SN6 SHORT NOTE 5M ★ High-yield

Classify uterine stimulants and relaxants. Add a note on magnesium sulphate in eclampsia.

Pharmacology · 2nd Professional — Para-Clinical

Uterine Stimulants (oxytocics)

Drug	Mechanism	Uses and cautions
Oxytocin	Acts on oxytocin receptors, which increase greatly in number toward term; raises intracellular calcium giving rhythmic coordinated contractions with relaxation between	Induction and augmentation of labour; prevention and treatment of post-partum haemorrhage (the first-line uterotonic); milk ejection. Given by titrated infusion. Causes water intoxication with hyponatraemia and convulsions at high dose (it resembles ADH), and hypotension with rapid bolus
Ergometrine and methylergometrine	Ergot alkaloid causing sustained tonic contraction of the uterus	Post-partum haemorrhage and to prevent it in the third stage. Never used before delivery, since sustained contraction would cause fetal asphyxia and uterine rupture. Contraindicated in hypertension and pre-eclampsia (it causes vasoconstriction), and in cardiac disease
Prostaglandins	PGE2 (dinoprostone), PGE1 (misoprostol), PGF2alpha (carboprost)	Cervical ripening and induction; medical abortion (mifepristone then misoprostol); post-partum haemorrhage. Misoprostol is heat-stable, oral and cheap — a major advantage in rural India where oxytocin cannot be refrigerated. Carboprost is contraindicated in asthma
Carbetocin	A long-acting oxytocin analogue	Single dose after caesarean section; a heat-stable formulation removes the cold-chain requirement
Tranexamic acid	Antifibrinolytic — not a uterotonic	Given within 3 hours reduces death from post-partum haemorrhage; cheap and widely available

CLINICAL PEARL

Oxytocin and ergometrine produce different kinds of contraction, and that is why one is used before delivery and the other never is. Oxytocin gives rhythmic contractions with relaxation between, during which the placenta is perfused — compatible with a living fetus. Ergometrine gives a sustained tonic contraction with no relaxation, which would asphyxiate the fetus and could rupture the uterus. Hence ergometrine only after the baby is out.

Uterine Relaxants (tocolytics)

Drug	Mechanism	Notes
Nifedipine	Calcium channel blockade	Now the tocolytic of choice in many settings — oral, cheap and effective; causes flushing, headache and hypotension
Atosiban	Oxytocin receptor antagonist	Best tolerated and most selective; expensive
Beta2 agonists	Ritodrine, isoxsuprine, salbutamol, terbutaline	Largely abandoned — maternal tachycardia, arrhythmia, hyperglycaemia, hypokalaemia and pulmonary oedema, which has caused maternal deaths
Indomethacin	Prostaglandin synthesis inhibition	Effective but risks premature closure OF the ductus arteriosus and oligohydramnios; used only before 32 weeks and briefly
Magnesium sulphate	Calcium antagonism at the neuromuscular junction	A poor tocolytic, but given for fetal neuroprotection before preterm delivery, and it is the treatment of choice for eclampsia

The purpose of tocolysis is not to prolong pregnancy indefinitely, but to gain 48 hours — enough for antenatal corticosteroids to mature the fetal lungs and for transfer to a unit with neonatal facilities. Judged against that aim, all the agents perform adequately and the choice rests on maternal safety

Magnesium Sulphate in Eclampsia

The drug of choice for preventing and treating eclamptic seizures, superior to diazepam and phenytoin in large trials
Monitoring during infusion — knee (patellar) jerks must be present, respiratory rate above 12 to 16, and urine output adequate; loss of reflexes is the earliest sign of toxicity
Toxicity progresses from loss of reflexes to respiratory depression and then cardiac arrest
Antidote: intravenous calcium gluconate, which must be immediately available whenever magnesium is infused
Excreted renally, so the dose is reduced in oliguria
Definitive treatment of eclampsia is delivery; magnesium controls seizures while that is arranged

Applied Aspects

Active management of the third stage prevents post-partum haemorrhage — a uterotonic at delivery of the anterior shoulder, controlled cord traction and uterine massage; this is among the most effective interventions in maternal health
Post-partum haemorrhage remains a leading cause of maternal death in India, and heat-stable misoprostol and carbetocin address the specific problem of maintaining a cold chain for oxytocin in rural facilities
Never give ergometrine to a hypertensive or pre-eclamptic woman
Give tranexamic acid early — within 3 hours — in post-partum haemorrhage; benefit falls sharply with delay
Oxytocin must be titrated and monitored; uterine hyperstimulation causes fetal distress and rupture, particularly in a scarred uterus
Misuse of oxytocin outside hospitals to hasten labour is a recognised and dangerous practice in parts of India, causing rupture and fetal death; it is restricted by law

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