Antihypertensives in pregnancy
Antihypertensives and magnesium sulphate in pregnancy: methyldopa, labetalol, hydralazine
Blood pressure drugs in pregnancy for FCPS Part 1: how methyldopa, labetalol, nifedipine and hydralazine work, and when to use or avoid each. Covers magnesium sulphate mechanism, therapeutic levels, toxicity monitoring and reversal.
Principles
- The uteroplacental circulation has no autoregulation: placental flow follows maternal BP. Lower severe hypertension steadily, not abruptly; a sudden fall causes fetal bradycardia.
- Treat to prevent maternal stroke when BP reaches about 160/110 mmHg; severe systolic pressure is the main cause of intracranial haemorrhage.
- In pre-eclampsia plasma volume is already reduced by capillary leak, so diuretics are avoided except for pulmonary oedema.
- Avoid: ACE inhibitors and ARBs (fetal renal failure, oligohydramnios, skull hypoplasia), long-term atenolol (growth restriction) and diuretics.
- Chronic hypertension discovered in the first trimester (for example 180/90 mmHg at 9 weeks): stop any ACE inhibitor and use labetalol, nifedipine or methyldopa.
Drug comparison
| Drug | Mechanism | Use | Key adverse effects |
|---|---|---|---|
| Labetalol | Blocks beta-1, beta-2 and alpha-1 (beta:alpha about 3:1 oral, 7:1 IV) | First line oral; IV for severe hypertension | Bronchospasm (avoid in asthma), neonatal hypoglycaemia and bradycardia |
| Nifedipine | L-type calcium channel blocker; arteriolar dilatation | Oral first line or alternative; acute oral use | Headache, flushing, reflex tachycardia; interacts with magnesium |
| Methyldopa | Prodrug converted in the brain to alpha-methylnoradrenaline, a central alpha-2 agonist that cuts sympathetic outflow | Chronic hypertension; longest fetal safety record | Sedation, depression (stop within 2 days of delivery), positive Coombs test, hepatitis |
| Hydralazine | Direct arteriolar smooth muscle relaxant | IV for severe hypertension (for example 190/110 mmHg before emergency caesarean) | Reflex tachycardia, headache, palpitations, fluid retention; drug-induced lupus |
Labetalol
- Its alpha-1 blocking component lowers systemic vascular resistance without the compensatory vasoconstriction of a pure beta-blocker.
- It crosses the placenta. Beta-blockade blunts the baby's catecholamine surge at birth, which normally drives glycogenolysis in the neonatal liver, so neonatal hypoglycaemia can occur; this is not hyperinsulinism (unlike the infant of a diabetic mother). Capillary glucose is checked for 24 hours; bradycardia may also occur.
- In a woman with asthma use nifedipine (or hydralazine); even cardioselective beta-blockers are unsafe in severe asthma.
- IV labetalol is also the usual choice for a sudden intraoperative hypertensive surge once anaesthesia is confirmed adequate; esmolol or GTN are alternatives. In an asthmatic patient under anaesthesia, a vasodilator such as hydralazine or GTN avoids beta-blockade.
- Sotalol is the beta-blocker with class III (potassium channel) antiarrhythmic action; labetalol and carvedilol add alpha-blockade.
Methyldopa
- Taken up by adrenergic neurones and converted (by dopa decarboxylase and dopamine beta-hydroxylase) to alpha-methylnoradrenaline, which stimulates central alpha-2 receptors in the brainstem and reduces sympathetic outflow. It is not a dopamine agonist or a peripheral alpha-blocker.
- Peak effect takes 4–6 hours, so it is unsuitable for acute control.
- A positive direct antiglobulin test (IgG on red cells, a warm-type picture) occurs in 10–20% of users; true haemolysis is rare. Clonidine shares the central alpha-2 action but is not the usual choice in pregnancy and causes rebound hypertension if stopped suddenly.
Hydralazine
- Dilates arterioles with little venous effect (unlike GTN, a venodilator). The fall in BP unloads baroreceptors, causing reflex sympathetic tachycardia, palpitations and renin release.
- The reflex tachycardia can be blocked with a beta-blocker such as propranolol: this is physiological (functional) antagonism and a pharmacodynamic interaction, not pharmacokinetic. By contrast, naloxone reverses morphine by competitive antagonism at the same receptor.
- A rapid IV bolus can drop BP so sharply that the fetus becomes bradycardic.
- Drug-induced lupus with anti-histone antibodies, mainly in slow acetylators; kidneys and CNS are usually spared.
- Nitrates and sildenafil must not be combined (both raise cGMP): profound hypotension.
Magnesium sulphate
- Drug of choice to prevent and treat eclamptic seizures, better than diazepam or phenytoin. In severe pre-eclampsia (BP 168/90 with epigastric pain and blurred vision) it is given to prevent eclampsia.
