INSPIRATORY MUSCLE STRENGTH AND RESPIRATORY DRIVE IN PATIENTS WITH RHEUMATOID-ARTHRITIS

In 15 patients with rheumatoid arthritis (RA) and in 12 age- and sex-matched normal subjects, we evaluated inspiratory muscle strength and respiratory control system. Inspiratory muscle strength was assessed by measuring maximal inspiratory pressure (MIP). Respiratory drive was assessed by evaluating surface electromyographic activity of the diaphragm (EMGd) during both room-air breathing and hypercapnic rebreathing. Compared to the predicted value (mean ± 1.65 SD), MIP was significantly reduced in nine patients (60%). All told, we noticed a significant inverse relationship in the patients between MIP and duration of steroid therapy (p < 0.01). During room-air breathing, both EMGd and mouth occlusion pressure (P0.1), expressed both in actual values and as percentage of MIP, were significantly greater in patients than in the control group (p < 0.001 for both). Both EMGd and P0.1 (%MIP) response slopes to CO2 were significantly greater in patients than in the normal control group (p < 0.01 and p < 0.001, respectively) and significantly related to the functional stage of disease. During quiet breathing and for a PET(CO2) of 60 mm Hg, both EMGd (p < 0.01 and p < 0.05, respectively) and P0.1 (%MIP) (p < 0.01 and p = 0.001, respectively) were inversely related to MIP. These results indicate that RA patients and rheumatoid myositis could explain the reduction in MIP, whereas neural afferents arising from respiratory muscle, lung, or joint receptors could be involved in the observed increase in neural drive.