Tests of respiratory muscle function.

Respiratory muscle strength and endurance should be assessed when dyspnea, respiratory failure, or poor performance on routine pulmonary function tests are unexplained. Respiratory muscle strength can be measured non-invasively from maximal mouth pressures, but measurement of transdiaphragmatic pressure refines the assessment. The maximal voluntary ventilation test is the only simple index of ventilatory or respiratory muscle endurance. Other tests for endurance and fatigue are more difficult, but some newer techniques may be applicable to the clinical laboratory. Some patients who exhibit tachypnea, marked use of neck inspiratory muscles, and chest-abdomen asynchrony may be developing respiratory muscle fatigue.