Uterine Pharmacology
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Direct answer
Uterine pharmacology splits into drugs that contract the uterus and drugs that relax it. The contractors — oxytocin for induction and postpartum haemorrhage, ergometrine for haemorrhage control, and the prostaglandins (misoprostol, dinoprostone, carboprost) for ripening, termination and refractory bleeding — act on term-pregnant or postpartum myometrium. The relaxers (tocolytics) — nifedipine, atosiban and indomethacin — delay preterm labour for 48 hours to complete steroid lung maturation and in-utero transfer. Every drug in this chapter is judged by one question: does the patient need the baby delivered, the bleeding stopped, or the pregnancy prolonged?
What you must remember
- Oxytocin is the posterior pituitary hormone; infusion for induction (titrated in milliunits), 5 units IV/IM or 10 units per litre infusion for postpartum haemorrhage; half-life 3–5 minutes so it stops quickly when the infusion stops.
- High-dose prolonged oxytocin has antidiuretic-like action — water intoxication with convulsions is the classical complication.
- Ergometrine 0.2–0.5 mg IM contracts the uterus durably but is contraindicated in hypertension, pre-eclampsia and cardiac disease because it vasoconstricts; never before delivery of the fetus.
- Misoprostol (PGE1) 800 micrograms sublingual is the field treatment for postpartum haemorrhage; it also ripens the cervix and, with mifepristone, terminates pregnancy — hence contraindicated in pregnancy intended to continue.
- Dinoprostone (PGE2) gel or pessary ripens the unfavourable cervix; carboprost (15-methyl PGF2alpha) is the third-line postpartum haemorrhage drug, avoided in asthmatics (bronchoconstriction).
- Tocolysis buys 48 hours: nifedipine (calcium channel blockade) is first-line in most units; atosiban (oxytocin antagonist) is the utero-selective alternative; indomethacin works before 32 weeks but risks premature ductus arteriosus closure and oligohydramnios.
- Ritodrine and other beta-2 tocolytics are largely abandoned — maternal tachyarrhythmia and pulmonary oedema.
- Magnesium sulphate tocolysis is weak; its obstetric role is eclampsia seizure prophylaxis (and fetal neuroprotection under 32 weeks).
- Active management of the third stage — oxytocin 10 units IM at delivery of the anterior shoulder — halves postpartum haemorrhage risk.
How to work through a postpartum haemorrhage
A woman bleeds heavily five minutes after a vaginal delivery, and the drug sequence is the exam. First, uterine massage and emptying the bladder — an atonic uterus is mechanical before it is pharmacological, and the four Ts (tone, tissue, trauma, thrombin) run the differential. Second, oxytocin 5 units IV slowly (or 10 units IM where no line runs) — fastest onset, safest in hypertension. Third, if the uterus remains boggy, ergometrine 0.25 mg IM — unless pre-eclampsia, hypertension or cardiac disease bars it, and always after the placenta is out. Fourth, misoprostol 800 micrograms sublingual where injections are unavailable — the reason community births in Indian programmes carry misoprostol. Fifth, carboprost 250 micrograms IM every 15 minutes up to eight doses — withheld in asthma. Sixth, tranexamic acid 1 g IV within three hours reduces death from bleeding in the WOMAN trial logic and belongs early, not last. Between the steps, bimanual compression and the operating theatre decision run in parallel — the pharmacology is a ladder, not a menu, and each rung is excluded by one contraindication.
Where students slip
The classic error is ergometrine (or any uterotonic other than oxytocin) before the fetus is delivered — ergot alkaloids cause sustained tetanic contraction that traps the fetus and ruptures the uterus; oxytocin alone is the inducing agent and even it requires a dilute, titrated infusion. The second slip is misreading indications: misoprostol for a wanted pregnancy in a patient also taking it for NSAID-induced ulcer prophylaxis — the answer is that women of childbearing age on misoprostol must be pregnant-excluded or counseled, a pharmacovigilance favourite. Third, tocolytic selection by gestational age: indomethacin below 32 weeks only, nifedipine first-line generally, atosiban when cardiovascular disease makes beta-2 agonists and nifedipine risky. Finally, remembering that the goal of tocolysis is 48 hours of steroids and transfer, not prevention of preterm birth itself.
Frequently asked questions
Why is oxytocin preferred over ergometrine for labour induction?
Oxytocin at low doses produces rhythmic contractions with relaxation between, allowing fetal oxygenation; ergometrine produces tetanic sustained contraction that is fetal-lethal and is reserved for postpartum use.
Which postpartum haemorrhage drug is contraindicated in asthma and why?
Carboprost, a PGF2alpha analogue, causes bronchoconstriction and can precipitate severe bronchospasm in asthmatics; misoprostol and oxytocin are safe alternatives.
What is the role of mifepristone and misoprostol in medical termination?
Mifepristone 200 mg orally blocks progesterone receptors and primes the decidua; misoprostol 36–48 hours later completes evacuation — the standard medical abortion regimen up to 63 days in Indian programme practice.
Why has nifedipine replaced beta-2 agonist tocolytics?
It delays delivery comparably with fewer maternal cardiovascular events — no maternal tachycardia, arrhythmia or pulmonary oedema — and costs less, with monitoring for hypotension and fetal heart rate.
What is the 48-hour purpose of tocolysis?
To complete a course of antenatal corticosteroids for fetal lung maturation and enable in-utero transfer to a neonatal unit — not to prevent preterm birth, which no tocolytic achieves beyond that window.