The purpose of preoperative fasting is to reduce the amount of particulate matter in the stomach and to lower the gastric fluid volume and acidity in case aspiration of stomach contents occurs. Gastric emptying times vary depending on the type and quantity of food consumed. Clear liquids (eg, water, coffee, pulp-free juice, and carbohydrate drinks) empty fastest, breast milk empties after 4 hours, nonhuman milk by 6 hours, and large meals that include meat or fatty substances empty by 8 hours after consumption. Small meals, such as dry toast with black coffee and pulp-free juice, have been shown to clear within 4 hours.
Perioperative fasting protocols vary between institutions and between patients and may depend on comorbidities that influence gastric emptying and how urgently surgery is needed. For example, some institutions and anesthesiologists wait only 2 hours when performing surgery on babies fed breast milk. Patients with diabetes mellitus, particularly those with autonomic neuropathy, are at risk for gastroparesis (more than 50% of patients with long-term diabetes); therefore, these patients may have a prolonged gastric emptying time. Pregnant patients have a higher-than-normal risk of aspiration. Patients with known gastroesophageal reflux disease and those with peptic ulcer disease may also have an increased risk of aspiration.
Note that a pediatric patient who fasts for 10–12 hours preoperatively may become hypotensive as a result of dehydration. The use of clear liquids orally up to 2 hours before surgery does not lead to a higher incidence of aspiration or other gastrointestinal complications in the setting of general or local anesthesia and is encouraged for the pediatric population.
Oral administration of an H2 blocker such as ranitidine or famotidine 2–4 hours before surgery reduces the percentage of patients with low gastric pH or high gastric volume. Metoclopramide promotes intestinal motility and decreases reflux, which may be especially useful in a nonfasting patient who requires urgent surgery, but it is associated with a higher risk for extrapyramidal adverse effects.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.