Show the model answerAttempt it first — that is what makes it stick
10 marks
Classification, mechanisms and adverse effects
What earns the marks10 marks
| Three drugs, not a survey | The question says commonly used — one drug per mechanism |
|---|---|
| Non-particulate antacid | Sodium citrate — neutralises acid already present |
| H2 receptor antagonist | Ranitidine — reduces volume and acidity at source |
| Prokinetic | Metoclopramide — empties the stomach, raises LOS tone |
| For each | Mechanism, dose, and adverse effects |
| State the negatives | What each drug does NOT do earns marks too |
| Class | Example | Dose | Mechanism | Adverse effects |
|---|---|---|---|---|
| Non-particulate antacid | Sodium citrate | 0.3 M, 30 mL orally, less than 10 minutes before surgery | Neutralises gastric acid already present. Water-soluble, faster onset than aluminium- or magnesium-containing antacids, but a short duration of action — timing matters | Belching; may cause a metabolic alkalosis in excess. No effect on gastric emptying |
| H2 receptor antagonist | Ranitidine | 50 mg IV | Specific, competitive antagonism at gastric parietal-cell H2 receptors, reducing the volume and acidity of gastric secretions | Cardiac arrhythmias with rapid IV administration; caution in porphyria (reports inconclusive); rarely thrombocytopenia, leucopenia, reversible LFT abnormalities |
| Prokinetic (dopamine D2 antagonist) | Metoclopramide | 10 mg IV | D2 antagonism together with a separate cholinergic action increases gastric emptying and lower oesophageal sphincter tone, coordinating gastric–pyloric–small intestinal motility | Extrapyramidal effects, sedation, hypotension and arrhythmias with rapid IV bolus |
What each drug does not do
- Sodium citrate — acts only on acid already secreted. No effect on gastric emptying or lower oesophageal sphincter tone. Short-lived, so it must be given close to induction to still be working.
- Ranitidine — reduces the volume and acidity of secretions at source. Also no effect on gastric emptying or LOS tone.
- Metoclopramide — the mechanistic odd one out. Does not reduce acid; it clears the stomach faster and raises the barrier to reflux.
Commonly lost: Some candidates stated that metoclopramide decreases LOS tone or reduces gastric acidity — the opposite of what it does.
Answers that earned nothing
| Drug | Why it earned no credit |
|---|---|
| Cimetidine | An older H2 antagonist that inhibits hepatic cytochrome P450 and is rarely used now. Ranitidine is its successor and the drug in current use |
| Erythromycin | A motilin-receptor agonist used as a prokinetic — not an acid-reducing or antacid drug, so outside this classification |
| Proton pump inhibitors | Not the expected answer: the question asks for drugs in common perioperative use, not every acid-suppressing agent |
Commonly lost: Many wrote at length about cimetidine for no credit, and some wrongly credited erythromycin as an aspiration-prophylaxis agent.
If this came up in the viva
Viva points
Name three drugs used for aspiration prophylaxis and how their mechanisms differ.
Answer
Sodium citrate, a non-particulate antacid, neutralises acid already in the stomach. Ranitidine, an H2 antagonist, reduces the volume and acidity of acid secreted by gastric parietal cells, with no effect on gastric emptying or lower oesophageal sphincter tone. Metoclopramide, a D2 antagonist and prokinetic, increases gastric emptying and lower oesophageal sphincter tone instead — three genuinely different mechanisms rather than three doses of the same idea.
Metoclopramide is used for both aspiration prophylaxis and PONV. Is that the same pharmacological action twice, or two different ones?
Answer
Two different mechanisms in the same molecule. The prokinetic effect that makes it useful for aspiration prophylaxis — increased gastric emptying and increased lower oesophageal sphincter tone — comes from a separate cholinergic action on the gut. The antiemetic effect is dopamine (D2) receptor antagonism at the CTZ, a distinct receptor and a distinct site. Attributing both actions to the same receptor is a common examiner-flagged error.
Metoclopramide and domperidone are both D2 antagonists. Why does only one of them cause extrapyramidal effects?
Answer
Metoclopramide crosses the blood–brain barrier and acts directly on central D2 receptors — the same property that produces its extrapyramidal and sedative effects. Domperidone shares the same D2-antagonist mechanism but does not cross the blood–brain barrier, so it is far less likely to cause them.