Characteristics of Glottic Closure Reflex in a Canine Model These observations indicate to us that we cannot demon- strate either the crossed R1 or R2 adductor reflex in fully anesthetized animals and humans. On the other hand, the presence of crossed reflexes in awoken subjects suggests the possibility that this response may be dependent on anesthesia. When these reflexes are exaggerated or hypersensitive, it may be responsible for several disorders such as spasmodic dysphonia, idiopathic laryngospasm, reflex apnea, apnea of infancy, stuttering, sudden infant death syndrome, etc.3,9-14 The loss of these reflexes is an equally or more important problem. Absence of these protective responses may place patients into a devastating situation, in which the chance of aspiration increases, resulting in an aspiration pneumonia fatal to the patient’s life. Therefore, a precise understanding of this effect may improve the prevention of aspiration in patients at risk who are emerging from prolonged sedation15,16 or under heavy psychotropic control.17 The purposes of this study are to confirm our hypothesis that when the supramedullary influences are abolished by general anesthesia, the crossed adductor response is also abolished18 and to provide an explanation for increased risk of aspiration in unconscious patients through the understan- ding of this laryngeal reflex responses. MATERIALS AND METHODS Seven young mongrel dogs were used in this experiment. Subjects were approximately 6 to 8 months of age and their weights were at around 20 kg. A canine larynx model was selected because of its anatomic and physiologic similarities to humans.4 Atropine and Xylazine, 0.05 and 2 mg/kg, respectively, were administered together as induction agents. Anesthesia was maintained through an endotracheal intubation. electro- cardiography (EKG), respiratory rate, and pulse oximetry were constantly monitored. And core body temperature was maintained at 38˚C using a heating pad. Sufficient amount of time, at least 90 minutes, was allowed for the pharmacological effects of induction agents to clear. During this period, preparatory procedures described below were carried out prior to the initiation of the study protocol. After routine antiseptic preparation, a midline neck incision was made that ectended from the hyoid bone to the sternal notch. A low tracheotomy was done below the level of the fourth tracheal ring, and an endotracheal tube (internal diameter 6 mm) was inserted into the trachea and secured to the skin. Subsequent inhalational anesthesia with isoflurane was maintained through the tracheotomy. Both the right and left iSLNs were identified and sectioned proximal to the entrance to the thyrohyoid membrane. Sufficient portion of the thyroid cartilage was removed to allow full access to and visualization of both vocal cords. MP 100 WSP data acquisition system (Biopac Systems, Inc., Santa Barbara, CA, USA) provided nerve stimulation and electromyogram (EMG) recording capabilities. Bipolar platinum-iridium stimulating electrodes were applied to the proximal end of each severed iSLN. Monopolar platinum recording electrodes were inserted into the mid-portion of TA muscles bilaterally. Reference electrodes were placed in the ipsilateral strap muscles, and a ground electrode was placed in the sternocleidomastoid muscle. Because the crossed adductor response may be obscured by bilateral iSLN stimulation, the iSLN was stimulated unilaterally in sequential fashion. In Protocol 1, the left iSLN in each subject was electri- cally stimulated sequentially under varying depths of ane- sthesia. Each subject was exposed to five different anesthesia depths (less than 0.5, 0.5, 0.75, 1.0, and more than 1.0 MAC) and minimal alveolar concentration; 1 MAC is defined as 1.5% of inhaled isoflurane concentration,19 and is a concen- tration to suppress electroencephalogram (EEG) waves characterized by increased activation of delta wave, and 2 MAC is a concentration to silent wave.20,21 No other phar- macologic agent was used during the period of data collec- tion. At least 20 minutes of equilibration time were allowed following change in each anesthetic level. Stimulation parameters consisted of single, rectangular pulse of 0.1 mA intensity with 0.1 msec pulse duration. Stimulation intensity was increased incrementally by 0.1 mA until a consistent EMG response was seen in the TA muscle or until the maximum stimulation intensity was attained. An average of six trials were performed in each experimental paradigm. In Protocol 2, identical procedures to Protocol 1 were repeated on the right side. The ratios of R1, crossed R1 and R2 responses obtained in Protocols 1 and 2 were calculated and the latencies of each response were averaged. Statistical analysis was performed by Student’s t-test. RESULTS Protocol 1 When the left iSLN was electrically stimulated, the average latencies of ipsilateral and contralateral evoked responses with their standard deviations are summarized in Table 1. We could calculate the percentage of responses with the number of evoked responses compared to stimulus presen- tations under each level of anesthesia. It is noted that ipsila- teral R1 responses occured at all depths of anesthesia with almost 100 % efficiency (Fig. 1A, B). However, the number of contralateral R1 responses underwent a dramatic decline with anesthesia levels over 0.75 MAC. By 1 MAC, only 1 out of 17 stimulations resulted in contralateral R1 evoked Yonsei Med J http://www.eymj.org Volume 50 Number 3 June 2009 381