Determinants of maximal inspiratory pressure in chronic obstructive pulmonary disease

Inspiratory muscle strength in COPD could be reduced either because of mechanical disadvantage consequent to increased lung volume or because respiratory muscles share in generalized muscle weakness. To assess the relative contributions of these factors, we measured maximal inspiratory and expiratory pressures (PImax, PEmax, cmH₂O) at RV at TLC, respectively, in 32 patients with COPD. The TLC, RV, and diaphragm length index at RV (DLI, cm/cm height) were determined roentgenographically and compared with values from 22 normal subjects studied at comparable lung volume. Half the patients with COPD had normal and half had low values of PEmax, but both groups had similar values of TLC, RV, and DLI. In patients with COPD, PImax correlated (p <0.001) with PEmax (r = 0.73) and DLI (r = 0.64). The slope of the PImax-DLI relationship was essentially the same in both groups of patients with COPD as it was in the normal subjects. However, at any value of DLI, PImax was in the normal range in patients with normal PEmax, but significantly lower in patients with low PEmax (p <0.001). Expressing PImax as a combined function of PEmax and DLI yielded the highest correlation (r = 84, p <0.001), with PEmax explaining 46% and DLI explaining 35% of the variance in PImax not explained by the other variable alone (p <0.001). The Pa(CO₂) was elevated in 13 of 18 patients whose PImax was <55 cm H₂O, and inversely correlated with PImax (r = -0.66, p <0.005). We conclude that inspiratory muscle weakness in COPD results from mechanical disadvantage, is frequently aggravated by generalized muscle weakness, and contributes to hypercapnia.