Identifying bronchoconstriction from the ratio of diaphragm EMG to tidal volume

Background: A fall of 20 % in forced expiratory volume in the first second (FEV1) with a cumulative dose of histamine <= 7.8 mu mol is considered to indicate bronchial hyperactivity, but no method exists for patients who cannot perform spirometry properly. Here we hypothesized that increases in respiratory central output measured by chest wall electromyography of the diaphragm (EMGdi-c) expressed as a function of tidal volume (EMGdi-c/ VT) would have discriminative power to detect a 'positive' challenge test. Methods: In a physiological study EMGdi was recorded from esophageal electrode (EMGdi-e) in 16 asthma patients and 16 healthy subjects during a histamine challenge test. In a second study, EMGdi from chest wall surface electrodes (EMGdi-c) was measured during a histamine challenge in 44 asthma patients and 51 healthy subjects. VT was recorded from a digital flowmeter during both studies. Results: With histamine challenge test the change in EMGdi-e/VT in patients with asthma was significantly higher than that in healthy subjects (104.2 % +/- 48.6 % vs 0.03 % +/- 17.1 %, p < 0.001). Similarly there was a significant difference in the change of EMGdi-c/VT between patients with asthma and healthy subjects (90.5 % +/- 75.5 % vs 2.4 % +/- 21.7 %, p < 0.001). At the optimal cut-off point (29 % increase in EMGdi-c/VT), the area under the ROC curve (AUC) for detection of a positive test was 0.91 (p < 0.001) with sensitivity 86 % and specificity 92 %. Conclusions: We conclude that EMGdi-c/VT may be used as an alternative for the assessment of bronchial hypersensitivity and airway reversibility to differentiate patients with asthma from healthy subjects.