Use of the forced oscillating airflow technique to measure the resistance of the equine upper airway: effects of laryngoventriculectomy and laryngoplasty

Investigations were carried out to determine whether measurements of total respiratory resistance (trr) made in resting animals could detect changes due to laryngeal hemiplegia. Control values of trr were obtained in eight ponies and in six of these the measurements were repeated after division of the left recurrent laryngeal nerve in the mid-cervical region; two were retained as controls. A further set of measurements were made before two of the operated animals were subjected to left laryngoventriculectomy (Hobday operation). A laryngoplasty (‘tie-back’ operation) was performed on another two animals. Measurements of trr were made at three-monthly intervals on all eight ponies for the next two years. Endoscopic examination of the larynx was carried out at the time of the measurements. All ponies showed a marked, but regular, variation in trr over the two-year observation period. Nerve section was shown to be associated with an increase in trr in five ponies. The Hobday operation decreased trr in one animal and had no marked effect in the second. The tie-back operation was associated with a decreased trr in one pony and an increase in the second. Autopsies were carried out on all six operated ponies and measurements of trr and lower respiratory resistance (lrr) were made immediately before the animals were killed. Evidence of regeneration of the recurrent laryngeal nerve was seen in one of the ponies which underwent the Hobday operation. Histological evidence of regeneration was found in another pony. It was concluded that the measurement technique was capable of demonstrating, in quietly standing ponies, changes in upper airway resistance (trr-lrr) due to partial obstruction of the laryngeal lumen.