Effect of bilateral blockade of the pharyngeal branch of the vagus nerve on soft palate function in horses.

This research investigates the impact of blocking both sides of the pharyngeal branch of the vagus nerve on the soft palate function in horses during exercise. Findings suggest persistent, reversible abnormalities occurred in all horses post nerve blockage and this may cause a respiratory obstruction impacting the horses’ performance.

Research Methodology

• The study involved five Standardbred horses.
• While the horses exercised at 75 and 100% of the speed that resulted in their maximum heart rate, their peak tracheal inspiratory and expiratory pressures and airflow were measured.
• This was done both with and without blocking the pharyngeal branch of the vagus nerve.
• The researchers also measured the respiratory frequency-to-stride frequency coupling ratio by comparing foot fall measurements with respiratory frequency.
• The pharyngeal branch of the vagus nerve was blocked on both sides as the nerve passed through the auditory tube diverticulum (guttural pouch) across the longus capitus muscle.

Key Findings

• Post nerve blockade, all horses had persistent, reversible dorsal displacement of the soft palate (DDSP). This effect lasted between 1 to 3 hours; normal nasopharyngeal function returned within 3 hours.
• Compared to control values, peak expiratory tracheal pressure increased, expiratory impedance also increased, but minute ventilation decreased.
• Respiratory frequency-to-stride frequency coupling ratio decreased, so that horses took one breath per stride without the nerve block, and approximately one breath every two strides following the block.

Conclusions

• The study concludes that DDSP can create flow-limiting expiratory obstruction and it may be caused by neuromuscular dysfunction involving the pharyngeal branch of the vagus nerve.
• DDSP might affect the horse’s performance by causing respiratory obstructions and by altering their breathing strategy.
• The research provides a repeatable, reversible model of DDSP, allowing further investigation into the disease.
• The dysfunction of the neuromuscular group, specifically the pharyngeal branch of the vagus nerve and certain muscles, may play a role in the development of clinical DDSP.