Correlation of Forced Expiratory Maneuver to Impulse Oscillometry During Methacholine Challenge Testing in Adults

Background: In adults, impulse oscillometry is more sensitive in detecting response to low levels and therapeutic levels of a short-acting bronchodilator than FEV1 in both asthmatic and COPD. There is limited data on the use of IOS during bronchoprovocation testing. The objective of this study is to correlate FEV1 and IOS values during methacholine challenge testing in patients with exertional dyspnea and normal resting spirometry. Methods: Increasing doses of Methacholine (MCT) were administered at the following concentrations: 0.0625 mg/mL, 0.25 mg/mL, 1.0 mg/mL, 4 mg/mL, 8 mg/mL, and 16 mg/mL via five breaths through a Salter model 0700 dosimeter. Both a forced expiratory maneuver with FEV1, FVC, and FEF25-75 were recorded in addition to IOS values (R5, R20, X5, AX) with each dose of MCT and post-bronchodilator. Correlation of spirometry and IOS values were made at baseline and maximum dose. Results: To date, 29 patients with exertional dyspnea and normal baseline spirometry without baseline obstruction have undergone Methacholine challenge testing. 22 patients had no reactivity at maximum dose of 16 mg/mL while 7 patients demonstrate mild to moderate bronchial hyperreactivity (BHR). Baseline values were not significantly different for FEV1, FVC, R5, R20 and X5 between the groups. At maximum dose of the BHR negative vs. BHR positive group, comparison of changes during IOS, X5 (-2.70 vs. -4. 24, P= 0.07), R5 (178% vs. 254%, P= 0.02), and R20 (136% vs. 148%, P= 0.40). These findings in the BHR positive group correlated to a -2.45 decrease in X5, 31% decrease in R5, 9% decrease in R20, and 62% increase in AX (all P values > 0.05). Discussion: This study demonstrated while X5 changed significantly during MCT, there was a more significant change using the R5 value which increased by 30% at PC20. All other IOS parameters changed during the tests but only changes in X5 were significantly different between BHR negative and BHR positive groups. The receiver operating characteristic (ROC) curve for changes in X5 (to predict a 20% decrease in FEV1) showed a best decision level for a 50% decrease in X5 with a sensitivity of 36% and a specificity of 85%. Conclusions: Impulse oscillometry may be an adjunct during methacholine challenge by correlating decrease in FEV1 to increases primarily in R5. Further study is required to determine if other IOS values may also demonstrate significant correlation with the PC20.