Minimum &;starvation time&; required in general anaesthesia
Traditionally, patients are considered to be at risk if their gastric volume is greater than 25 mL (0.4 mL/kg) and their gastric pH is less than 2.5. Some investigators believe that controlling acidity is more impoant than volume and that the criteria should be revised to a pH less than 3.5 with a volume greater than 50 mL. Patients who have eaten immediately prior to emergency surgery are obviously at risk. Traditionally, "NPO after midnight" implied a preoperative fast of at least 6 h. The current opinion allows clear liquids until 2 h before the induction of anesthesia. According to the American Society of Anesthesiologists (ASA) guideline, breast milk is permitted up to 4 h before anesthesia. Infant formula, nonhuman milk, and a light meal are permitted up to 6 h before induction. Patients consuming a heavy meal including meat, fats, and fried foods should fast for 8 h. Ceain patient populations are paicularly likely to have large volumes of acidic gastric fluid: patients with an acute abdomen or peptic ulcer disease, children, the elderly, diabetic patients, pregnant women, and obese patients. Fuhermore, pain, anxiety, or opioids may delay gastric emptying. Note that pregnancy and obesity place patients in double jeopardy by increasing the chance of aspiration (increased intraabdominal pressure and distoion of the lower esophageal sphincter) and the risk of aspiration pneumonia (increased acidity and volume of gastric contents). Aspiration is more common in patients undergoing esophageal, upper abdominal, or emergency laparoscopic surgery. Ref: Morgan & Mikhail&;s clinical anesthesiology 6e
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Fundamental concepts