Anesthesia Pharmacology Chapter 21:  Gastrointestinal Pharmacology

Antacids as Preoperative Medication

• In adults: possibly, relatively high-risk of pulmonary complications if aspiration volume:

  1. > 25 ml and

  2. pH < 2.5

• Antacids: neutralize gastric acid content

  • Single dose -- 15-30 minutes before induction: about 100% effective in increasing gastric fluid pH above 2.5

  • Sodium citrate (0.3M) --nonparticulate antacid-- commonly given before the procedure when an increase in gastric fluid pH is required

  • Nonparticulate antacids:

    • do not themselves produce pulmonary damage if aspiration of gastric fluid containing these antacids occur.

    • Colloid antacid suspensions: more effective than nonparticulate agents in increasing gastric acid pH

      •  Aspiration of gastric fluid containing particulate antacids produce significant/persistent damage even if gastric pH has been increased:

        •  Damage:

          •   pulmonary edema

          •   arterial hypoxemia

• Onset: no "lag time"-- as seen with H₂ receptor antagonists

  • Effective on fluid already present in the stomach

  • Good choice in emergency situations (assuming patients can take oral medication)

• Other issues:

  • Antacids increase gastric fluid volume (unlike H₂ receptor antagonists)

    • Most evident following repeated dosing -- e.g., during labor {concurrent opioid administration may promote gastric emptying delay}

  • Do not withhold antacids because of possible increases in gastric volume --increasing the pH is more important

• Other drugs:

  • Omeprazole -- gastric acid suppression by inhibition of parietal cell proton pump

  • 40 mg IV (adult); 30 minutes prior to induction has been employed

  • 40-80 mg oral -- 2-4 hours before surgery is required

  • Duration of action of effects on gastric pH: about 24 hours

  • pH increases; variable effects on gastric volume with omeprazole and H₂ receptor antagonists

Moyers, J.R., Preoperative Medication, Chapter 21, In: Clinical Anesthesia 3rd edition, (Barash, P.G., Cullen, B.F. and Stoelting, R.K., eds) Lippincott-Ravin, Philadelphia, New York, 1997, pp. 519-529