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It would be extremely misleading to leave the impression that pharmacological prophylaxis is a substitute for sound judgement and good anaesthetic technique. Yet, existing reports demonstrate that significant morbidity and occasional death all too commonly still arise secondary to the unpredictable event of aspiration, z The primary purpose of this refresher course is to review the pharmacology, efficacy and safety of those drugs which are utilized clinically to alter the volume and/or character of gastric fluid contents. Although documentation is lacking, it is hoped that administration of these drugs will lessen either the incidence or severity of aspiration pneumonitis. Factors affecting the severity of aspirationThe acidity of the aspirate is probably the most important factor contributing to the severity of pneumonitis. In numerous animal models, increasing acidity, whether alone or in combination with food particles, results in increasing tracheal mucosal and lung parenchymal damage. 2-4 The literature has been replete, however, with the suggestion that an aspirate with a pH greater than 2.5 is relatively benign. Several anecdotal reports as well as recent animal work refute this concept. 5-7 In fact, the initial physiologic derangement may be just as severe and potentially as lethal. Nevertheless, recovery and healing rates appear to be faster if acidity is reduced.In this same light, there has been propagation in our literature of the notion that a volume of aspirate less than 0.4 ml.kg -t (20 to 25ml in an adult) is relatively safe. Perhaps just as misleading is the converse of this statement -that an aspirate volume exceeding this derived number if uniformly fatal. The concept of a "critical" volume was based on work done in rhesus monkeys in which the aspirated fluid was highly acidic. 8 In fact, recent work in rats demonstrated that the critical volume depends on the pH of the apirate. Even low volumes have a high mortality rate if the pH is low; whereas higher volumes than noted above can be tolerated if the gastric fluid is effectively buffered. 9Aspiration of particulate matter such as partially digested food particles has also been known for some time to result in severe pulmonary physiologic derangements, z~ Such aspirates, in addition to increasing shunt fraction and decreasing arterial oxygen tension, often result in alveolar hypoventilation with accompanying hypercarbia. In the long term, a granulomatous response develops around these particles. Similarly, bacterial contamination of an aspirate, as may occur in a patient with a bowel obstruction, is associated with a very high mortality rate. 11Finally, experience in both fresh and salt water drowning victims have demonstrated the importance of the tonicity of an aspirate. Recent work in dogs reconfirmed that as osmolarity of the aspirate increases, so too does the degree of pulmonary physiologic derangement, t2 It thus becomes apparent that multiple factors influence the significance of an aspirate.

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Haemodynamic

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