Labetalol is a combined a- and B-adrenoceptor antagonist.The proportion of B-blockade to a-blockade when given orally is 3:1,and 7:1 when given IV. It lowers the blood pressure by blocking a-adrenoceptors in arterioles and thereby reduces the peripheral resistance. Concurrent B-blockade protects the heart from reflex sympathetic drive normally induced by peripheral vasodilatation.
Uses
All grades of hypertension, particularly useful when there is tachycardia
Pre-eclampsia
Contraindications
Asthma (worsens)
Cardiogenic shock (further myocardial depression)
Second- or third-degree heart block
Administration
• Orally: 100–800 mg 12 hourly
• IV bolus: 10–20 mg over 2 min, repeat with 40 mg at 10-min intervals as necessary, up to 300 mg in 24 hours
Maximum effect usually occurs within 5 min and the duration of action is usually 6 hours
• IV infusion: 20–200 mg/h
Rate: 4–40 ml/h (20–200 mg/h), adjust rate until satisfactory decrease in BP obtained
Available in 20-ml ampoules containing 100 mg labetalol (5 mg/ml)
Draw up three ampoules (60 ml) into a 50-ml syringe
How not to use labetalol
Incompatible with sodium bicarbonate
Adverse effects
Postural hypotension
Bradycardia
Heart failure
Cautions
Rare reports of severe hepatocellular damage (usually reversible)
Presence of labetalol metabolites in urine may result in false-positive test for phaeochromocytoma
Organ failure
Hepatic: reduce dose
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