How is gastric volume reduced and the pH of gastric fluid increased by perioperative medication management?

Updated: Jan 09, 2018
  • Author: Nafisa K Kuwajerwala, MD; Chief Editor: William A Schwer, MD  more...
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Answer

Many patients who come to the operating room are at risk for aspiration pneumonitis. Examples include patients with large gastric fluid volumes, such as parturients, nonfasting patients scheduled for emergency surgery, obese patients, patients with trauma, and patients with hiatal hernia or history of gastroesophageal reflux. Many different agents have been used to alter gastric volume and increase the pH of gastric fluid. Anticholinergics, H2 receptor antagonists, antacids, and gastrokinetic agents have all been used to reduce the possibility of aspiration pneumonitis. Regardless of pharmacotherapy used, clinicians should maintain a low threshold for placement of a nasogastric tube prior to induction while patient has intact airway reflexes to ensure proper gastric emptying prior to induction of anesthesia.

Anticholinergics are ineffective in increasing gastric fluid pH or reducing gastric fluid volume. [14] In fact, intravenous doses of anticholinergics may cause relaxation of the gastroesophageal junction and increase risk of aspiration.

The histamine receptor antagonists cimetidine, ranitidine, famotidine, and nizatidine reduce gastric acid secretion via competitive antagonism of the histamine receptor, thereby increasing gastric fluid pH. These agents may be used for the allergic patient or in preparing a patient for exposure to an allergenic trigger, such as radiologic dye.

Antacids are used 20-30 minutes before induction to increase gastric fluid pH higher than 2.5. The nonparticulate antacid, 0.3 M sodium citrate, is used because aspiration of gastric contents containing these antacids will not cause damage to lungs. In contrast, aspiration of gastric fluid containing particulate antacids may lead to pulmonary damage, despite the increase in gastric fluid pH.

Proton-pump inhibitors suppress gastric acid secretion in a dose-dependent manner by binding to the proton pump of the parietal cell. Intravenous doses of 40 mg 30 minutes before induction are commonly used. Unfortunately, investigators have found increases in gastric pH and inconsistent effects on gastric volume with administration of omeprazole and ranitidine. [15]

Gastrokinetic agents are used to reduce gastric fluid volume. Metoclopramide is a dopamine antagonist that stimulates upper gastrointestinal motility, increases gastroesophageal sphincter tone, and relaxes the pyloric sphincter. It has no effect on gastric acid secretion. An intravenous dose of 5-10 mg is usually given 15-30 minutes before induction over 3-5 minutes to prevent abdominal cramping. An oral dose of 10 mg achieves onset within 30-60 minutes. Use is contraindicated in patients with bowel obstruction.

A summary of fasting recommendations to reduce the risk of pulmonary aspiration includes the ingested material and the minimum fasting time for all age groups. These recommendations apply only to healthy patients who are undergoing elective procedures and are not intended for women in labor. Following the guidelines does not guarantee complete gastric emptying. See the following [16, 17] :

  • Clear liquids (eg, water, fruit juices without pulp, carbonated beverages, clear tea, black coffee) - 2 hours

  • Breast milk - 4 hours

  • Infant formula - 6 hours

  • Nonhuman milk - 6 hours

  • Light meal (toast and clear liquids) - 6 hours


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