Drug Classes of Labetalol, Hydralazine, and Nifedipine
Labetalol is a combined alpha-1 and non-selective beta-adrenergic blocker, hydralazine is a direct-acting arterial vasodilator, and nifedipine is a dihydropyridine calcium channel blocker. 1, 2, 3
Labetalol - Combined Alpha and Beta Blocker
- Labetalol combines selective, competitive alpha-1-adrenergic blocking and non-selective, competitive beta-adrenergic blocking activity in a single agent 1
- The ratio of alpha to beta blockade is approximately 1:3 following oral administration and 1:7 following intravenous administration 1
- It produces dose-related falls in blood pressure without reflex tachycardia, presumably through its mixture of alpha and beta-blocking effects 1
- Labetalol is considered a first-line agent for severe hypertension in pregnancy, with dosing of 20 mg IV bolus, then 40 mg 10 minutes later, followed by 80 mg every 10 minutes for 2 additional doses to a maximum of 220 mg 4
- For non-severe hypertension in pregnancy, oral labetalol is recommended as a first-line option 4
Hydralazine - Direct-Acting Arterial Vasodilator
- Hydralazine is a direct-acting peripheral arterial vasodilator that works on vascular smooth muscle 2
- Unlike beta-blockers or calcium channel blockers, hydralazine does not have receptor-mediated activity but directly relaxes arterial smooth muscle 2
- For acute severe hypertension in preeclampsia, the recommended dosing is 5 mg IV bolus, then 10 mg every 20 to 30 minutes to a maximum of 25 mg, repeated in several hours as necessary 4, 5
- Hydralazine causes more sympathetic activation than calcium antagonists, leading to reflex tachycardia 6
- It is considered a first-line agent for severe hypertension in pregnancy alongside labetalol and nifedipine 4
- For non-severe chronic hypertension in pregnancy, hydralazine may be used as a second or third-line agent 4
Nifedipine - Dihydropyridine Calcium Channel Blocker
- Nifedipine is a calcium ion influx inhibitor (calcium channel blocker) that inhibits transmembrane influx of calcium ions into vascular smooth muscle and cardiac muscle 3
- It acts as a peripheral arterial vasodilator by binding to voltage-dependent channels in vascular smooth muscle, resulting in inhibition of calcium influx 3
- For acute severe hypertension in preeclampsia, oral immediate-release nifedipine is dosed at 10 mg, repeated every 20 minutes to a maximum of 30 mg 4
- Caution is warranted when using nifedipine with magnesium sulfate due to risk of precipitous blood pressure drop 4
- Long-acting nifedipine is recommended as a first-line agent for non-severe hypertension in pregnancy 4
- Extended-release formulations should never be crushed, as this destroys the controlled-release mechanism and can result in rapid bioavailability of the total drug amount, potentially causing severe hypotension 7
Clinical Context for Pregnancy Hypertension
- All three agents are considered acceptable first-line therapies for severe hypertension in pregnancy (BP >160/110 mmHg), which requires urgent treatment within 30-60 minutes 4
- For non-severe hypertension in pregnancy, methyldopa, labetalol, and long-acting nifedipine are preferred first-line agents, while hydralazine is typically reserved for acute management 4
- ACE inhibitors, angiotensin receptor blockers, and spironolactone are contraindicated in pregnancy due to risk of fetal damage 4