Duration of Action of Hydralazine
Intravenous hydralazine has a duration of action of 2-4 hours, with blood pressure beginning to decrease within 10-30 minutes after administration. 1
Pharmacokinetic Profile
Intravenous Administration
- The hypotensive effect of IV hydralazine persists for 2-4 hours after administration, though some guidelines cite a range of 1-4 hours 1, 2
- Blood pressure reduction begins within 10-30 minutes of IV administration 1
- IV hydralazine is typically administered as an initial 10 mg via slow IV infusion and can be repeated every 4-6 hours as needed 1
Oral Administration
- After oral administration, peak plasma levels are reached at 1-2 hours 3
- Plasma levels of hydralazine decline with a half-life of 3-7 hours 3
- The duration of action for oral hydralazine varies significantly based on renal function and acetylator status 4
Factors Affecting Duration of Action
Renal Function Impact
- In patients with creatinine clearance <35 ml/min, the duration of action is significantly prolonged to 14.3 ± 1.4 hours compared to 7.9 ± 0.5 hours in patients with adequate renal function 4
- This represents nearly double the duration and requires adjustment of dosing intervals 4
Acetylator Status
- Hydralazine undergoes polymorphic acetylation, with slow acetylators having higher plasma levels and longer duration of action 3
- Slow acetylators generally require lower doses to maintain blood pressure control 3
- Fast acetylators show more pronounced waning of hypotensive effect at 24 hours with once-daily dosing 5
Clinical Dosing Implications
Repeat Dosing Intervals
- For IV administration in non-obstetrical patients, dosing every 4-6 hours is appropriate based on the 2-4 hour duration of action 1
- For obstetrical patients with severe hypertension, doses of 5-10 mg IV can be repeated every 20-30 minutes as needed 2
- Patients should be monitored throughout the 2-4 hour period for potential hypotensive effects 1
Important Caveats
- The blood pressure response to hydralazine is unpredictable and highly variable, making it less desirable as a first-line agent for acute treatment in most patients 6, 2
- Changes in systolic blood pressure are related to baseline values but remain highly variable, with risk of hypotension 7
- Hydralazine causes reflex tachycardia, which can be problematic in certain patient populations 6, 2