Labetelol
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Labetalol
Mechanism of Action
Labetalol is a combined α₁- and non-selective β-adrenergic receptor antagonist. Its key property is its dual blockade:
| Receptor | Effect |
|---|---|
| β₁ (cardiac) | ↓ Heart rate, ↓ contractility, ↓ AV conduction |
| β₂ (vascular/bronchial) | Bronchospasm risk; blocks vasodilatory β₂ |
| α₁ (vascular) | Peripheral vasodilation → ↓ SVR |
The α:β blockade ratio is approximately 1:5–1:7 orally and 1:3 intravenously. This means beta-blockade predominates, but the alpha component provides vasodilation that prevents reflex tachycardia — a key advantage over pure beta-blockers.
Pharmacokinetics
- Bioavailability: ~25% oral (extensive first-pass metabolism)
- Onset: IV — 2–5 min; oral — 20 min to 2 hours
- Duration: IV — 2–4 hours; oral — 8–12 hours
- Metabolism: Hepatic (glucuronide conjugation)
- Half-life: ~6–8 hours
- Elimination: Primarily renal (metabolites); dose adjustment in hepatic impairment
Clinical Indications
1. Hypertension (Chronic)
- Oral dosing: 100 mg twice daily, titrated up to 2400 mg/day in divided doses
- Useful in patients where both heart rate control and vasodilation are desired
2. Hypertensive Emergency (IV)
- Bolus: 20 mg IV over 2 minutes; may repeat 40–80 mg every 10 min (max cumulative 300 mg)
- Infusion: 0.5–2 mg/min, titrated to BP response
- Advantages: Rapid onset, titratable, does not cause reflex tachycardia (Harrison's, p. 2087)
3. Hypertension in Pregnancy / Preeclampsia
- First-line oral antihypertensive in pregnancy (alongside nifedipine and methyldopa) per ESC/ESH guidelines
- Preferred over atenolol (which causes fetal growth restriction)
- Safe in breastfeeding (Management of Elevated Blood Pressure and Hypertension, p. 59)
4. Aortic Dissection
- IV labetalol is a first-line agent — reduces both heart rate and BP simultaneously (Harrison's, p. 2106)
- Target: HR < 60 bpm, SBP 100–120 mmHg
5. Pheochromocytoma Crisis (IV)
- IV labetalol (1–2 mg/kg bolus over 1 min followed by infusion) can manage adrenergic crisis with the advantage of avoiding tachycardia via beta-blockade
- Caution: Should NOT be used as initial/sole therapy for pheochromocytoma — its α:β ratio is insufficient to prevent paradoxical hypertension from unopposed α-stimulation; alpha-blockade must be established first (ESC Guidelines, p. 72; PPGL Surveillance Guidelines, p. 11)
6. Other
- Ischemic heart disease (rate control)
- Sympathomimetic overdose/toxicity
Contraindications
| Absolute | Relative |
|---|---|
| Decompensated heart failure | Mild-moderate reactive airway disease |
| Cardiogenic shock | Peripheral vascular disease |
| Severe bradycardia / high-degree AV block | Diabetes (masks hypoglycemia symptoms) |
| Sick sinus syndrome | Chronic obstructive pulmonary disease |
| Asthma (significant bronchospasm risk) | — |
Adverse Effects
- Cardiovascular: Bradycardia, orthostatic hypotension, heart block
- Respiratory: Bronchospasm (non-selective β₂ blockade)
- CNS: Fatigue, dizziness, headache
- Metabolic: Blunted hypoglycemic awareness in diabetics
- Hepatic: Rare drug-induced liver injury (hepatotoxicity) — monitor LFTs with long-term use
- Scalp tingling/paresthesias: A characteristic complaint reported by some patients
- Abrupt withdrawal can precipitate rebound hypertension or angina
Special Populations
| Population | Consideration |
|---|---|
| Pregnancy | First-line; safe (avoid atenolol) |
| Renal impairment | No dose adjustment needed |
| Hepatic impairment | Reduce dose; heavily hepatically metabolized |
| Elderly | Start low; orthostatic hypotension risk |
| Pheochromocytoma | IV use only after adequate alpha-blockade is established |
| Heart failure | Avoid in decompensated HF; stable HF — use with caution |
Key Clinical Pearls
- Unlike pure beta-blockers, labetalol does not increase SVR and typically does not worsen peripheral vascular disease as much
- It does not cause reflex tachycardia — useful in aortic dissection and post-op hypertension
- In pregnancy, it is preferred IV agent for acute severe hypertension alongside hydralazine and nifedipine
- Do not use as initial monotherapy for pheochromocytoma — always establish alpha-blockade first to avoid paradoxical hypertensive crisis
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