The Effect of Hyperinflation on Respiratory Muscle Strength and Efficiency in Healthy Subjects and Patients with Asthma

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Abstract
We evaluated respiratory muscle strength and efficiency in 15 patients with asthma. There was a significant reduction in mean Plmax (89.3 .+-. 4.7 versus 110.5 .+-. 9.4 cm H2O, p < 0.001) and efficiency (2.41 .+-. 0.2 versus 4.2 .+-. 0.6%, p < 0.01). This reduction in strength and efficiency was seen only in the male patients. Following bronchodilator, there was a significant increase in Plmax (from 89.3 .+-. 4.7 to 96.2 .+-. 5.4 cm H2O2, p < 0.005) and efficiency (from 2.41 .+-. 0.2 to 3.22 .+-. 0.2%, p < 0.001). There was no correlation between the change in strength and efficiency and the degree of improvement in FEV1 following bronchodilator. However, there was a significant correlation with the fall in lung volume. To determine whether hyperinflation would result in a reduction in respiratory muscle strength and efficiency, we induced a mean increase in end-expiratory lung volume of 0.66 L by applying continuous negative pressure around the chest in 10 healthy individuals. This was associated wtih a significant fall in P1lmax (from 110.5 .+-. 9.4 to 100.5 .+-. 8.93 cm H2O, p < 0.001) and efficiency (from 4.2 .+-. 0.6 to 2.6 .+-. 0.5%, p < 0.005). The data suggest that the strength and efficiency of the respiratory muscles are reduced in asthmatic males but not in the females. The strength and efficiency of the respiratory muscles improve significantly following bronchodilator, and this improvement is related to reduction in lung volume. Similarly, breathing at an elevated lung volume was associated with a reduction in the strength and efficiency of the respiratory muscles in healthy subjects. It was suggested that hyperinflation affects respiratory muscle performance, and this may play a major role in the reduced strength and efficiency found in asthma.