THE EFFECTS OF 3 MODELS OF AIRWAY DISEASE ON TIDAL BREATHING FLOW-VOLUME LOOPS OF THOROUGHBRED HORSES

The effects of histamine and methacholine aerosols and of a fixed insp iratory resistance on tidal breathing flow-volume loops (TBFVL) were i nvestigated using 18 unsedated, standing, healthy thoroughbred horses. Tile data were first analysed using traditional flow-volume loop indi ces and then reduced using standardized factor scoring coefficients ob tained in a previous study in this laboratory using similar experiment al techniques. On the basis of resting TBFVL analysis, the degree of p ulmonary dysfunction caused by inhalation of histamine and methacholin e aerosols with concentrations of 10 and 2 mg/ml, respectively, was si milar. The fixed resistance also caused significant changes in the res ting spirogram and TBFVL indices, suggesting that this model may prove valuable for further studies involving upper respiratory tract (URT) conditions. Administration of histamine and methacholine aerosols resu lted in significant changes in all factor scores, although most of the observed changes were due to the effects of these aerosols on the res piratory rate. These findings re-emphasize the importance of the effec ts of respiratory rate on pulmonary mechanics. Application of the resi stance resulted in significant changes in factor score 3, the 'inspira tory' factor, which lends support to the validity of this model for UR T conditions. The close agreement between the factor scores obtained u nder controlled conditions in this study and in a previous study in th is laboratory confirms that the factor analysis used for both of these studies provides an adequate means of reducing TBFVL data obtained fr om thoroughbred horses. The large intra- and inter-individual variatio n observed both with the indices of TBFVL and with the factor scores l imits the potential of these variables for detecting individual animal s with obstructive airway disease. Re-evaluation of these indices unde r the stress of exercise may reduce the variability observed in these data and may increase the magnitude of differences between different a nimals, providing a means of detecting individual animals with subclin ical obstructive airway conditions.