Cricothyroid muscle responses to increased chemical drive in awake normal humans.

To examine the response of the cricothyroid muscle (CT) to increased chemical drive, we measured its electromyogram simultaneously with that of the alae nasi (AN) in seven normal awake subjects. During both progressive hyperoxic hypercapnia and hypoxia, peak integrated inspiratory activity (moving time average, MTA) of the CT and AN increased as a power function of mean inspiratory flow (ratio of tidal volume to inspiratory time, VT/TI), given by MTA = a(VT/TI)b + c (where a, b, and c are constants). The exponent b varied from 0.009 to 3.4 among subjects but was correlated between CT and AN both during hypercapnia (r = 0.86) and hypoxia (r = 0.81). The onset of inspiratory activity of the CT and AN preceded that of inspiratory flow. Expressed as a percentage of expiratory time, the CT lead time rose from 7% at rest to 20% during hyperpnea. The corresponding values for the AN were from 22 to 52% (both P less than 0.03). Thus the pattern of response of the CT and AN is similar and related to that of the inspiratory muscles in a curvilinear manner. The findings suggest that during chemical stimulation the electrical activity of the CT is analogous to that of the AN, an upper airway dilator.