Respiratory muscle and pulmonary function in polymyositis and other proximal myopathies

We studied 53patients withproximal myopathy todetermine atwhatlevel ofmuscle weakness hypercapnic respiratory failure islikely, andwhichtestsofpulmonary function or respiratory muscle strength wouldbest suggest this development. Respiratory muscle strength was determined frommaximal static efforts andinhalf thepatients, bothinspiratory andexpiratory muscle strengths wereless than50%ofnormal. Inthe37patients without lungdisease respiratory muscle weakness was accompanied bysignificant decreases invital capacity, total lungcapacity, andmaximumvoluntary ventilation; bysignificant increases inresidual volume andarterial carbon dioxide tension (Paco2); andgreater likelihood ofdependence on ventilators, atelectasis, and pneumonia. Hypercapnia was particularly likely whenrespiratory muscle strength was less than 30%ofnormal inuncomplicated myopathy, andwhenvital capacity was less than55%ofthe predicted value inanypatient. Myopathies which affect proximal limbmuscles may alsoaffect respiratory muscles;'-' buttherelation- ships between theextent towhichrespiratory muscles areaffected, abnormality ofpulmonary function, andtheonset ofrespiratory failure havenot beendefined. We addressed these questions ina groupof 53 patients withvarious proximal