Validation of the clinical utility of sGaw as a response variable in methacholine challenge testing

Abstract

Background and Objective Airway hyperresponsiveness (AHR) is commonly assessed by a methacholine challenge test (MCT), during which a provocative concentration causing a 20% reduction in forced expiratory volume in 1 second (FEV1) (PC20) < 8 mg/ml is considered a positive response. However, a fall in specific airway conductance (sGaw) may also have clinical significance. The purpose of this study was to assess whether AHR determined by a provocative concentration causing a 40% reduction in sGaw (PC40) < 8 mg/ml corresponds to a clinical diagnosis of asthma. Methods We analysed the changes in spirometry, lung volumes and sGaw during MCT in 211 randomly selected patients being evaluated for AHR to support a clinical diagnosis of asthma. Results The mean (SD) age of the group was 53 (15) years, with 141 women (67%). Overall lung function was normal, with FEV1 = 92 (15) % predicted, total lung capacity = 97 (13) % predicted and sGaw = 0.19 (0.15–0.23) L/s/cm H2O/L, (median, 25–75 IQR). There were many more patients who responded by PC40 only (n = 120) than who responded by PC20 (n = 52). There was no significant difference in asthma diagnosis between the PC20 (98%) and PC40 (93%) groups, and we estimate 34% of patients with a diagnosis of asthma would have been classified as having no AHR if only the FEV1 criterion was used. Conclusion Changes in sGaw during MCT indicate clinically significant AHR in support of a clinical diagnosis of asthma among patients being evaluated for asthma.

Airway Hyperresponsiveness

“…Существует прямая связь между гиперреактивностью дыхательных путей, функцией легких и антропометрическими параметрами человека, а следовательно, размерами дыхательных путей. Так, на метахолин чаще реагируют женщины вследствие более высокой, по сравнению с мужчинами, восприимчивости к различным раздражающим агентам и меньшего размера дыхательных путей [4]. В то же время гиперреактивность дыхательных путей более распространена у мальчиков до начала полового созревания [5].…”

Bronchial provocation testing: Should we look beyond FEV1?

“…What should be regarded as a positive change has been subject to debate, 7 and due to its inherent variability, a larger percent change in sGaw than FEV 1 is required for a positive test. In this Respirology article, 2 the authors found a PC40 to be highly sensitive for a subsequent asthma diagnosis (93%); however, physicians were not blinded to the outcome of the challenge test and so the high sensitivity of the PC40 should be interpreted in this context. It is possible (and relevant) that this high specificity is an indication that the PC40 was incorporated into the physician's diagnosis.…”

“…Finally, like oscillometry, sGaw should not, and indeed cannot, be used as a surrogate for spirometry as the association between FEV 1 and sGaw is weak at best (fig. 8 of Reference 2). This discrepancy likely reflects that FEV 1 and sGaw describe related but distinct physiological changes in the airways, and the article by Parker et al provides evidence that both outcomes (FEV 1 and sGaw) are of probable clinical relevance.…”

“…In a recent publication in Respirology, Parker et al 2 explored the clinical utility of using specific airway conductance measurements (sGaw) as an outcome measure for methacholine challenge testing (MCT), and whether a fall in sGaw of 40% (PC40) provided clinically relevant information to assist with the diagnosis of asthma. Performed in a body plethysmograph during tidal breathing, the technique for sGaw is relatively easy for patients to perform and minimizes changes to airway calibre that can result from the deep inhalation during spirometry.…”

Recommendations from The Medical Education Editor

“…In the May issue of Respirology, Parker et al reported the findings of their study of the utility of specific airway conductance (sGaw) as a response variable in direct bronchoprovocation tests using methacholine. 6 They reviewed the results of 211 challenge tests from a random selection of 300, with 89 excluded due to missing data. Their cohort had a mean age of 53 years, 67% were female, the median BMI was 29 kg/m 2 , and 38% were current or former smokers.…”