Respiratory activity of the cricothyroid muscle in conscious humans.

The cricothyroid muscle serves as an accessory muscle of respiration in anesthetized animals. The functional significance of this activity is unknown. To determine respiratory cricothyroid muscle activity in conscious humans, cricothyroid muscle electromyogram was measured percutaneously, along with posterior cricoarytenoid muscle electromyogram and esophageal pressure in six volunteers. Low level cricothyroid muscle activity with quiet breathing was observed in four subjects. In all subjects, cricothyroid muscle activity was recruited by voluntary deep breathing, brief airway occlusion, and hypercapnia. Continuous positive airway pressure at 7.5-cm H2O increased expiratory cricothyroid muscle activity in all subjects, and suppressed inspiratory activity in five. These results indicate that respiratory reflexes of the cricothyroid muscle are operative in conscious humans.