Within-breath input impedance by impulse oscillometry in severely asthmatic horses

Impulse oscillometry system (IOS) detects pulmonary obstruction in horses measuring average total respiratory system impedance (Zrs). In humans, analysis of within-breath changes of Zrs allows identification of the presence of expiratory flow limitation. Aims of this study were to describe the inspiratory and expiratory Zrs parameters measured by IOS in asthmatic horses and to determine whether they allow the detection of residual airway obstruction in asthmatic horses in disease remission. Seven severely asthmatic horses in disease exacerbation (HE), seven asthmatic horses in clinical remission (HR) and seven control horses (HC) from a cohort of experimental age-matched animals underwent IOS testing. Only data at 3, 5 and 7 Hz with coherence >0.85 at 3 Hz and >0.9 at 5 and 7 Hz were considered in this study. Mean, inspiratory and expiratory resistance (R) and reactance (X) and the difference between inspiratory and expiratory X (ΔX) were calculated at each frequency. The three groups were compared using Kruskal-Wallis and Dunn’s multiple comparison tests. HE differed from HC for all R parameters at 3 Hz and all X parameters at all frequencies (P<0.01-0.001). HE differed from HR (P<0.05) for R3, R3e, X3e, X5e. HR differed from HC for X7i (P<0.05). ΔX was higher (P<0.05) in HE than in HC (5 Hz) or HR (all frequencies, P<0.01-0.001). Results indicate that as reported in humans during tidal expiratory flow limitation, Xrs during the expiratory phase is more negative than during inspiration in HE. Difference in X7i appears to be promising to discriminate between HC and HR.