Measurements of the lowest mouth pressures developed during maximum static inspiratory efforts are related to the recoil force of the lung and occasionally are influenced by glottic closure. The measurement of minimal pleural pressures (P pl min) over the entire range of inspiratory capacity eliminates both problems and, in addition, provides a good estimate of the subject's cooperation. Using this technique, we have investigated the inspiratory muscle force in 120 healthy adults (60 men, 60 women) aged 21 to 76 years, and 15 healthy children (eight boys, seven girls) aged 7 to 13 years. Twelve patients with interstitial lung disease were studied for comparison. In the healthy adults, at any fixed (fractional) lung volume, P pl min increased, that is, became less negative with advancing age, both in males and females (all r greater than 0.56, p less than 0.001). This pattern was not modified after correction of the data for the static recoil pressure of the chest wall, indicating that the inspiratory muscle force actually decreases with age. In any age group, and after correction of lung volume for the difference in stature, the P pl min values in women were between 80 and 90% of the values found in men; moreover the children generated pressures that were as low as those developed by the younger adults. This is probably because women and children have a smaller thorax than men and are therefore able to generate low pressures, despite weaker muscles. When the reduction in lung volume was taken into account, the relationship between lung volume and P pl min was normal in the patients with interstitial lung disease. These patients showed a close relationship between the degree of lung volume restriction and the increase of the static recoil pressure of the lung at full inflation, suggesting that their thorax is normally compliant. It appears therefore that these patients have normal inspiratory muscle force, at least when they are not in an advanced stage of the disease.
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