During acute asthma there is an increase in functional residual capacity (FRC) that not uncommonly reaches values that exceed the patient's total lung capacity (TLC) during clinical remission. Have the respiratory muscles a critical role in this phenomenon? According to the recent literature, respiratory muscles may influence, and be influenced by, the degree of hyperinflation of bronchial asthma. The increase in lung volume causes an increase in the static transpulmonary pressure, and thus prevents airway closure. This explains only the rise in residual volume and not the increase in FRC. Martin et al. demonstrated that during induced bronchoconstriction and hyperinflation, the most positive expiratory pleural pressure was less than the predicted chest wall relaxation pressure, indicating persistent inspiratory muscle activity throughout expiration. This was directly confirmed, using electromyography by Muller et al. This persistent use of the inspiratory muscles throughout expiration, together with the increased resistive load, leads to a rise in respiratory muscle work; the recruitment of the intercostal/accessory and abdominal muscles seems to exceed that of the diaphragm, protecting it, but placing the inspiratory rib cage at a greater risk of fatigue in acute asthma. However, as a defence mechanism, it has been shown in dogs (Jiang et al.) that the mechanical effectiveness of the parasternal intercostals, during acute hyperinflation, is greater at TLC than at FRC. The strength of the respiratory muscles has been studied in stable asthmatic patients by several authors, who found no impairment in inspiratory or expiratory muscle strength or endurance, whilst Weiner et al. demonstrated a reduced strength and efficiency in asthmatic males, but not females. The only study performed during acute natural bronchospasm, by Lavietes et al., showed a mild degree of respiratory muscle weakness; no correlation was found between strength and the degree of airway obstruction, while respiratory muscle force was closely related to hyperinflation. We have also shown, in stable asthmatic patients, that airway obstruction does not play a major role in determining respiratory muscle strength and neuromuscular drive.
|Number of pages||4|
|Journal||European Respiratory Review|
|Publication status||Published - 1993|
ASJC Scopus subject areas
- Pulmonary and Respiratory Medicine