Gestational Antihypertensives

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 the receptors β₁, β₂ y α₁-adrenergic receptors in a mixed pattern (blockade ratio of 1:3 orally and 1:7 intravenously). α blockade₁ induces rapid generalized arterial vasodilation, reducing peripheral resistance without compromising uterine artery flow, while simultaneous β blockade₁ prevents compensatory reflex tachycardia, stabilizing the hemodynamic profile. Indicated in gestational hypertensive crisis (preeclampsia) and chronic maintenance.
• Alpha-methyldopa (selective α₂ Central): A prodrug absorbed and converted in the brain to α-methylnorepinephrine. It stimulates inhibitory presynaptic adrenergic α receptors₂ in the brainstem, reducing sympathetic vasoconstrictor signals to the peripheral vasculature. Vascular tone relaxes progressively. Indicated for outpatient chronic control of gestational hypertension and chronic hypertension in pregnancy.
• Hydralazine (Direct-acting vasodilator): Directly relaxes vascular smooth muscle in systemic resistance arterioles (possibly through potassium channel modulation and membrane hyperpolarization), rapidly reducing blood pressure. Indicated in preeclamptic acute hypertensive emergency via slow intravenous administration.

Precision Dosing

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

IV Labetalol Crisis Protocol (Intermittent Bolus)

1. Administrar primer bolo de 20 mg IV lento (en 2 minutos).
2. Reassess BP after 10 minutes. If it remains ≥ 160/110, administer a second bolus of 40 mg IV.
3. If the crisis persists after another 10 minutes, administer a third bolus of 80 mg IV (a fourth 80 mg IV bolus may be repeated, without exceeding a cumulative maximum of 220-300 mg).
4. As soon as BP falls below 150/100, stop the boluses and start maintenance therapy.

Pregnancy and Breastfeeding

Labetalol (Category C) and Alpha-Methyldopa (Category B) are the safest guideline-supported agents for gestational hypertension. Compatible with breastfeeding with minimal excretion into breast milk.

Security

Contraindications and Dangers of Contraindicated Drugs in Pregnancy

Critical Alert: Total Contraindication of ACEI and ARB 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.