How Respiratory Muscle Strength Correlates with Cough Capacity in Patients with Respiratory Muscle Weakness

PURPOSE The purpose of this study is to investigate how respiratory muscle strength correlates to cough capacity in patients with respiratory muscle weakness. MATERIALS AND METHODS Forty-five patients with amyotrophic lateral sclerosis (ALS), 43 with cervical spinal cord injury (SCI), and 42 with Duchenne muscular dystrophy (DMD) were recruited. Pulmonary function tests including forced vital capacity (FVC) and respiratory muscle strength (maximal expiratory pressure, MEP; maximal inspiratory pressure, MIP) were performed. The correlation between respiratory muscle strength and cough capacity was analyzed. RESULTS In the SCI group, FVC in a supine position (2,597 +/- 648 mL) was significantly higher than FVC in a sitting position (2,304 +/- 564 mL, p < 0.01). Conversely, in the ALS group, FVC sitting (1,370 +/- 604 mL) was significantly higher than in supine (1,168 +/- 599 mL, p < 0.01). In the DMD group, there was no statistically significant difference between FVC while sitting (1,342 +/- 506 mL) and FVC while supine (1,304 +/- 500 mL). In addition, the MEP and MIP of all three groups showed a significant correlation with peak cough flow (PCF) (p < 0.01, Pearson's correlation analysis). In the SCI group, MIP was more closely correlated with PCF, while in the ALS and DMD groups, MEP was more closely correlated with PCF (p < 0.01, multiple regression analysis). CONCLUSION To generate cough flow, inspiratory muscle strength is significantly more important for SCI patients, while expiratory muscle function is significantly more important for ALS and DMD patients.