be assessed by monitoring developed tension during repetitive contractions of the quadriceps muscle. This index of muscle fatigability correlates relatively closely with maximal exercise performance and provides the best index of fatigue currently available. However, further studies are needed to document the clinical utility of this methodology.