- Mechanisms: blocks the NMDA receptor channel and calcium entry into neurons; causes cerebral vasodilatation; at the neuromuscular junction it reduces presynaptic calcium entry and acetylcholine release.
- Calcium antagonises magnesium: its actions are inhibited by calcium, which is why calcium gluconate reverses toxicity.
- Excreted almost entirely unchanged by the kidneys, so oliguria (under 100 mL in 4 hours) causes accumulation.
| Serum magnesium | Effect |
|---|---|
| 0.7–1.0 mmol/L (1.7–2.4 mg/dL) | Normal |
| 2–3.5 mmol/L (about 4.8–8.4 mg/dL) | Therapeutic (often quoted as 4–8 mg/dL) |
| About 5 mmol/L (12 mg/dL) | Loss of patellar reflexes (earliest warning) |
| 6–7.5 mmol/L (15–18 mg/dL) | Respiratory depression |
| Over 12 mmol/L (30 mg/dL) | Cardiac arrest |
- The most important bedside monitor is deep tendon reflexes (biceps jerk if an epidural blocks the knee), with respiratory rate and urine output hourly.
- Toxicity (CNS depression, depressed reflexes, hypotension, bradycardia, respiratory depression): stop the infusion, support airway and breathing, and give 10 mL of 10% calcium gluconate IV over about 10 minutes.
- Interactions: potentiates non-depolarising neuromuscular blockers; with nifedipine causes additive hypotension and weakness.
- Given before birth under about 30–32 weeks it reduces cerebral palsy (fetal neuroprotection); the baby may be briefly floppy. IV magnesium is also used in acute severe asthma and torsades de pointes.
Acute and long-term choices
| Situation | Choice |
|---|---|
| Severe hypertension, acute | IV labetalol, oral nifedipine or IV hydralazine |
| Chronic or gestational hypertension | Oral labetalol, nifedipine or methyldopa |
| Asthma | Nifedipine (avoid labetalol) |
| Postpartum | Switch from methyldopa; enalapril, nifedipine or labetalol (compatible with breastfeeding) |
| Seizure prophylaxis or treatment | Magnesium sulphate |
Remember For severe hypertension in pregnancy think labetalol, nifedipine or hydralazine; for fits think magnesium; for magnesium toxicity think calcium gluconate.
Exam trap Methyldopa is not for acute control and is stopped after delivery because of depression.
Exam trap Neonatal hypoglycaemia after labetalol is impaired glycogenolysis, not hyperinsulinism.
How FCPS asks about this
- Mechanism of methyldopa: its metabolite alpha-methylnoradrenaline stimulates central alpha-2 receptors.
- Therapeutic magnesium level: about 2–3.5 mmol/L (4–8 mg/dL); reflexes lost at about 5 mmol/L.
- Most important parameter to monitor for magnesium toxicity: deep tendon reflexes.
- Antidote for magnesium toxicity: 10 mL of 10% calcium gluconate IV.
- Drug for severe hypertension before emergency caesarean: IV labetalol or hydralazine.
- Why check glucose in a baby exposed to labetalol: beta-blockade impairs neonatal glycogenolysis.
- Antihypertensive to avoid in asthma: labetalol; use nifedipine.
Test yourself on this topic
5 of the 26 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A woman who took methyldopa throughout pregnancy for chronic hypertension delivers at 38 weeks. The postnatal team changes her to another antihypertensive within two days of birth. Which adverse effect of methyldopa is the main reason?
Answer: Depression
Methyldopa reduces central noradrenergic activity and is sedating; it can cause low mood and depression. The puerperium is already a time of high risk for mood disorders, so guidelines advise stopping methyldopa soon after delivery and using another agent (for example enalapril, labetalol or nifedipine). Haemolytic anaemia is rare, raised prolactin is a minor effect, and fluid retention is not the reason for switching.
Methyldopa after delivery
- Sedation and low mood
- Puerperium: high risk of depression
- Switch within 2 days of birth
- Positive DAT common, haemolysis rare
Remember Stop methyldopa after birth: protect the mother's mood.
Exam trap A positive direct antiglobulin test on methyldopa usually does not mean haemolysis.
Why the other options are wrong
- Hyperprolactinaemia Can occur but is not the reason for switching.
- Haemolytic anaemia Rare; a positive DAT is common but haemolysis is uncommon.
- Fluid retention Minor and not the usual reason.
- Bradycardia Not a typical problem with methyldopa.
Twenty minutes after IV hydralazine for severe pre-eclampsia, a woman's BP falls from 170/115 to 145/95 mmHg, but her pulse rises from 90 to 125/min and she has a throbbing headache and palpitations. What is the cause of the rise in pulse?
Answer: Baroreceptor-driven sympathetic reflex
Hydralazine lowers BP by dilating arterioles. The fall in arterial pressure reduces baroreceptor firing in the carotid sinus and aortic arch, so the vasomotor centre increases sympathetic outflow, raising heart rate and contractility, and renin release rises. The drug itself has no direct action on cardiac β receptors. Pairing a vasodilator with a β-blocker such as labetalol reduces this reflex.
