2 Upper airway reflexes

Abstract

The maintenance of a clear unobstructed upper airway and the rapid return of normal laryngeal reflexes after the end of general anaesthesia form two of the most fundamental principles of modern anaesthetic practice. Upper airway reflexes are of considerable importance to clinical anaesthetists. The sensitivity of the upper airway reflexes are important during induction of anaesthesia, when heightened upper airway reflexes may lead to the development of life threatening laryngospasm. Following anaesthesia the larynx plays a primary role in protecting the lungs from aspiration of foreign material. The irritant nature of inhalation anaesthetic agents varies, with halothane being one of the least irritant. Newer agents including isoflurane and desflurane have been found to be associated with a much higher incidence of airway irritation. The intravenous anaesthetic induction agents thiopentone and propofol seem to have different actions on the sensitivity of upper airway reflexes during induction of anaesthesia, the upper airway appearing to be less irritable following propofol. Laryngospasm is a common and potentially dangerous complication of general anaesthesia, the incidence is higher in young children particularly between the ages of 1 and 3 months. Upper respiratory tract infection increases the incidence of laryngospasm. This is thought to be due to the viral infection causing shedding of epithelial cells, leading to increased exposure of intraepithelial sensory receptors to inhaled irritants. Histologically, sensory nerve fibres are found in almost all areas of the laryngeal mucosa. A particularly high density of sensory free nerve endings are found in the posterior supraglottis. Stimulation of irritant receptors in the upper respiratory tract causes a reflex motor response, afferent signals travel in the vagus nerve and project centrally to the caudal and posterior parts of the nucleus tractus solitarius. The efferent response results in vocal cord closure, changes in respiratory rhythm and coughing, which together act to protect the lower airway from foreign material.