Non-invasive tension-time-index and ventilator use in Duchenne muscular dystrophy

Non-invasive determination of the tension-time-index (TTMUS) can identify predisposition to respiratory muscle fatigue in neuromuscular disease. We correlated TTMUS with age and extent of need of ventilator use for patients with Duchenne muscular dystrophy (DMD). Maximal inspiratory pressure (MIP), inspiratory pressure 0.1 second after onset of inspiration (P0.1), and the breathing pattern during spontaneous breathing were measured in 46 subjects with DMD and in 46 healthy males of same age. TTMUS (TTMUS=TI/TTOT x PI/MIP) was determined by calculating PI from P0.1 (PI=5 x P0.1 x TI). The data were compared to normal values, and related to age and ventilator use. TTMUS was significantly higher in the entire DMD-group than in controls (0.21±0.11 vs. 0.06±0.02, p<0.001) and increased with age in the patients (p<0.001). TTMUS was significantly higher in subjects ventilated 8–20 hrs per day than in nocturnal-only users, and in patients ventilated >20 hrs per day than in those ventilated 8–20 hrs per day (p<0.001). TTMUS surpassed 0.23 in 95% of subjects ventilated 8–20 hrs. TTMUS exceeded 0.37 in 95% of individuals ventilated greater than 20 hrs per day, whereas it was below this value in 95% of patients ventilated 8–20 hrs per day. TTMUS increases significantly with disease progression and is a sensitive indicator of risk of inspiratory muscle fatigue and ventilator use. Longitudinal determination of TTMUS in patients with DMD may help to justify extent of need for mechanical ventilation.