Posterior cricoarytenoid activity and glottic size during hyperpnea in humans

We measured the electromyographic activity of the posterior cricoarytenoid (PCA) muscle simultaneously with glottic width (dg) in five normal human subjects during hyperpnea induced by hypoxia (7% CO2 in N2) or hypercapnia (9% CO2 in 50% O2). The glottic aperture was measured during inspiration at the time corresponding to peak inspiratory PCA activity and during expiration at the time corresponding to the minimum tonic activity. During hyperpnea, peak and tonic PCA activity increased simultaneously with widening of the vocal cords in both phases of the respiratory cycle. The PCA activity during both inspiration and expiration showed a single curvilinear relationship with dg of the form dg = A - Be-k.PCA (where A, B, and k are constants) in three of the five subjects. At 50% of maximum PCA activity, dg already reached 95% of its maximum value, which was less than that recorded during a voluntary forced expiratory maneuver. The single curvilinear relationship between PCA activity and dg could be due to the length-tension relationship of the PCA muscle and/or changes in its mechanical coupling, as well as simultaneous agonist and antagonist laryngeal muscle activity during progressive chemical stimulation. Also, further widening of the glottis during forced expiration suggests recruitment of additional muscles, e.g., the arytenoideus.