Epistemis

Gestational Antihypertensives

  • Critical Obstetric Therapies

Labetalol / Alfa-Methyldopa / Hydralazine (e.g. *Trandate*, *Aldomet*).

Pharmacokinetics

Pharmacokinetics

  • Labetalol: Bioavailability of 25% orally by first hepatic pass; onset of action in 5-10 minutes intravenously, with a plasma half-life of 4 to 6 hours.
  • Alpha-Methyldopa: Erratic absorption due to active transport of neutral amino acids. Bioavailability of 25%. The onset of action is slow, requiring 4 to 6 hours to demonstrate the maximum blood pressure drop due to its central synthesis mechanism. Plasma half-life of 2 hours, but prolonged clinical effect of up to 12-24 hours.
  • Hydralazine: Onset of action within 10-20 minutes by slow IV route, with a rapid duration of action of 3 to 6 hours.

Indicators and dose

Mechanism of Action and Specific Obstetric Indications

  • Labetalol (Combined Alpha and Beta Blocker): Competitively blocks β1, β2 and α1-adrenergic receptors in a mixed manner (blocking ratio of 1:3 for the oral route and 1:7 for the IV route). α1 blockade induces rapid generalized arterial vasodilation that reduces peripheral resistance without compromising uterine artery flow, while simultaneous β1 blockade prevents the development of compensatory reflex tachycardia, ideally stabilizing the hemodynamic profile. Indicated in gestational hypertensive crisis (preeclampsia) and chronic maintenance.
  • Alpha-Methyldopa (Selective α2 Central Agonist): It is a prodrug that is absorbed and transformed in the brain to α-methylnorepinephrine. It acts by stimulating central α2 inhibitory pre-synaptic adrenergic receptors in the brainstem, decreasing the outflow of sympathetic vasoconstrictor signals to the peripheral vasculature. Vascular tone progressively relaxes. Indicated for the chronic outpatient control of gestational hypertension and chronic hypertension in pregnancy.
  • Hydralazine (direct acting vasodilator): Directly relaxes the vascular smooth muscle of systemic resistance arterioles (possibly modulating potassium channels and hyperpolarizing the membrane), acutely and rapidly decreasing blood pressure. Indicated in pre-eclamptic hypertensive emergency crisis by slow intravenous route.

Precision Dosing

  • Chronic maintenance (Alpha-Methyldopa): 250 mg to 500 mg orally every 8 to 12 hours (maximum dose 3 g/day).
  • Chronic maintenance (Labetalol): 100 mg to 400 mg orally every 8 to 12 hours (maximum dose 2,400 mg/day).
  • Treatment of Hypertensive Crisis (BP 160/110 mmHg):

Crisis Protocol with IV Labetalol (Intermittent Bolus)

1. Administer first bolus of 20 mg IV slowly (over 2 minutes).
2. Reassess BP in 10 minutes. If 160/110 persists, inject a second bolus of 40 mg IV.
3. If the crisis persists after another 10 minutes, inject a third bolus of 80 mg IV (a fourth bolus of 80 mg IV can be repeated, without exceeding a maximum of 220-300 mg accumulated).
4. As soon as BP drops below 150/100, stop bolusing and start maintenance.

Pregnancy and Breastfeeding

Labetalol (Category C) and Alpha-Methyldopa (Category B) are the safest drugs validated by global guidelines for gestational hypertension. Safely compatible with breastfeeding, with minimal levels detected in milk.

Security

Contraindications and Dangers of Contraindicated Drugs in Pregnancy

Critical Alert: Total Contraindication of ACEI and ARA-II in Obstetrics

Angiotensin-converting enzyme inhibitors (ACE inhibitors, e.g., Enalapril, Captopril) and angiotensin II receptor antagonists (ARBs, e.g., Losartan, Valsartan) are absolutely contraindicated in the second and third trimesters of pregnancy due to nephrotoxicity and severe fetal teratogenesis:

  • Fetal Renal Dysgenesis: The fetal renin-angiotensin-aldosterone system is essential to maintain renal glomerular perfusion of the fetus. Its irreversible blockade suppresses fetal renal clearance, inducing anuria and severe oligohydramnios.
  • Fetal Cranial Vault Hypoplasia and IUGR: Secondary to hypotension and generalized gestational bone ischemia.
  • Secondary Potter syndrome: Intrauterine fetal compression due to the absence of amniotic fluid, causing lethal pulmonary hypoplasia and joint deformities.

Adverse Effects (ADR)

  • Labetalol: Transient maternal or fetal bradycardia, extreme fatigue, orthostatic dizziness, and minor rebound neonatal hypoglycemia.
  • Alpha-Methyldopa: Deep sedation and severe drowsiness (very common after starting the dose due to alteration of central catecholamines), minor psychotic depression, autoimmune hemolytic anemia (direct Coombs positive in 10-20% of users after months of treatment) and transient elevation of transaminases.
  • Hydralazine: Intense pulsating headache, rapid palpitations due to reflex tachycardia, nausea, vomiting and minor sodium retention. It can precipitate severe hypotension with loss of fetal well-being if injected quickly.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

PreviousMaternal-Fetal Antimicrobials
System
Gynecology and Obstetrics
Cluster
Critical Obstetric Therapies
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