Respiratory Muscle Fatigue in COPD Patients

Abstract

For man to take a spontaneous breath, the inspiratory muscles must generate sufficient force to overcome the elastance of the lungs and chest wall (lung and chest wall elastic loads) as well as the airway and tissue resistance (resistive load). This requires an adequate output of the centers controlling the muscles, anatomic and functional nerve integrity, unimpaired neuromuscular transmission, an intact chest wall and adequate muscle strength. This can be schematically represented by considering the ability to take a breath as a balance between inspiratory load and neuromuscular competence (Fig. 1). Under normal conditions this system is polarized in favour of neuromuscular competence, i.e., there are reserves that permit considerable increases in load. However, for a man to breathe spontaneously the inspiratory muscles should be able to sustain the above mentioned load over time and also adjust the minute ventilation in such a way that there is adequate gas exchange. The ability of the respiratory muscles to sustain this load without the appearance of fatigue is called endurance and is determined by the balance between energy supplies (Us) and energy demands (Ud) (Fig. 2).