Temporal Agreement of Histamine Bronchoprovocation Testing in Horses Using Open Plethysmography
Open plethysmography with histamine challenge is a simple, non-invasive field test for measuring airway reactivity in horses with suspected Inflammatory Airway Disease (IAD). Horses (n=9, adult mares) were tested with a commercial system (Open PlethTM, Ambulatory Monitoring Inc.), and underwent bronchoprovocation via nebulization with exponentially increasing concentrations of histamine from 2-32mg/ml. Baseline Δflow (a measure of gas trapping due to airway obstruction), and the concentration of histamine needed to obtain a 35% increase in Δflow (PC35), were measured. A PC35 of <6mg/ml histamine is associated with airway hyper-reactivity. Measurements were obtained at days 1 and 3, and again after 90 days with agreement being reflected in the association of repeated measures of PC35, determined by Fischer's Exact Test and concordance analysis (rho 'ρ'). Animals were categorized into 1 of 5 groups of responsiveness at each time point based on their individual test results. Each animal maintained the same degree of airway reactivity, excepting 2 with increased reactivity at 90 days. Short-term measurements (days 1 and 3) had strong association (P=0.002) and concordance (ρ=0.97). Long-term association (90 day interval) was also significant, (P=0.038), while concordance was low (ρ=0.63). By observation, differences in ambient temperature may have impacted longer time frame results, perhaps due to cold air-provoked bronchospasm in some animals. Airway reactivity measured by histamine bronchoprovocation and open plethysmography is highly reproducible over a short time period, but may be unacceptable over a period of 90 days with large ambient temperature variation.