Impact of anaesthetic procedures on colonization defences of the tracheobronchial tree

Abstract

Summary Surgery and general anaesthesia can predispose to gram-negative colonization of the tracheobronchial tree and subsequent pneumonia for multiple reasons. Not only do the anaesthetic gases and the surgery itself play a part in potentially impairing colonization defences of the lower airway, but the presence of an endotracheal tube, respiratory assistance devices, coexisting illnesses and medication effects can also lead to the growth of organisms in the lung. When the airway is instrumented, bacteria have access directly to the tracheobronchial tree, and the injury associated with intubation may favour the exposure of an increased number of epithelial surface receptors for bacterial binding. In addition, the stimulation of mucus along with the stagnation of secretions can promote colonization. Secretion stagnation may result from the presence of an endotracheal tube, the mucosal drying that can accompany the use of poorly humidified gases or medications such as atropine, and the reduction in mucociliary clearance that can occur (at least transiently) with the use of anaesthetic gases such as halothane. Mucus contains receptors for bacterial adherence, and if the mucus is not rapidly removed from the lung the bound bacteria may serve as a nidus for subsequent airway colonization. Systemic illnesses and medications can also lead to an increase in tracheal cell binding capacity for bacteria, further serving to promote colonization. In addition to an increased receptiveness of the airway for bacterialbinding and colonization, surgery and anaesthesia may be accompanied by an increased bacterial inoculum that can reach the tracheobronchial tree. The endotracheal tube may serve as a reservoir for bacteria that remains sequestered from host defence mechanisms. Respiratory therapy nebulizers may disseminate bacteria, and ventilator tubing condensate may become overgrown with organisms, which can inadvertently be washed back to the patient. With the recognition that the stomach may serve as a source for organism entry into the lung, it may be important to minimize the numbers of bacteria that colonize the stomach. The use of acid neutralization therapy to prevent intestinal bleeding can dramatically increase the number of bacteria in the stomach, and such therapy may be a risk factor for both colonization and pneumonia. Once the airway becomes colonized by enteric gram-negative bacteria, pneumonia may follow, especially if colonization occurs in the setting of impaired systemic and respiratory host defences. Such impairment can occur as the result of anaesthetic agents or of coexisting illnesses and therapeutic interventions. Thus, when a postoperative patient develops pneumonia, many aetiologic factors may be operative. Although general anaesthesia is one of these factors, it rarely leads to pneumonia by itself, and when tracheobronchial colonization or pneumonia develop, invariably some of the other associated factors are present.