Effects of hypoxia and hypercapnia on cricothyroid muscle response to airway pressure.

We studied the effects of hypercapnia (FICO2 = 0.07) and hypoxia (FIO2 = 0.13) on the cricothyroid muscle response (peak integrated CT EMG) to graded inspiratory resistance (RI). Five anesthetized dogs spontaneously breathed through the upper airway (UAB) or a side arm of a tracheotomy cannula bypassing the larynx (TB). With room air UAB, graded increases in RI increased CT EMG, upper airway pressure change (Paw) and esophageal pressure change (Pes, indicating increased inspiratory drive), but decreased VT. For room air TB, the effects of RI were much less but still significant. Hypercapnia increased VT, fR, Pes, Paw and CT EMG for any given RI during UAB more than during TB. Hypoxia increased fR but did not increase VT, Pes, Paw, or CT EMG. The results are consistent with a model predicting CT activity as additive functions of inspiratory drive and laryngeal receptor stimulation by Paw and no direct effects of chemoreceptor stimulation on the CT, independent of those resulting from changes in inspiratory drive and upper airway pressure.