Updated: 12/17/2021

H2 Blockers

Review Topic
  • Snapshot
    • A G1P0 patient presents to the obstetrics ward for induction of labor. Six hours after placement of an epidural, the fetal heart tracings reveal late decelerations. The obstetrician on call consents the patient for Cesarean delivery. The patient last drank fluids four hours ago, and has no significant drug allergies. For aspiration prophylaxis, the anesthesiologist administers the patient a combination of oral sodium citrate, intravenous metoclopramide, and intravenous ranitidine.
  • Introduction
    • Mechanism of action
      • indirectly inhibits proton pump by blocking stimulation via H2 receptors on parietal cells and enterochromaffin-like cells
        • leads to ↓ H+ secretion by parietal cells
      • no effect on gastic emptying time
    • Clinical use / indications
      • peptic ulcer 
      • aspiration prophylaxis
      • gastritis 
      • GERD 
      • Zollinger-Ellison syndrome 
    • Toxicity
      • cimetidine
        • potent inhibitor of P-450
        • antiandrogenic effects
          • results in gynecomastia, impotence, ↓ libido in males
        • crosses blood-brain barrier
          • results inconfusion, dizziness, headaches
        • crosses placenta
        • thrombocytopenia 
      • cimetidine and ranitidine ↓ creatinine clearance
      • other H2 blockers have fewer side effects

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