Effect of inhaled carbon dioxide on laryngeal abduction

Abstract

Hypercapnia produces a profound effect on respiratory drive and upper airway function to maintain airway patency. Previous work has evaluated the effects of hypercapnia on the sole arytenoid abductor, the posterior cricoarytenoid (PCA), using indirect measures of function, such as electromyography and direct nerve recording. Here we describe a novel method to evaluate PCA function in anesthetized animals and use this method to determine the effects of hypercapnia on PCA function. Eight dogs were anesthetized, and a laryngeal mask airway was used, in combination with high-speed videoendoscopy, to evaluate laryngeal function. A stepwise increase in inspired partial pressure of CO2 produced marked arytenoid abduction above 70-mmHg end-tidal CO2 (ETCO2 ) (P < 0.001). Glottic length increased above 80-mmHg ETCO2 (P < 0.02), and this lead to underrepresentation of changes in glottic area, if standard measures of glottic area (normalized glottic gap area) were used. Use of a known scale to determine absolute glottic area demonstrated no plateau with increasing ETCO2 up to 120 mmHg. Ventilatory parameters also continued to increase with no evidence of a maximal response. In a second anesthetic episode, repeated bursts of transient hypercapnia for 60 s with an ETCO2 of 90 mmHg produced a 43-55% increase in glottic area (P < 0.001) at or shortly after the end of the hypercapnic burst. A laryngeal mask airway can be used in combination with videoendoscopy to precisely determine changes in laryngeal dimensions with high temporal resolution. Absolute glottic area more precisely represents PCA function than normalized glottic gap area at moderate levels of hypercapnia.

Laryngeal mask airway and transient hypercapnic hyperpnea for video‐endoscopic assessment of unilateral laryngeal paralysis in dogs

“…Hypercapnic hyperpnea stimulates ventilation and arytenoid abduction in dogs . Coupled with video endoscopy, this technique could quantify the severity of the DCA dysfunction and facilitate the diagnosis of laryngeal hemiplegia.…”

“…Doxapram HCl is typically used to increase ventilation and arytenoid abduction . The administration of CO 2 may provide an advantageous alternative because the stimulus can be repeated at least 15 minutes apart without indications of additive effects between exposures . To the best of our knowledge, there are no data evaluating additive effects of repeated doses of doxapram HCl for this purpose, and different respiratory stimulants have not been compared to determine how they facilitate diagnosis of laryngeal paralysis in dogs.…”

“…A flexible high definition video endoscope (3‐mm outer diameter; FLEX‐XC; Karl Storz, Tuttlingen, Germany) was inserted through the free port of the 3‐way adaptor into the lumen of the LMA and placed rostral to the larynx so that the arytenoid cartilages, vocal folds, and dorsal and ventral commissures could be observed (Figure ). Arytenoid abduction was evaluated during spontaneous breathing at rest (eupnea) and during hypercapnic hyperpnea, which was induced by administering CO 2 to the mixture of inspired gases so that the inspired fraction of CO 2 was 12% …”

“…The second and third minutes of the video included CO 2 administration plus 1 minute after discontinuation of CO 2 . These intervals were chosen because it was previously shown that maximal response to CO 2 occurs shortly after 1 minute of CO 2 administration . This procedure was replicated after an interval of at least 15 minutes so that video recordings of laryngeal activity could be obtained in duplicate for each dog.…”

“…To circumvent these limitations, respiratory stimulants such as doxapram HCl are often used to increase ventilation and facilitate the observation of deficits in DCA . Alternatively, carbon dioxide (CO 2 ) supplementation can be used to increase respiratory drive, tidal volume, and arytenoid abduction and, hence, might be useful to assist in recognizing DCA deficits.…”