Methacholine challenge in pre-school children--which outcome measure?

The aim of our study was to evaluate the utility of interrupter resistance (R(int)), transcutaneous oximetry and auscultation as outcome measures for a recently suggested tripling-dose methacholine (Mch) challenge in pre-school children. We studied 57 children aged 3-6 years. R(int) was measured at baseline and after each Mch dose. Oxygen saturation (SaO(2)) and transcutaneous oxygen pressure (tcpO(2)) were monitored during the challenge. Mch concentrations of 0.22, 0.66, 2.0, 6.0 and 18.0 mg/ml were nebulised during tidal breathing. The challenge was terminated if there was wheeze, SaO(2) below 91% or persistent cough; this final Mch dose was considered as PCW. Nine healthy children, 17 with cough and 25 with wheeze performed the study up to the point of PCW or all five Mch inhalations. If a change of 20% of predicted R(int) or termination by wheeze, desaturation or cough is taken as a completed test, then 39 out of 51 children (78%) had adequate R(int) measurements on each occasions from start to completion. The success rate for tcpO(2) measurements was similar: 38 out of 51 (76%) had complete tcpO(2) data until a 15% fall of tcpO(2) or clinical endpoint was reached. Using the above-mentioned cut-off levels significant change in R(int) or tcpO(2) preceded PCW in most of the cases. Both R(int) and tcpO(2) measurements may allow detection of bronchial hyper-responsiveness at lower Mch doses and also provide a less subjective measure, but will not be feasible in all children.