Determinants of cough efficiency in Duchenne muscular dystrophy.

OBJECTIVE: With the progression of Duchenne muscular dystrophy (DMD) cough becomes inefficient leading to recurrent chest infections. Several factors determine the effectiveness of cough in DMD patients. The aim of this study was to investigate how weakened inspiratory muscles alter operating lung and thoraco-abdominal volumes and whether they contribute to cough efficiency. PATIENTS AND METHODS: Pulmonary function, respiratory muscle strength, and peak cough flow (PCF) were assessed in 36 DMD patients (age 17.0 ± 5.0). Total and compartmental chest wall volumes were measured by Opto-Electronic Plethysmography in the DMD patients and 15 age-matched controls during quiet breathing and maximal voluntary cough maneuvers. The DMD population was divided into three groups: PCF < 160 L/min (inefficient cough), PCF > 270 L/min (efficient cough), and 160 < PCF < 270 L/min (intermediate cough efficiency).
RESULTS: During the inspiration preceding cough, patients with efficient cough presented normal volume variations whereas patients with intermediate cough efficiency showed low abdominal volume variation (P < 0.01). Patients with inefficient cough were characterized by reduced total (P < 0.05) and compartmental (ribcage: P < 0.01; abdomen: P < 0.001) chest wall volumes during the inspiration preceding cough and reduced abdominal contribution to tidal volume during quiet breathing (ΔVAB (%VT), P < 0.001). ROC analysis revealed that among all spirometric, respiratory muscle strength and chest wall parameters ΔVAB (%VT ) was the best discriminator between inefficient and efficient cough.
CONCLUSIONS: Inefficient cough in DMD is associated to reduced operating lung and chest wall volume secondary to weakened inspiratory muscles. Abdominal contribution to tidal volume during spontaneous breathing represents a non-volitional and noninvasive index able to discriminate efficient and inefficient cough.