RESPIRATORY EFFORT PERCEPTION AT REST AND DURING CARBON-DIOXIDE REBREATHING IN PATIENTS WITH DYSTROPHIA-MYOTONICA

Background - Breathlessness appears to be closely related to the perce ption of the outgoing motor command to breathe and should be increased in the presence of muscle weakness. However, breathlessness is not a common symptom in patients with chronic muscle disease who have weak r espiratory muscles. The factors that determine the perception of respi ratory effort in such patients have not been examined. Methods - The i nspiratory effort sensation during resting breathing and progressive h ypercapnia was investigated in 12 patients with dystrophia myotonica w ith weak respiratory muscles (nine men and three women of mean (SD) ag e 41.1 (10.5) years; maximum inspiratory pressure 43.1 (17.2) cm H2O) and an age and sex matched control group of normal subjects of mean ag e 39.6 (10.6) years and a maximum inspiratory pressure of 123 (15.2) c m H2O. Results - During resting breathing with a mouthpiece no differe nces were seen in inspiratory effort sensation, mouth occlusion pressu re, or tidal volume, but inspiratory time and cycle duration were sign ificantly shorter in the patients with dystrophia. Minute ventilation (VE) was significantly higher in the patients (15.8 (4.0) 1/min v 12.5 (2.6) 1/min), while resting breathing was no more variable in the pat ients than in controls. The ventilatory response to carbon dioxide (VE /PCO2) was not significantly lower in the patients (14.9 (6.9) 1/min/k Pa) than in the controls (17.4 (4.3) 1/min/kPa). Effort sensation resp onses to carbon dioxide driven breathing were similar in the control s ubjects and the patients. With regression analysis of pooled data neit her maximum inspiratory pressure nor disease state contributed to perc eived inspiratory effort during hypercapnia. Conclusions - Moderately severe global respiratory muscle weakness does not appear to influence the ventilatory response to rising carbon dioxide tension or the perc eption of inspiratory effort in patients with dystrophia myotonica.