Clinical implications of maximal respiratory pressure determinations for individuals with Duchenne Muscular dystrophy

Objective: To analyze the relationship between disease progression, pulmonary volumes, respiratory muscle strength (maximum inspiratory [MIP] and expiratory [MEP] pressures), and arterial blood gases for patients with Duchenne muscular dystrophy (DMD). Design: An inception cohort study of pulmonary volumes, MIPs, and MEPs, correlated with age and Paco(2) levels and with each other using linear and nonlinear regression analyses. Setting: Outpatient clinic. Patients: Fifty-two consecutive DMD patients who presented for regular evaluations at a regional DMD center. Results: Maximum expiratory pressures were 47.7% +/- 10.9% of normal in the 16 7- to 14-year-old patients and decreased linearly thereafter (MEP% = -2.7 x age +73.8; p < .001). Declines in MEP also correlated linearly with expiratory reserve volume (p < .001) and inversely with residual volume (p < .001). By contrast, MIP was 66.3% +/- 19.0% in the 35 7- to 14-year-old patients and then declined to 30.2% +/- 19.5% after age 14. No linear relationships were found with age but declines did correlate linearly with inspiratory reserve volume (p < .001) and total lung capacity (p < .001). Paco(2) elevations correlated best with decreases in MIP (p < .0001) and appeared when MTP was below 30cmH(2)O. Conclusions: Lung volume changes in DMD patients correlate with respiratory muscle weakness, and although inspiratory muscle dysfunction plays a key role in the development of chronic ventilatory insufficiency, reductions in expiratory muscle strength are the first signs of dysfunction and lead to the first episodes of respiratory failure. (C) 1997 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation