Interrupter technique versus plethysmography for measurement of respiratory resistance in children with asthma or cystic fibrosis

The purpose of the present study was to compare measurements of respiratory system resistance by the interrupter method (Rrsint) with those of airway resistance by plethysmography (Raw) in nonobstructed children with asthma o r cystic fibrosis (ratio of forced expiratory volume in 1 sec to vital capa city, FEV1/VC greater than or equal to 80% with a forced expiratory flow ra te between 25-75% of forced vital capacity, FEF25-75 greater than or equal to 75% of normal values) and in obstructed children with the same diseases (FEV1/VC <80% and/or FEF25-75 <75% of normal values). Eighty-one children ( 47 asthmatics and 34 suffering from cystic fibrosis) aged 5-18 years (mean 11.2 +/- SD 3.4 years) were included in the study. For the overall group, we observed generally lower values for Raw (4.7 +/- 2.8 cmH(2)O.L-1.s) than for Rrsint20 (extrapolation of the mouth pressure d uring occlusion to 40 ms after interruption) (5.6 +/- 1.7 cmH(2)O.L-1.s) (P < 0.02), or for Rrsint40 (extrapolation of the mouth pressure during occlu sion to 60 ms after interruption) (6.6 +/- 2.2 cmH(2)O.L-1.s) (P < 0.001), but there was no difference between Rrsint20 and Raw in the obstructed subg roup. Moreover, we observed a correlation between the difference (Rrsint20 - Raw) expressed in percentage of predicted values and the degree of obstru ction estimated by FEV1/VC (r = 0.56, P < 0.001). The differences between t he specific resistances (sRrsint20 - sRaw, sRrsint40 - sRaw) were also corr elated with the severity of the obstruction (r = 0.65, P < 0.001 and r = 0. 57, P < 0.001, respectively). We observed also that the tendency to underes timate resistance by Rrsint in obstructed children was not the same in chil dren with asthma and cystic fibrosis. We conclude that the tendency of Rrsint, as measured with our method, to un derestimate airway obstruction appears to increase in proportion to the sev erity of the airway obstruction. Pediatr Pulmonol. 2000; 29:213-220. (C) 20 00 Wiley-Liss, Inc.