Noninvasive Determination of the Tension-Time Index in Duchenne Muscular Dystrophy

Objective: Noninvasive determination of the tension-time index of the inspiratory muscles (TT MUS ) can identify predisposition to respiratory muscle fatigue in neuromuscular disease. We correlated Π MUS 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. Π MUS (Π MUS = T I /T TOT × 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: Π MUS was significantly higher in the entire Duchenne muscular dystrophy group than in controls (0.21 ± 0.1 1 vs. 0.06 ± 0.02, P 20 hrs per day than in those ventilated 8-20 hrs per day (P 20 hrs per day, whereas it was less than this value in 95% of patients ventilated 8-20 hrs. Conclusions: Π MUS increases significantly with disease progression and is a sensitive indicator of risk of inspiratory muscle fatigue and ventilator use. Longitudinal determination of Π MUS in patients with Duchenne muscular dystrophy may help to justify the extent of need for mechanical ventilation.