Effects of hypoxia on lung volume in the garter snake

Ventilation and lung volume were measured in unanesthetized garter snakes using a headout body plethysmograph. The breathing cycle consisted of expiration, inspiration and a prolonged post-inspiratory breath-holding period, which lasted until the onset of the next expiration. Hypoxia induced increases in VT and in the volume at which the breath was held between cycles. Hypercapnia usually elicited hyperpnea after an initial, transient inhibition of breathing movements, but was accompanied by little or no increase in breath-holding lung volume. Snakes with T-tube tracheostomies failed to maintain the normal positive intrapulmonary pressure during the breath-holding period and failed to increase breath-holding lung volume in hypoxia, despite increased inspiratory excursions. These studies demonstrate a pronounced increase in lung volume during hypoxia and indicate that this response requires an intact upper airway. These findings in a reptile may be related in evolutionary terms to the increase in lung volume and post-inspiratory laryngeal narrowing that occur in mammals during hypoxia.