Preoperative fasting is used to reduce the risk of pulmonary aspiration. From the ASA guidelines on preoperative fasting, meta-analysis of randomized controlled trials demonstrated in adult patients given clear liquids 2-4 hours before a procedure versus 4 hours had smaller gastric volumes and higher pH values. In children who received clear liquids 2-4 hours prior to a procedure versus fasting for >4 hours , meta-analysis of randomized controlled trials demonstrated only higher gastric pH values, but was equivocal regarding gastric volume.
In contrast, observational studies with ingestion of breast milk and infant formula 4 hours prior to a procedure were equivocal regarding higher gastric volumes or lower gastric pH.
References
- American Society of Anesthesiologists Committee Practice guidelines for preoperative fasting and the use of pharmacologic agents to reduce the risk of pulmonary aspiration: application to healthy patients undergoing elective procedures: an updated report by the American Society of Anesthesiologists Committee on Standards and Practice Parameters. Anesthesiology: 2011, 114(3);495-511 PubMed Link
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