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TRANSLATIONAL PHYSIOLOGY

Noninvasive measurement of the tension-time index in children with neuromuscular disease

Division of Pulmonary Medicine, Children's Hospital of Philadelphia, and University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104

Submitted 27 November 2002 ; accepted in final form 2 May 2003

Respiratory muscle weakness is common in children with neuromuscular disease (NMD). We hypothesized that weakness puts them at risk for respiratory muscle fatigue, a harbinger of chronic respiratory failure. We therefore measured a noninvasive index of respiratory muscle fatigue, the tension-time index of the respiratory muscles (TT_mus), in 11 children with NMD and 13 control subjects. Spirometric flow rates and maximal inspiratory pressure were significantly lower in the NMD group than in controls (43 ± 23 vs. 99 ± 21 cmH₂O, P < 0.001). The mean TT_mus was significantly higher in the NMD group than in controls (0.205 ± 0.117 vs. 0.054 ± 0.021, P < 0.001). The increase in TT_mus was primarily due to an increase in the ratio of average mean inspiratory pressure to maximal inspiratory pressure, indicating decreased respiratory muscle strength reserve. We found a significant correlation between TT_mus and the residual volume-to-total lung capacity ratio (r = 0.504, P = 0.03) and a negative correlation between TT_mus and forced expiratory volume in 1 s (r = -0.704, P < 0.001). In conclusion, children with NMD are prone to respiratory muscle fatigue. TT_mus may be useful in assessing tolerance during weaning from mechanical ventilation, identifying impending respiratory failure, and aiding in the decision to institute therapies.

respiratory muscles; muscular dystrophy

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