EFFECT OF BILATERAL BLOCKADE OF THE PHARYNGEAL BRANCH OF THE VAGUE NERVE ON SOFT PALATE FUNCTION IN HORSES

Objective-To determine the effect of bilateral blockade of the pharyng eal branch of the vagus nerve on soft palate function in horses. Anima ls-5 Standardbreds. Procedure-Peak tracheal inspiratory and expiratory pressures and airflow were measured while horses exercised at the spe eds corresponding to 75 and 100% of the speed that resulted in maximal heart rate, with and without pharyngeal branch of the vagus nerve blo ckade. Respiratory frequency-to-stride frequency coupling ratio was me asured by correlating foot fall measurements with respiratory frequenc y. The pharyngeal branch of the vagus nerve was blocked bilaterally as the nerve coursed through the auditory tube diverticulum (guttural po uch) across the longus capitus muscle. Results-Persistent, reversible dorsal displacement of the soft palate (DDSP) occurred in all horses a fter nerve blockade, and lasted from 1 to 3 hours; normal nasopharynge al function returned within 3 hours. Compared with control values, pea k expiratory tracheal pressure increased (P = 0.001), expiratory imped ance increased (P = 0.007), and minute ventilation decreased (P = 0.04 ). Respiratory frequency-to-stride frequency coupling ratio decreased (P = 0.009) so that horses took 1 breath/stride without the nerve bloc k and, approximately, 1 breath/2 strides with the block. Conclusion-DD SP creates flow-limiting expiratory obstruction and may be caused by n euromuscular dysfunction involving the pharyngeal branch of the vagus nerve. It may alter performance by causing expiratory obstruction and by altering breathing strategy in horses. Clinical Relevance-A repeata ble, reversible model of DDSP exists that allows further study of the disease. Dysfunction of the neuromuscular group, pharyngeal branch of the vagus nerve and palatinus and palatopharyngeus muscles, may be imp licated in the pathogenesis of clinical DDSP.