Effect of sternothyrohyoid myectomy on upper airway mechanics in normal horses

The effect of transection of the sternothyroideus and sternohyoideus muscles on upper airway mechanics was investigated in exercising horses. Upper airway mechanics of six Standardbred horses were measured at rest and during exercise before and 24 h and 2 wk after sternothyrohyoid myectomy. Transnasal tracheal and pharyngeal catheters connected to differential pressure transducers were used to measure tracheal and pharyngeal pressures. A pneumotachograph mounted on the rostral end of an airtight face mask was used to measure airflow. Horses ran at 50, 75, and 100% of maximal O2 consumption on a treadmill. Twenty-four hours after sternothyrohyoid myectomy, no significant difference was detected in tracheal, pharyngeal, or translaryngeal inspiratory and expiratory pressures and impedances, inspiratory and expiratory flows, and respiratory frequency. Two weeks after sternothyrohyoid myectomy, there was a statistically significant increase in translaryngeal inspiratory pressure (P = 0.035) and tracheal inspiratory pressure (P = 0.032) compared with preoperative measurements. Two weeks after sternothyrohyoid myectomy, there was a statistically significant increase in translaryngeal inspiratory resistance (P = 0.017) and tracheal inspiratory resistance (P = 0.023) compared with preoperative values. Increased translaryngeal and tracheal inspiratory pressures and resistances after sternothyrohyoid myectomy suggest that the sternothyroideus and sternohyoideus muscles act to increase or maintain upper airway patency and stability in normal horses.