What is the role of hydralazine in managing severe hypertension?

Hydralazine for Severe Hypertension

Hydralazine is an acceptable second-line agent for acute severe hypertension in pregnancy (BP ≥160/110 mmHg), but labetalol and nifedipine are preferred first-line options due to hydralazine's unpredictable response, prolonged duration of action, and higher rates of maternal adverse effects. 1

Obstetric Hypertensive Emergencies

First-Line Treatment Options

• Severe hypertension (≥160/110 mmHg) requires urgent treatment within 60 minutes in a monitored setting, with acceptable first-line agents including oral nifedipine or intravenous labetalol. 1
• Hydralazine (5 or 10 mg IV) is considered a standard antihypertensive agent but is typically reserved as an alternative when first-line agents are unavailable or contraindicated. 1

Hydralazine Dosing in Pregnancy

• The recommended regimen is 5 mg IV bolus initially, followed by 10 mg every 20-30 minutes to a maximum of 25 mg, repeated in several hours as necessary. 2
• Blood pressure typically begins to decrease within 10-30 minutes, with effects lasting 2-4 hours. 2, 3
• Treatment should be initiated within 60 minutes of the first severe hypertension episode to meet quality metrics. 1

Comparative Effectiveness

Advantages of Hydralazine

• Hydralazine achieves target BP (≤150/100 mmHg) faster than labetalol, with a mean time of 45.8 minutes versus 72.7 minutes. 4
• Fewer doses are required compared to labetalol (mean 1.72 vs 3.72 doses), with 45% of patients achieving target BP with a single dose. 4
• Overall success rate of 91.7% in achieving target BP. 4

Disadvantages and Safety Concerns

• Hydralazine is associated with significantly more maternal adverse effects including palpitations, maternal tachycardia, hypotension, and oliguria compared to other agents. 5, 6
• Meta-analysis shows increased risk of cesarean sections (RR 1.30), placental abruption (RR 4.17), maternal oliguria (RR 4.00), and adverse fetal heart rate changes (RR 2.04). 6
• The unpredictable blood pressure response and prolonged duration of action (2-4 hours) make titration difficult and increase risk of excessive hypotension. 2, 7, 3
• Continuous IV infusion should be avoided due to reports of rapid uncontrolled BP decline causing fetal distress in 5 of 7 patients in one study. 8

Clinical Considerations

When to Use Hydralazine

• Hydralazine is specifically indicated for hypertensive emergencies in eclampsia when first-line agents are unavailable. 2
• It may be considered when labetalol is contraindicated (e.g., severe asthma, heart block) or nifedipine cannot be administered orally. 1

Monitoring Requirements

• Hold or reduce antihypertensive drugs if diastolic BP falls below 80 mmHg to avoid hypoperfusion. 7
• Monitor for reflex tachycardia, headache, flushing, and vomiting—common adverse effects of hydralazine. 2, 3
• Assess for symptoms of hypoperfusion including dizziness, chest pain, or altered mental status. 7
• Wait 2-4 hours to assess full effect before administering additional doses due to prolonged duration of action. 2, 7

Important Contraindications and Precautions

• Use with extreme caution in patients with coronary artery disease, as myocardial stimulation can precipitate anginal attacks or myocardial infarction. 3
• Avoid in patients with mitral valvular disease, as the hyperdynamic circulation may increase pulmonary artery pressure. 3
• Monitor for peripheral neuritis (paresthesia, numbness, tingling) and consider pyridoxine supplementation if symptoms develop. 3
• Complete blood counts and antinuclear antibody titers are indicated during prolonged therapy due to risk of blood dyscrasias and drug-induced lupus. 3

Non-Obstetric Hypertensive Emergencies

Hydralazine is no longer recommended as a first-line agent for non-obstetric hypertensive emergencies due to its unpredictable response and prolonged action. 1, 7

• Preferred alternatives include nicardipine (5-15 mg/h IV), labetalol (20-80 mg IV bolus every 10 min), or clevidipine for more predictable control with shorter half-lives. 2, 7
• The 2007 European Society of Cardiology guidelines specifically state that IV hydralazine should no longer be considered for hypertensive emergencies due to more perinatal adverse effects compared to other drugs. 1

Key Pitfalls to Avoid

• Never use continuous IV hydralazine infusion in pregnancy—bolus dosing only to prevent uncontrolled hypotension and fetal distress. 8
• Do not administer additional doses before waiting at least 20 minutes to assess response, given the delayed and prolonged effect. 2, 3
• Avoid combining with MAO inhibitors or diazoxide due to risk of profound hypotensive episodes. 3
• Do not use as first-line therapy when labetalol or nifedipine are available and appropriate. 1, 6