Effect of expiratory glottic constriction on lung volume and pattern of breathing in adult dogs.

We examined the effect of laryngeal constriction on the pattern of breathing in 4 anaesthetized adult mongrel dogs. By means of a T-shaped tracheostomy tube the larynx could be repeatedly excluded or included in the breathing circuit. Marked expiratory activity of the thyroarytenoid muscle (TA), the main glottic constrictor, was induced by injection of 100-400 ml of air into the pleural space or by inhalation of histamine aerosol (2 dogs) which resulted in rapid shallow breathing. In 21 pairs of runs in the 4 dogs switching from tracheostomy breathing to oral breathing decreased mid-expiratory flow and frequency by 85 +/- 2% (P less than 0.001) and 48 +/- 4% (P less than 0.001), respectively. Although expiratory duration (TE) increased, end-expiratory lung volume also increased by 40 +/- 8 ml or 21 +/- 4% of the tidal volume (VT) during tracheostomy breathing (P less than 0.001). In contrast, VT remained unchanged (P = 0.9). Instantaneous ventilation decreased due to both the prolongation of TE and an increase in inspiratory duration. Our results indicate that laryngeal braking can be recruited in adult dogs and interacts with reflex mechanisms that modulate respiratory timing, thereby significantly influencing end-expiratory lung volume, ventilation and the pattern of breathing. Simulation of the laryngeal mechanism by expiratory resistive loading at the tracheostomy below the larynx points to a non-reflex mechanical effect of the larynx as a resistance in series.