Introduction: Assessing diaphragmatic function is essential for the investigation of dyspnoea and for the diagnosis and follow-up of neuromuscular diseases. Magnetic phrenic nerve stimulation remains the gold standard but requires sophisticated equipment, considerable expertise and invasive measurements. 

Aims: Determine whether a simplified phrenic nerve stimulation protocol for measuring mouth twitch pressure (P_mo,tw) can reliably discriminate normal from clinically significant diaphragmatic dysfunction.

Methods: 10 participants [age 59.4(± 8.4) years] with known diaphragmatic dysfunction (DD) and 17 age-, sex- and BMI-matched healthy volunteers (HV) underwent cervical magnetic stimulations. P_mo,tw was measured simultaneously with a standardised method using a calibrated pressure transducer and a simplified protocol using a portable pressure meter. 

Results: Mean(±SD) standardised P_mo,tw was 6.24(±2.68) cmH2O in the DD group and 12.57(±3.1) cmH₂O in the HV group. Mean simplified P_mo,tw was 4.98 (±2.5) and 11.97(±4.3) cmH₂O respectively. The mean difference between the standardised and simplified P_mo,tw was 0.85 cmH₂0, 95%CI [-2.2, 3.9]. Both methods identified individuals with diaphragmatic dysfunction reliably (AUC of 0.97 and 0.95). Using the simplified method, a P_mo,tw lower than 9 cmH₂O predicted diaphragmatic dysfunction with a sensitivity of 90%, specificity of 82%, and an AUC of 0.86.

Conclusion: This simplified protocol for P_mo,tw measurement can be used to reliably predict clinically significant diaphragmatic dysfunction, compared to the standardised technique. Further studies are required to establish age and gender normative data for this new method.