THYROARYTENOID MUSCLE-ACTIVITY DURING HYPOCAPNIC CENTRAL APNEAS IN AWAKE NONSEDATED LAMBS

In this study, we examined whether the glottis is open or closed durin g central apnea and the effect of arterial P-o2 (Pa-o2) on this contro l. We hyperventilated nine 11- to 30-day-old awake nonsedated lambs vi a a tracheostomy for 1 min to induce central apnea. Four gas mixtures (8, 15, 21, and 30% O-2) were used. At the end of the hyperventilation period, the lambs were allowed to breathe spontaneously through intac t upper airways. Using a pneumotachograph attached to a face mask, we measured airflow, and we continuously recorded electromyographic (EMG) activity of the thyroarytenoid (TA), the main glottic adductor muscle . We also studied the lateral cricoarytenoid muscle (LCA, laryngeal ad ductor), the posterior cricoarytenoid muscle (PCA, laryngeal abductor) , the cricothyroid muscle (CT), and the diaphragm. We found that hyper ventilation consistently induced hypocapnic central apnea in all nine lambs in hyperoxic conditions [30% inspiratory fraction of O-2 (FIo2)] in eight of nine lambs in normoxia or mild hypoxia (15 and 21% FIo2), and in four of seven lambs in hypoxia (8% FIo2). During baseline room air breathing, there was no glottic adductor muscle expiratory EMG ac tivity or expiratory airflow braking. Continuous Th EMG activity began early during hyperventilation and continued throughout the central ap nea, regardless of Pa(o2)irst subsequent breathing efforts were marked by expiratory flow braking and expiratory activity of the TA. The LCA and the TA demonstrated the same EMG activity pattern. Conversely, th e activity of the glottic abductor (PCA), the CT, and the diaphragm (p hasically active in inspiration during baseline room air breathing) di sappeared during the hyperventilation period, was nil throughout centr al apnea, and resumed with the first inspiratory efforts. Thus, in awa ke nonsedated lambs, hypocapnic central apneas were accompanied by con tinuous glottic adduction regardless of Pa-o2. This combination could prevent alveolar gas from flowing out of the lung, thus permitting con tinuous alveolar blood gas exchanges during central apnea.