Drugs acting on uterus

Which of the following is the most potent uterotonic agent for controlling postpartum haemorrhage due to uterine atony?

• Oxytocin

• Ergometrine

• Carboprost tromethamine

• Misoprostol

Answer explanation

• Correct (C): Carboprost is a prostaglandin F₂α analogue with potent uterotonic effects, especially in refractory postpartum haemorrhage. Acts via Gq protein–coupled receptors → ↑IP₃/DAG → ↑intracellular Ca²⁺ → strong myometrial contraction.

• A (Oxytocin): First-line agent for uterine atony; less potent than carboprost in refractory cases. Acts via Gq receptors.

• B (Ergometrine): Potent vasoconstrictor and uterotonic, but less effective in carboprost-resistant cases; contraindicated in hypertension/preeclampsia.

• D (Misoprostol): PGE₁ analogue, slower onset, useful where injectables unavailable.

Which receptor subtype mediates the uterotonic effect of oxytocin?

• Gs-coupled β₂ receptors

• Gq-coupled oxytocin receptors

• Gi-coupled prostaglandin receptors

• Voltage-gated calcium channels

Answer explanation

• Correct (B): Oxytocin receptors are Gq/11 protein–coupled → activate phospholipase C → ↑IP₃/DAG → ↑Ca²⁺ release → myometrial contraction.

• A: β₂ receptors mediate relaxation (tocolysis).

• C: PGF₂α receptors are Gq, but not the main pathway for oxytocin.

• D: Voltage-gated Ca²⁺ channels are involved in contraction but not directly receptor-mediated by oxytocin.

Which drug used for uterine relaxation acts primarily via β₂-adrenoceptor stimulation?

• Atosiban

• Ritodrine

• Nifedipine

• Indomethacin

Answer explanation

• Correct (B): Ritodrine is a β₂-selective agonist → ↑cAMP → ↓intracellular Ca²⁺ → myometrial relaxation.

• A: Atosiban is an oxytocin receptor antagonist.

• C: Nifedipine is a calcium channel blocker (tocolytic).

• D: Indomethacin is a COX inhibitor (↓prostaglandin synthesis)

Which is the main side effect of oxytocin when administered as a rapid IV bolus?

• Bronchospasm

• Severe hypotension

• Seizures

• Hypertension

Answer explanation

• Correct (B): Rapid IV bolus → vasodilation via vascular smooth muscle Gq activation + reflex tachycardia → severe hypotension, especially under regional anaesthesia.

• A: Not common with oxytocin.

• C: Seizures are not typical.

• D: Hypertension is associated with ergometrine, not oxytocin.

Which uterotonic is contraindicated in patients with severe asthma?

• Oxytocin

• Ergometrine

• Carboprost

• Misoprostol

Answer explanation

• Correct (C): Carboprost (PGF₂α) can cause bronchospasm via smooth muscle constriction in airways.

• A: Oxytocin has minimal bronchoconstrictive effect.

• B: Ergometrine causes vasoconstriction but not significant bronchospasm.

• D: Misoprostol generally safe in asthma.

Which is the mechanism of action of atosiban?

• β₂-receptor agonism

• Voltage-gated Ca²⁺ channel blockade

• Oxytocin and vasopressin V₁a receptor antagonism

• COX-2 inhibition

Answer explanation

• Correct (C): Atosiban competitively inhibits oxytocin and vasopressin V₁a receptors → ↓myometrial contractions.

• A: β₂ agonists cause relaxation but not via antagonism.

• B: Ca²⁺ channel blockers relax uterus but by direct channel blockade.

• D: COX-2 inhibition decreases prostaglandin synthesis

A woman with postpartum haemorrhage after vaginal delivery has preeclampsia. Which drug should be avoided?

Answer explanation

Ergometrine causes potent vasoconstriction and can precipitate severe hypertension in preeclampsia.

Which tocolytic drug is associated with risk of maternal pulmonary oedema?

Answer explanation

β₂ agonists like ritodrine cause sodium and water retention, ↑capillary permeability, and tachycardia → risk of pulmonary oedema.

Which uterotonic has the fastest onset when given intravenously?

Answer explanation

• Oxytocin acts within seconds when given IV, with a short half-life (~3–5 min).

• B: Ergometrine onset is ~1–5 min IV.

• C: Misoprostol onset is slower (oral/sublingual ~8–16 min).

• D: Carboprost IM onset is ~2–5 min.

Which drug inhibits prostaglandin synthesis and is used as a tocolytic, but can cause premature closure of the fetal ductus arteriosus?

Answer explanation

Indomethacin is a non-selective COX inhibitor → ↓prostaglandin synthesis → uterine relaxation, but can cause fetal ductal constriction and oligohydramnios.