Reflex tachycardia
- Arteriolar dilation, BP falls
- Less baroreceptor firing
- More sympathetic outflow
- Tachycardia, palpitations, renin rise
Remember Pure vasodilator: the heart speeds up to compensate.
Exam trap Hydralazine tachycardia is reflex, not a direct drug action on the heart.
Why the other options are wrong
- Direct β1 stimulation by hydralazine Hydralazine is not a β agonist.
- Hydralazine-induced hypocalcaemia It does not lower calcium.
- Release of histamine from mast cells Not the mechanism of hydralazine tachycardia.
- Early pulmonary oedema No breathlessness or crackles are described.
A woman at 33 weeks with pre-eclampsia has marked leg oedema and BP 158/104 mmHg. A visiting doctor suggests furosemide to treat both. Why are diuretics avoided for this purpose in pre-eclampsia?
Answer: Her plasma volume is already reduced
In pre-eclampsia, widespread endothelial damage causes capillary leak, so fluid moves into the tissues while the intravascular (plasma) volume is reduced and the blood is haemoconcentrated. A diuretic shrinks the plasma volume further, which can reduce uteroplacental perfusion and worsen renal function, while doing little for the oedema. Diuretics are reserved for pulmonary oedema.
Pre-eclampsia volume status
- Capillary leak: oedema
- Plasma volume reduced
- Haemoconcentration
- Diuretics only for pulmonary oedema
Remember Oedematous but dry inside the vessels.
Exam trap Leg oedema alone is common in normal pregnancy and is not a reason for diuretics.
Why the other options are wrong
- Furosemide is teratogenic at this stage Teratogenicity is not the issue at 33 weeks.
- Furosemide raises uterine artery resistance directly Its harm comes from volume depletion, not a direct arterial action.
- It causes fetal hyperkalaemia Loop diuretics tend to lower potassium.
- It worsens proteinuria by acting on the glomerulus Proteinuria in pre-eclampsia is from glomerular endotheliosis, not diuretics.
An hour after starting magnesium sulphate for eclampsia, a woman is drowsy with a respiratory rate of 7/min, absent reflexes and a pulse of 52/min. The infusion is stopped. Which drug is given next?
Answer: 10 mL of 10% calcium gluconate IV
Calcium antagonises magnesium at nerve terminals and cardiac cells, so IV calcium gluconate (1 g, as 10 mL of a 10% solution over about 10 minutes) rapidly reverses respiratory depression and bradycardia while ventilation is supported. Naloxone reverses opioids and has no effect on magnesium. Atropine may raise the heart rate but does not correct the respiratory and neuromuscular effects. Dextrose and bicarbonate are not antidotes.
Magnesium toxicity treatment
- Stop the infusion
- Support airway and breathing
- Give calcium gluconate 10%, 10 mL IV
- Over about 10 minutes
Remember Too much magnesium: give calcium.
Exam trap Calcium chloride also works but is more irritant to veins; gluconate is standard on the labour ward.
Why the other options are wrong
- 50 mL of 50% dextrose IV For hypoglycaemia, not magnesium toxicity.
- 0.4 mg naloxone IV Reverses opioids only.
- 1 mg atropine IV Treats bradycardia alone, not the neuromuscular block.
- 10 mL of 8.4% sodium bicarbonate IV Not an antidote to magnesium.
A woman on high-dose oral labetalol for pre-eclampsia delivers at 37 weeks. The paediatrician asks for the baby's capillary glucose to be checked for the first 24 hours. What is the reason?
Answer: β-blockade impairs neonatal glycogenolysis
Labetalol crosses the placenta. At birth the newborn depends on a catecholamine surge acting through β receptors to break down liver glycogen and mobilise fat while feeding is established. Residual β-blockade blunts this response, so neonatal hypoglycaemia (and sometimes bradycardia) can occur. Babies of mothers on β-blockers are therefore monitored for low glucose.
β-blockers and the newborn
- Labetalol crosses the placenta
- Blunts the catecholamine surge at birth
- Less glycogenolysis
- Risk: neonatal hypoglycaemia, bradycardia
Remember Mother on a β-blocker: check the baby's sugar.
Exam trap Neonatal hypoglycaemia here is not due to hyperinsulinism, unlike the infant of a diabetic mother.
Why the other options are wrong
- Labetalol stimulates fetal insulin secretion It does not stimulate insulin release.
- Labetalol causes neonatal adrenal suppression It has no corticosteroid effect.
- Labetalol blocks placental glucose transfer Placental glucose transfer is by GLUT1 and is not blocked.
- Labetalol causes neonatal hypothyroidism It does not affect the fetal thyroid.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Drugs in obstetrics and gynaecology, Antihypertensives and anticonvulsants in pregnancy: methyldopa, labetalol, hydralazine, magnesium sulphate
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