AIR-FLOW MECHANICS IN MODELS OF EQUINE OBSTRUCTIVE AIRWAY DISEASE UNDER CONDITIONS SIMULATING EXERCISE

Effects of respiratory tract obstructions on ventilatory mechanics in horses exercising at high speeds were tested with a fibreglass replica of the airways (nares to mainstem bronchi) of an adult horse. Segment al pressures were recorded at six sites along the model at four differ ent unidirectional flows (1300 - 4100 litre min(-1)), and the respecti ve resistances (R) to airflow were calculated. The external nares and the larynx made the greatest contributions to the total resistance (R- TOT) when no obstruction was present. Modifying the model to simulate severe pharyngeal lymphoid hyperplasia (PLH) had no effect on R at the larynx or at any point in the trachea under these flow conditions. Tw o 16 litre anaesthetic rebreathing bags were attached to the bronchial end of the model, and tidal ventilation generated by a piston pump. U pper (nares to pharynx) and lower tract R (R-U and R-L) and R-TOT, and dynamic compliance were determined for pump volumes (V-p) of six and 12 litres, at pumping frequencies (f(p)) of 20 - 100 min(-1) while the airway was clear, and after modifying it to simulate either PLH or pa rtial bronchial obstruction. Model condition had no effect on R-U. How ever, R-L and R-TOT Were higher in the PLH simulated condition when f( p) greater than or equal to 90 and V-p = 12 litres (P<0.05). This sugg ested that severe PLH may significantly interfere with airflow distal to the site of the lesions during high frequency, high volume ventilat ion of the type seen in galloping horses. With partial bronchial obstr uction R-L and R-TOT Were increased when f(p)>34 with each V-p. The ap plicability of the model was verified by comparing results from the un obstructed state with those from normal horses exercising on a treadmi ll.