Effects of hypercapnia and hypoxia on laryngeal resistance to airflow.

Ventilation, laryngeal resistance and electromyograms of the diaphragm, posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscles were recorded in anesthetized, spontaneously breathing cats during 100% O2 administration and during steady state inhalation of hypercapnic and hypoxic gas mixtures. As shown previously, hyperoxic hypercapnia lowered expiratory laryngeal resistance (RlarE). Isocapnic hypoxia also lowered RlarE, and hypercapnia superimposed on hypoxia decreased it further. Hypocapnia raised RlarE. Changes in inspiratory laryngeal resistance (RlarI) were similar to those in RlarE, but smaller. When ventilation was stimulated to the same extent by hypoxia and by hypercapnia, RlarE was lower under hypoxic than hypercapnic conditions in most animals. The electromyograms showed that the respiratory oscillations in laryngeal resistance and the laryngeal responses to hypercapnia and hypoxia were determined chiefly by the activity of the PCA muscle, the abductor of the vocal cords. The TA-a representative adductor muscle-was silent under all conditions studied. The results, considered with previous work, indicate that the larynx plays a part in determining the breathing pattern under resting conditions and during respiratory stimulation by hypercapnia and hypoxia.