Duration of Action of Intravenous 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
Pharmacokinetics of IV Hydralazine
- The average maximal decrease in blood pressure typically occurs 10-80 minutes after administration of IV hydralazine 2
- The hypotensive effect of IV hydralazine persists for 2-4 hours after administration 1
- IV hydralazine preferentially dilates arterioles compared to veins, which minimizes postural hypotension while promoting increased cardiac output 2
- The medication works through direct relaxation of vascular smooth muscle, altering cellular calcium metabolism that interferes with calcium movements responsible for maintaining vascular contractility 2
Clinical Considerations for IV Hydralazine Use
- IV hydralazine is typically administered as an initial 10 mg via slow IV infusion (maximum initial dose 20 mg) and can be repeated every 4-6 hours as needed 1
- According to FDA labeling, the usual dose is 20-40 mg, repeated as necessary, with blood pressure checked frequently after administration 2
- The unpredictability of response and prolonged duration of action make hydralazine less desirable as a first-line agent for acute treatment in most patients 1
- In studies examining IV hydralazine's effects, the medication reduced blood pressure by approximately 24/9 ± 29/15 mmHg with significant variability between patients 3
Important Caveats and Limitations
- Patient response to IV hydralazine can be highly variable, with significant differences in plasma concentrations and clinical effects between slow and rapid acetylators 4
- Hydralazine typically causes a greater reduction in diastolic than systolic blood pressure 2
- IV hydralazine often increases heart rate, stroke volume, and cardiac output due to its vasodilatory effects and reflex sympathetic activation 2
- Hypotension is one of the most common adverse effects, occurring in approximately 12% of hospitalized patients receiving IV hydralazine 5
- The medication should be used with caution in patients with coronary artery disease as it may precipitate myocardial ischemia due to reflex tachycardia 6
Monitoring Recommendations
- Blood pressure should be monitored frequently after IV hydralazine administration, as effects may begin within minutes 2
- For hypertensive emergencies, when using IV hydralazine, systolic blood pressure should be reduced by no more than 25% within the first hour 1
- After the initial reduction, if the patient is stable, blood pressure can be further reduced to 160/100 mmHg within the next 2-6 hours, and then cautiously to normal during the following 24-48 hours 1
- Due to its duration of action of 2-4 hours, patients should be monitored throughout this period for potential hypotensive effects 1