Respiratory muscle assessment

Respiratory muscles generate the pressure differences driving ventilation. Respiratory muscle weakness is hence an important clinical feature. In advanced stages, respiratory muscle weakness leads to respiratory pump failure. Respiratory muscle dysfunction (i.e. reduced strength or endurance) is to be distinguished from lung function abnormalities, and should be measured separately. Inspiratory muscle weakness may partially explain dyspnoea and exercise intolerance. In addition, reduced respiratory muscle force has been shown to be an important predictive factor for poor survival in chronic obstructive pulmonary disease (COPD) [1], cystic fibrosis [2] and congestive heart failure [3]. In advanced stages the functional consequence of respiratory muscle weakness is a reduction of the operational lung volume and patients may require mechanical ventilation. Expiratory muscle weakness leads to problems with speech, and mucus retention due to impaired cough efficacy. Measurement of respiratory muscle function is important in the diagnosis of respiratory muscle disease [4–6], or respiratory muscle dysfunction [7]. It may also be helpful in the assessment of the impact of chronic diseases [8–12] or their treatment [13–15] on the respiratory muscles. For example, specific inspiratory muscle training has been reported to be useful in COPD only when patients present with significant respiratory muscle weakness [15], and tapering of oral corticosteroid treatment successfully restored respiratory muscle strength and dyspnoea in patients with corticosteroid-induced myopathy [16]. The present chapter aims to provide clinicians with some aspects of respiratory muscle testing. More detailed, excellent reviews on the pathophysiology and aetiology of respiratory muscle weakness are available elsewhere for the interested reader [17, 18]. Indications, techniques commonly used in clinical practice and issues important in the interpretation of the test results are the main focus of this chapter.