Routine measurements of respiratory function, that is, volumes, flows, and indices of gas exchange, are nonspecific in relation to diagnosis but give useful indirect information about respiratory muscle performance. On occasion, the presence of respiratory muscle dysfunction is first suspected from the pattern of conventional respiratory function tests. More frequently, they are of use in assessing the severity, functional consequences, and progress of patients with recognized muscle weakness.
The most frequently noted abnormality of lung volumes in patients with respiratory muscle weakness is a reduction in vital capacity (VC). The pattern of abnormality of other subdivisions of lung volume is less consistent. Residual volume (RV) is usually normal or increased, the latter particularly with marked expiratory weakness (1). Consequently, total lung capacity (TLC) is less markedly reduced than VC, and the RV/ TLC and FRC/TLC ratios are often increased without necessarily implying airway obstruction.
The VC is limited by weakness of both the inspiratory muscles, preventing full inflation, and expiratory muscles, inhibiting full expiration. In addition to the direct effect of loss of muscle force, reductions in compliance of both the lungs (2) and chest wall (3) also contribute to the reduction of VC in patients with chronic respiratory muscle weakness. In severe weakness, the TLC and VC relate more closely to lung compliance than to the distending force (4, 5) (Figure 1). The mechanism of reduced lung compliance is unclear. Contrary to earlier suggestions, it is probably not simply due to widespread microatelectasis (6). Static lung volumes may also be affected in some patients by coexistent lung or airway disease. Vital capacity, thus, reflects the combined effect of weakness and the static mechanical load on the respiratory muscles.
In mild respiratory muscle weakness, VC is less sensitive than maximum respiratory pressures. However, the curvilinear relation between VC and maximum inspiratory pressure (5) (Figure 2) implies that, in more advanced disease, marked reductions in VC can occur with relatively small changes in maximum pressures.
In patients with isolated or disproportionate bilateral diaphragmatic weakness or paralysis, the VC shows a marked fall in the supine compared with the erect posture because of the action of gravitational forces on the abdominal contents. In some patients, this postural fall may exceed 50%. In most normal subjects, VC in the supine position is 5-10% less than when upright (7) and a fall of 30% or more is generally associated with severe diaphragmatic weakness (8).
Methodology and Equipment
Recommendations and requirements for the measurement of VC and other lung volumes are covered in detail elsewhere (9, 10).
VC has excellent standardization, high reproducibility and well-established reference values. It is easily performed, widely available, and economical. It is quite sensitive for assessing progress in moderate to severe respiratory muscle weakness. The rate of decline has been shown to predict survival in both amyotrophic lateral sclerosis (11) and Duchenne muscular dystrophy (12).
VC has poor specificity for the diagnosis of respiratory muscle weakness. In mild weakness, it is generally less sensitive to changes than are maximum pressures (13).
Serial measurements of VC should be routine in monitoring progress of patients with acute and chronic respiratory muscle weakness.
Measurement of postural change of VC gives a simple index of weakness of the diaphragm relative to the other inspira-tory muscles.
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