Unilateral diaphragm dysfunction (UDD) is an important underdiagnosed cause of dyspnea. Inspiratory muscle training (IMT) is the only conservative treatment for UDD, but the clinical utility and mechanisms of improvement are unknown. We characterized the effects of IMT on respiratory muscle function, activity-related dyspnea, and exercise tolerance in people with UDD.

15 people (73% male, 61±8yrs) were randomized to a 6-month IMT (45±6% of maximal inspiratory mouth pressure (P_I,max), n=10) or sham program (6±2% P_I,max, n=5) consisting of 30 breaths against an external load twice daily. UDD was confirmed by magnetic phrenic nerve stimulation and persisted throughout the intervention. P_I,max was measured using a mouth pressure device and maximal transdiaphragmatic pressure during a sniff manuever (P_di,sniff) with a dual-balloon esophageal catheter. Change in dyspnea was captured by the Transitional Dyspnea Index (TDI) and exercise tolerance by constant load cycle tests performed pre- and post-training.

The IMT group completed an average of 330 sessions (92% prescribed) and progressed to 62±23% P_I,max. The sham group completed an average of 310 sessions (86%) without progression. P_I,max and P_di,sniff improved more with IMT vs. sham (p=0.003 and p=0.046, respectively). Improvements in TDI score and cycle endurance time were also greater with IMT vs. sham (p=0.008 and p=0.04, respectively).

IMT yielded meaningful improvements in respiratory muscle function, dyspnea, and exercise tolerance in people with UDD. The benefits were not related to improvement in isolated diaphragm function, but may reflect increased strength and/or better coorindation of the extra-diaphragmatic inspiratory muscles.