Differences in pulmonary function measured by oscillometry between horses with mild–moderate equine asthma and healthy controls

Background: The diagnosis of mild-moderate equine asthma (MEA) can be confirmed by airway endoscopy, bronchoalveolar lavage fluid (BALf) cytology, and lung function evaluation by indirect pleural pressure measurement. Oscillometry is a promising pulmonary function test method, but its ability to detect subclinical airway obstruction has been questioned. Objectives: To evaluate the differences in lung function measured by oscillometry between healthy and MEA-affected horses. Study design: Prospective case-control clinical study. Methods: Thirty-seven horses were divided into healthy and MEA groups, based on history and clinical score; the diagnosis of MEA was confirmed by airway endoscopy and BALf cytology. Horses underwent oscillometry at frequencies ranging from 2 to 6 Hz. Obtained parameters included whole-breath, inspiratory, expiratory, and the difference between inspiratory and expiratory resistance (Rrs) and reactance (Xrs). Differences between oscillometry parameters at different frequencies were evaluated within and between groups by repeated-measures two-way ANOVA and post hoc tests with Bonferroni correction. Frequency dependence was compared between groups by t test. For significant parameters, a receiver operating characteristics curve was designed, cut-off values were identified and their sensitivity and specificity were calculated. Statistical significance was set at p < 0.05. Results: No significant differences in Xrs and Rrs were observed between groups. The frequency dependence of whole-breath and inspiratory Xrs significantly differed between healthy (respectively, -0.03 +/- 0.02 and -0.05 +/- 0.02 cmH(2)O/L/s) and MEA (-0.1 +/- 0.03 and -0.2 +/- 0.02 cmH(2)O/L/s) groups (p < 0.05 and p < 0.01). For inspiratory Xrs frequency dependence, a cut-off value of -0.06 cmH(2)O/L/s was identified, with 86.4% (95% CI: 66.7%-95.3%) sensitivity and 66.7% (95% CI: 41.7%-84.8%) specificity. Main limitations: Sample size, no BALf cytology in some healthy horses. Conclusions: Oscillometry can represent a useful non-invasive tool for the diagnosis of MEA. Specifically, the evaluation of the frequency dependence of Xrs may be of special interest.