Colonisation of the trachea in ventilated patients: What is the bacterial pathway?

Abstract

It is well known that the trachea of patients undergoing assisted ventilation in intensive therapy units (ITUs) rapidly becomes colonized with bacteria. This is undesirable since bacteria are then advantageously placed to invade the lung parenchyma. In addition, the results of culture of lung aspirates, which are inevitably contaminated with tracheal organisms, are made more difficult to interpret. The bacteria involved have been shown to originate from several different sources; (Gaya, 1974; Stoddart, 1975) e.g. respiratory equipment, nasogastric feeds, other patients and endogenous sources such as faeces and other colonized sites in the patient. Bacteria from these sources have been found on the hands of staff and other vehicles of spread, but the route of entry into the trachea is less clear. There are two potential ‘final common pathways’ by which bacteria may enter the trachea, (1) through the airway of the endotracheal or tracheostomy tube via the inhaled air or during lung suction, (2) via contaminated oropharyngeal or gastric secretions passing between an endotracheal tube and the wall of the trachea. Bacterial passage through a tracheostomy wound is probably unimportant, since endotracheal tubes are more commonly used and wounds receive protection from infection. This article examines the two ‘final common pathways’ in an attempt to demonstrate the potential importance and consequences of the second route for tracheal colonization in patients undergoing assisted ventilation. The air delivered to the patient may carry bacteria entrapped from a contaminated part of the respiratory equipment or from the air of the unit itself. However, much emphasis is placed on preventing bacterial contamination of respiratory equipment and ventilators are generally decontaminated after use by efficient processes (Lumley, 1976). Humidifier reservoirs are usually filled with a disinfectant at an appropriate concentration and are stored dry between patients. Respiratory tubing is changed frequently (Gaya, 1974). Less work has been done on the bacterial contamination of the air of ITUs, but common experience and the available evidence (Lowbury et al., 1969; Teres, 1973) tends to indicate that it is of little importance. In any case, in cross-infection episodes, the pattern of spread of tracheal isolates between patients is typical of contact spread. Contamination of inspired air from retrograde movement of tracheal secretion into the inspiratory line could only occur after tracheal colonization. The relative unimportance of the air and respiratory equipment as a pathway