The forced oscillation technique (FOT) is ideal for assessing airway hyperresponsiveness, as it does not require maximal breathing manoeuvres that may affect airway tone. Previous studies have examined FOT responses to methacholine, however new commercial FOT devices mean new cut-points are needed.
Compare FEV1 vs FOT responses to a methacholine challenge using a commercial FOT device, and propose potential cut-points that define a positive response.
We recruited patients referred for a methacholine challenge to the Respiratory Laboratory, Concord Hospital. At each dose, FOT measurements (tremoFlo C-100, Thorasys Medical Systems, Canada) were made prior to spirometry. The challenge was terminated when FEV1 fell by 20% of baseline (positive response) or a cumulative dose of 8 µmoles was reached (negative response). We calculated the dose response slope (DRS) for FEV1 and for resistance (Rrs) and reactance (Xrs) at 5 Hz. Relationships between logDRS FEV1 vs logDRS Rrs and Xrs were examined using Spearman correlations. Accuracy in detecting a positive response was assessed using receiver operating characteristics (ROC) curves.
Out of 29 patients, 9 were positive responders. DRS FEV1 correlated with DRS Rrs (r=0.68, p<0.001) and DRS Xrs (r=0.78, p<0.001). The area under the ROC curve was 0.850 for Rrs and 0.911 for Xrs. A cut-off of 67% increase in Rrs and 2.80 cmH2O s L-1 decrease in Xrs yielded optimal sensitivity and specificity for a positive response.
Changes in FOT correlated well with spirometry during methacholine challenge. FOT has potential as a substitute for spirometry for assessing AHR. Cut-points will need to be validated in a larger study.