Original Article DOI 10.3349/ymj.2009.50.3.380 pISSN: 0513-5796, eISSN: 1976-2437 Yonsei Med J 50(3): 380-384, 2009 Characteristics of Glottic Closure Reflex in a Canine Model Young-Ho Kim, Ju Wan Kang, and Kwang-Moon Kim Department of Otorhinolaryngology, Yonsei University College of Medicine, Seoul, Korea. Purpose: The most important function of the larynx is airway protection which is provided through a polysynaptic reflex closure triggered by the receptors in the glottic and supraglottic mucosa, evoking the reflex contraction of the laryngeal muscles especially by strong adduction of vocal cords. Based on the hypotheses that central facilitation is essential for this bilateral adductor reflex and that its disturbance can result in weakened laryngeal closure, we designed this study to elucidate the effect of central facilitation on this protective reflex. Materials and Methods: Seven adult, 20 kg mongrel dogs underwent evoked response laryngeal electromyography under 0.5 to 1.0 MAC (minimum alveolar concentration) isoflurane anesthesia. The internal branch of the superior laryngeal nerve was stimulated through bipolar platinum-iridium electrodes, and recording electrodes were positioned in the ipsilateral and contralateral thyroarytenoid muscles. Results: Ipsilateral reflex closure was consistantly recorded regardless of anesthetic levels. However, contralateral reflex responses disappeared as anesthetic levels were deepened. Additionally, late responses (R2) were detected in one animal at lower level of anesthesia. Conclusions: Deepened level of anesthesia affects central facilitation and results in the loss of the crossed adductor reflex, predisposing to a weakened glottic closure response. Precise understanding of this effect may possibly provide a way to prevent aspiration in unconscious patients. Key Words : Glottic closure reflex, anesthesia, central facilitation, aspiration Received: June 19, 2007 Revised: October 25, 2007 Accepted: November 10, 2007 Corresponding author: Dr. Young-Ho Kim, Department of Otorhinolaryngology, Yonsei University College of Medicine, 250 Seongsan-ro, Seodaemun-gu, Seoul 120-752, Korea. Tel: 82-2-2228-3607, Fax: 82-2-393-0580 E-mail: yhkimmd@yuhs.ac *Presented at American Bronchoesophagolo- gical Association Annual Meeting, COSM, Chicago, Il., May 19-20, 2006. © Copyright: Yonsei University College of Medicine 2009 380 INTRODUCTION The physiologic functions of the larynx can be classified into several systems such as respiratory, phonatory and protective reflex system. Although primitive, the protective reflex may be the most important in protecting the lung against aspiration. The larynx provides this protection through a reflex closure triggered by the receptors in the glottic and supraglottic mucosa, thereby evoking the reflex contraction of laryngeal muscles. This reflex is a polysynaptic brain stem response,1-3 and the starting point is the input of signals into the internal branch of the superior laryngeal nerve (iSLN). After electrical stimulation of one iSLN, the electromyographic potential can be recorded in the ipsilateral or contralateral thyroarytenoid (TA) muscle. Three categories of protective laryngeal responses have been observed. Firstly, an early response involves adduction of the ipsilateral vocal cord with a latency of approximately 10 to 18 msec in anesthetized cats, dogs, and pigs.4-7 This short latency-evoked response, termed R1, has also been noted consistently in humans under anesthesia.4 The second category of laryngeal responses involves simulta- neous, contralateral adduction with a short-latency, also known as the crossed adductor reflex. Although this response has consistently been found in anesthetized cats, it is less consistently found in dogs and pigs, and rarely in anesthetized human subjects.4 The third category of adductor response involving a longer latency reflex, termed R2, has been observed to produce bilateral responses, but its presence is most readily noted in awake human subjects with a latency of 50 to 80 msec.8 Yonsei Med J http://www.eymj.org Volume 50 Number 3 June 2009