Respiratory muscle dysfunction

Abstract

The respiratory system consists of two main parts, the lung and the ventilatory pump. The latter is formed by the bony structure of the thorax, the central respiratory controllers, the inspiratory and expiratory muscles and the nerve innervating these muscles. Whereas failure of the lung leads to hypoxemia, failure of the ventilatory pump, in particular a dysfunction of the respiratory muscles, leads to hypercapnic respiratory failure. Respiratory muscle fatigue occurs when respiratory muscle endurance is exceeded; that is, when the load against which the muscles must contract requires too great an effort for too long [ 1 ]. Healthy people never approach this threshold, below which diaphragm fatigue does not occur. In contrast, patients with severe chronic obstructive pulmonary disease (COPD) neuromuscular diseases may be close to the threshold at rest and exceed it even with minor exertion [2, 3]. In September 1988 a workshop was held at Kansas State University to reassess the state of knowledge on respiratory muscle fatigue [4]. Muscle fatigue was defined as a condition in which there is a loss in the capacity for developing force and/or velocity of a muscle resulting from muscle activity and which is reversible by rest. Muscle weakness is a condition in which the capacity of a rested muscle to generate force is impaired.