Differences in Respiratory Reflex Responses from the Larynx, Trachea, and Bronchi in Anesthetized Female Subjects

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Abstract
Background: Animal studies show that airway receptors responsible for eliciting respiratory protective reflexes are not uniformly distributed in the airways. Based on this information, it is possible that the protective reflex responses to airway irritation in humans may vary, depending on the site of stimulation. The purpose of this study is to examine whether the protective reflex responses evoked from the larynx are different from those evoked from the lower airways and to see how change in depth of anesthesia modifies the protective reflex responses evoked from individual sites. Methods: The airway mucosa of the larynx, tracheal carina, and bronchi were stimulated by injection of distilled water (0.5 ml) at two different depths of sevoflurane anesthesia (1.2 and 1.8 MAC) in 11 female subjects breathing spontaneously through the laryngeal mask airway. The respiratory responses were monitored by measuring ventilatory flow and airway pressure. Results: At 1.2 MAC of sevoflurane anesthesia, both laryngeal and tracheal stimulation caused protective responses, such as forceful expiratory efforts, apnea, and spasmodic panting, whereas bronchial stimulation caused little or no such responses. There was no significant difference in the incidence of different types of reflex responses between the larynx and the trachea. At 1.8 MAC of sevoflurane, the nature of the elicited responses was very similar to that observed at 1.2 MAC of sevoflurane, showing little dose-dependence of anesthetic effect. Conclusions: The respiratory reflex responses evoked by injection of water vary, depending on the site of stimulation. The incidence of various reflex responses was not affected by the changing depth of anesthesia. The sensitivity to airway irritation seems to be greater at the larynx and trachea than at the more peripheral airways.