What is the duration of effect of Hydralazine (Hydralazine hydrochloride)?

From the Guidelines

Hydralazine has a duration of effect that typically lasts 2-4 hours when administered intravenously, necessitating repeat dosing every 4-6 hours as needed. This is based on the most recent and highest quality study available, which is the 2017 ACC/AHA/AAPA/ABC/ACPM/AGS/APhA/ASH/ASPC/NMA/PCNA guideline for the prevention, detection, evaluation, and management of high blood pressure in adults 1. The study states that the blood pressure begins to decrease within 10-30 minutes of hydralazine administration, and the fall lasts 2-4 hours.

Key Points

• Hydralazine is a direct vasodilator that relaxes vascular smooth muscle, primarily affecting arterioles rather than veins.
• The medication has a short half-life, approximately 2-8 hours, which explains its brief duration of action.
• The drug undergoes acetylation in the liver, and the rate of this metabolism varies among individuals based on genetic factors.
• For patients requiring blood pressure control, the short duration of hydralazine means that missed doses can quickly lead to rebounds in hypertension.
• When used for hypertensive emergencies via IV route, continuous monitoring is necessary as the effect begins rapidly but doesn't last long.

Clinical Implications

• Hydralazine is often combined with longer-acting antihypertensives in clinical practice due to its short duration of action.
• The unpredictability of response and prolonged duration of action do not make hydralazine a desirable first-line agent for acute treatment in most patients, as stated in the guideline 1.
• However, hydralazine can still be a useful option for certain patients, and its use should be individualized based on the patient's specific needs and circumstances.

From the FDA Drug Label

HydrALAZINE is rapidly absorbed after oral administration, and peak plasma levels are reached at 1 to 2 hours. Plasma levels of apparent hydrALAZINE decline with a half-life of 3 to 7 hours. The duration of effect of hydralazine is related to its half-life, which is 3 to 7 hours 2.

• The peak plasma levels are reached at 1 to 2 hours after oral administration.
• The duration of action can be inferred to be around 12 to 28 hours, considering the half-life, but this is not directly stated in the label. However, caution should be exercised when interpreting this data, as individual variability in plasma levels and the effects of polymorphic acetylation may influence the actual duration of effect.

From the Research

Hydralazine Duration of Effect

• The duration of effect of hydralazine is not explicitly stated in the provided studies, but its pharmacokinetics and therapeutic use can provide some insights 3.
• Hydralazine is absorbed well from the gastrointestinal tract and has an extensive and complex metabolism depending on acetylator status 3.
• The half-lives, clearances, and bioavailability of the drug are not significantly altered in congestive heart failure compared with hypertensive patients 3.
• A wide range of dosages in heart failure has been noted (150 to 3000 mg/24h), and may be related to a saturation of the first-pass effect 3.
• Hydralazine improves hemodynamics in the short term in patients with increased peripheral vascular resistance, and has variable effects on pulmonary capillary wedge and left ventricular filling pressures 3.
• The effect of hydralazine on clinical outcomes remains uncertain, and there is insufficient evidence to conclude on its effects on mortality, morbidity, withdrawals due to adverse effects, serious adverse events, or systolic and diastolic blood pressure 4.
• Hydralazine may reduce blood pressure when compared to placebo in patients with primary hypertension, however this data is based on before and after studies, not randomized controlled trials (RCTs) 4.
• The use of hydralazine is often combined with other medications, such as beta-blockers and diuretics, to prevent reflex tachycardia and sodium retention 5, 6.