Evaluation of a new interrupter device for measuring bronchial responsiveness and the response to bronchodilator in 3 year old children.

The interrupter technique for measuring airway resistance is non-invasive and convenient, and therefore ideally suited for the assessment of induced changes in airway calibre in preschool children. The aim of this study was to evaluate a commercially available interrupter device (based on Microlab 4000), which calculates the interrupter resistance (Rint) from pressure and flow following a brief interruption of expiration during quiet breathing. The repeatability of Rint was assessed, and its response to methacholine challenge and the bronchodilator salbutamol were compared with an indirect technique, the fall in transcutaneous oxygen tension (Ptc,O2), using the sensitivity index (SI, i.e. the change after challenge expressed in multiples of the baseline standard deviation) in 12 wheezy children (aged 3 yrs +/- 2 months). The mean (SD) baseline value of Rint was 0.91 (0.20) kPa.L-1.s. Short-term repeatability and baseline variability were satisfactory for Rint (intraclass correlation coefficient = 0.6; mean intrasubject coefficient of variation = 13%). Although 10 of the 12 subjects obtained a significant response using Rint at maximal bronchoconstriction (i.e. SI > 2), overall, Rint was five times less sensitive than Ptc,O2 (geometric mean SI: Rint 3 vs Ptc,O2 16; p < 0.0001). Reversal of obstruction with administration of a bronchodilator was clearly demonstrated in almost all subjects: Rint after challenge (mean +/- SD) 1.25 (0.22) kPa.L-1.s; after salbutamol 0.78 (0.19) kPa.L-1.s; p < 0.001. In conclusion, the convenient interrupter resistance method appears more promising for detecting bronchodilator responses than induced bronchoconstriction in wheezy preschool children; however, measurement of transcutaneous oxygen tension provides a reliable indirect means of detecting induced airway obstruction in this age-group.