What is the dose and onset of action of hydralazine (Hydralazine) in managing severe hypertension in preeclampsia?

Hydralazine in Preeclampsia: Dosing and Onset of Action

Intravenous hydralazine has an onset of action of 10 minutes and should be administered at a starting dose of 5 mg IV, with additional doses of 5-10 mg given every 20-30 minutes as needed to a maximum of 25-30 mg. 1

Dosing Protocol for Hydralazine in Severe Preeclampsia

• Initial dose: 5 mg IV bolus
• Subsequent doses: 5-10 mg IV every 20-30 minutes
• Maximum total dose: 25-30 mg
• Duration of action: 12 hours
• Target blood pressure: Reduce systolic BP to <160 mmHg and diastolic BP to <110 mmHg

Pharmacodynamics and Monitoring

Hydralazine works as a direct vasodilator, causing preferential dilation of arterioles compared to veins. The blood pressure typically begins to fall within minutes after injection, with the maximal effect occurring between 10-80 minutes after administration 2. Close monitoring is essential as:

• The BP-lowering effect can be unpredictable
• The goal is to decrease mean arterial pressure by 15-25%
• Overly rapid BP reduction can compromise uteroplacental perfusion

Maternal and Fetal Considerations

When using hydralazine in preeclampsia, be aware of these important considerations:

• Maternal side effects: Headache, palpitations, tachycardia, nausea/vomiting, flushing
• Fetal concerns: Risk of fetal distress secondary to abrupt maternal hypotension; lower APGAR scores have been reported 1
• Monitoring requirements: Continuous fetal heart rate monitoring is necessary during treatment

Alternative First-Line Agents

While hydralazine is widely used in North America for severe preeclampsia, current guidelines suggest that IV labetalol may be preferred due to:

• More predictable blood pressure response
• Lower risk of maternal hypotension
• Fewer adverse effects on uteroplacental blood flow 1

A recent study from 2023 found that IV hydralazine achieved target blood pressure faster (45.8 vs 72.7 minutes) and required fewer doses compared to labetalol, though it had more maternal side effects 3.

Important Cautions

• Avoid continuous hydralazine infusions as they may cause rapid uncontrolled decreases in blood pressure leading to fetal distress 4
• Side effects of hydralazine (headache, palpitations) may mimic worsening preeclampsia, complicating clinical assessment 1
• Use with caution when combined with magnesium sulfate due to potential for enhanced hypotensive effects
• Hydralazine should not be added to infusion solutions and may discolor upon contact with metal 2

Clinical Decision Algorithm

1. Confirm severe hypertension: Sustained BP ≥160/110 mmHg for 15 minutes or more
2. Initiate treatment: Give hydralazine 5 mg IV bolus
3. Monitor response: Check BP every 10 minutes
4. Repeat dosing: If target BP not achieved after 20-30 minutes, give additional 5-10 mg
5. Consider alternative: If BP not controlled after 2-3 doses, consider switching to alternative agent (labetalol or nifedipine)
6. Transition to oral therapy: Once BP is controlled, transition to appropriate oral antihypertensive within 24-48 hours

By following this approach, you can effectively manage severe hypertension in preeclampsia while minimizing risks to both mother and fetus.