Background Unilateral diaphragm dysfunction (UDD) is an underdiagnosed cause of dyspnoea. Inspiratory muscle training (IMT) is the only conservative treatment for UDD, but the mechanisms of improvement are unknown. We characterised the effects of IMT on dyspnoea, exercise tolerance and respiratory muscle function in people with UDD.

Methods 15 people with UDD (73% male, 61±8 years) were randomised to 6 months of IMT (50% maximal inspiratory mouth pressure (P_I,max), n=10) or sham training (10% P_I,max, n=5) (30 breaths twice per day). UDD was confirmed by phrenic nerve stimulation and persisted throughout the training period. Symptoms were assessed by the transitional dyspnoea index (TDI) and exercise tolerance by constant-load cycle tests performed pre- and post-training. Oesophageal (P_es) and gastric (P_ga) pressures were measured with a dual-balloon catheter. Electromyography (EMG) and oxygenation (near-infrared spectroscopy) of respiratory muscles were assessed continuously during exercise.

Results The IMT group (from 45±6 to 62±23% P_I,max) and sham group (no progression) completed 92 and 86% of prescribed sessions, respectively. P_I,max, TDI scores and cycle endurance time improved significantly more after IMT versus sham (mean between-group differences: 28 (95% CI 13–28) cmH₂O, 3.0 (95% CI 0.9–5.1) points and 6.0 (95% CI 0.4–11.5) min, respectively). During exercise at iso-time, P_es, P_ga and EMG of the scalene muscles were reduced and the oxygen saturation indices of the scalene and abdominal muscles were higher post- versus pre-training only in the IMT group (all p<0.05).

Conclusion The effects of IMT on dyspnoea and exercise tolerance in UDD were not mediated by an improvement in isolated diaphragm function, but may reflect improvements in strength, coordination and/or oxygenation of the extra-diaphragmatic respiratory muscles.