Objective: Noninvasive determination of the tension-time index of the inspiratory muscles (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.

Design: Maximal inspiratory pressure, inspiratory pressure 0.1 sec after the onset of inspiration (P-0.1), and the breathing pattern during spontaneous breathing were measured in 46 subjects with Duchenne muscular dystrophy and in 46 healthy males of the same age. TTMUS (TTMUS = T-I/T-TOT x P-I/MIP) was determined by calculating P-I from P-0.1 (P-I = 5 x P-0.1 x T-I). The data were compared with normal values and related to age and ventilator use.

Results: TTMUS was significantly higher in the entire Duchenne muscular dystrophy 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 >20 hrs per day, whereas it was less than this value in 95% of patients ventilated 8-20 hrs.

Conclusions: 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 Duchenne muscular dystrophy may help to justify the extent of need for mechanical ventilation.