Inspiratory flow reserve in boys with Duchenne muscular dystrophy

Patients with advanced muscular dystrophy frequently develop ventilatory fa ilure. Currently respiratory impairment usually is assessed by measuring vi tal capacity and the mouth pressure generated during a maximal inspiratory maneuver (P-1,max), neither of which directly measures ventilatory capacity . We assessed inspiratory flow reserve in 26 boys [mean (SD) age 12.8 (3.8) years] with Duchenne muscular dystrophy (DMD) without ventilatory failure and in 28 normal boys [mean (SD) age 12.6 (1.9) years] by analyzing the rat io between the largest inspiratory flow during tidal breathing (V ' (1),max (t)) and during a forced Vital capacity maneuver (V ' (1),max((FVC))), (V ' (1),max(t)/V ' (1),max((FVC))). We have compared this ratio with the force d vital capacity (Nc) and P-1,max measured at functional residual capacity. Mean P-1,max was - 90(30)cmH(2)O, average 112% (range 57-179%) of predicted values in control boys and - 31(11)cmH(2)O, average 40% predicted values i n DMD boys (control vs DMD, P < 0.001). FVC was reduced in DMD boys [59(20) % predicted values vs 86(10)% predicted values in controls, P < 0.01]. Abso lute V ' (1),max((FVC)) was strongly related to FVC in both control and DMD boys; V ' (1),max((FVC)) (expressed as FVC(.)s(-1)) was not related to P-1 ,max in either group. The mean V-1,max(t)/V ' (1),max((FVC)) ratio was high er in DMD 0.22 (0.08) than in controls 0.12 (0.03) (P < 0.001) indicating a reduction in inspiratory flow reserve in DMD. Inspiratory flow reserve was within the normal range in 8 of 19 DMD patients with P-1,max less than 50% of predicted values. We conclude that measurement of inspiratory flow reserve (V ' (1),max(t)/V ' (1),max((FVC)) ratio) provides a simple and direct assessment of dynamic inspiratory muscle function which is not replicated by static measurement o f P-1,max or vital capacity and might be useful in assessment of respirator y impairment in boys with Duchenne muscular dystrophy. Follow-up studies ar e required to establish whether measures of inspiratory flow reserve are of clinical value in predicting subsequent ventilatory failure. (C) 2001 Wile y-Liss, Inc.