EFFECT OF STERNOTHYROHYOID MYECTOMY ON UPPER AIRWAY MECHANICS IN NORMAL HORSES

The effect of transection of the sternothyroideus and sternohyoideus m uscles on upper airway mechanics was investigated in exercising horses . Upper airway mechanics of six Standardbred horses were measured at r est and during exercise before and 24 h and 2 wk after sternothyrohyoi d myectomy. Transnasal tracheal and pharyngeal catheters connected to differential pressure transducers were used to measure tracheal and ph aryngeal pressures. A pneumotachograph mounted on the rostral end of a n airtight face mask was used to measure airflow. Horses ran at 50, 75 , and 100% of maximal O-2, consumption on a treadmill. Twenty-four hou rs after sternothyrohyoid myectomy, no significant difference was dete cted in tracheal, pharyngeal, or translaryngeal inspiratory and expira tory pressures and impedances, inspiratory and expiratory flows, and r espiratory frequency. Two weeks after sternothyrohyoid myectomy, there was a statistically significant increase in translaryngeal inspirator y pressure (P = 0.035) and tracheal inspiratory pressure (P = 0.032) c ompared with preoperative measurements. Two weeks after sternothyrohyo id myectomy, there was a statistically significant increase in transla ryngeal inspiratory resistance (P = 0.017) and tracheal inspiratory re sistance (P = 0.023) compared with preoperative values. Increased tran slaryngeal and tracheal inspiratory pressures and resistances after st ernothyrohyoid myectomy suggest that the sternothyroideus and sternohy oideus muscles act to increase or maintain upper airway patency and st ability in normal horses.