Vasodilator · Antihypertensive
Hydralazine
Indications
- Severe hypertension in pregnancy, preeclampsia and eclampsia (ACOG first-line)
- Severe hypertension when oral therapy is impossible or BP must fall urgently
- Oral, with isosorbide dinitrate, for HFrEF in self-identified Black patients with NYHA III–IV symptoms on optimal therapy
- Oral, with a nitrate, for HFrEF when RAAS blockers cannot be used (off-label)
Dosing
| Where | Dose |
|---|---|
| Blood pressure boluses | Hydralazine: 5–10 mg IV every 20–30 min (unpredictable; used in pregnancy) |
On the quick cards
Mechanism of action
Hydralazine relaxes arteriolar smooth muscle directly, but its molecular target is unsettled. The best-supported action is blocking IP3-triggered calcium release from the sarcoplasmic reticulum (Gurney 1995), leaving less calcium for myosin light-chain kinase; it also inhibits prolyl hydroxylases, stabilizing HIF-1α (Knowles 2004), of unclear acute relevance. Arterioles dilate far more than veins, and the baroreflex answers with sympathetic activation, tachycardia and renin release.
Pharmacology
- Cardiovascular: dilates arterioles, not veins, lowering diastolic more than systolic pressure; postural hypotension is uncommon
- Cardiovascular: no direct cardiac action, but reflex rises in heart rate, stroke volume and output can cause angina and ischemic ECG changes
- Renal: renal blood flow is kept; renin and aldosterone rise, retaining sodium and water
- CNS: cerebral blood flow is kept; headache and flushing are common
Pharmacokinetics
| Onset | IV: BP starts to fall within a few minutes |
|---|---|
| Peak | 10–80 min after an IV dose |
| Duration | Hours and unpredictable; the effect outlasts plasma levels as the drug binds arterial wall muscle |
| Half-life | About 1 h by selective assay (Ludden 1980), 2–3 h by older assays; up to 16 h with CrCl under 20 mL/min |
| Distribution | Vd at steady state about 1.8 L/kg; crosses the placenta |
| Protein binding | 88–90%, mainly albumin |
| Metabolism | Largely converted in blood to a pyruvic acid hydrazone; also hepatic NAT2 acetylation, which affects oral but not IV levels |
| Elimination | Urine, mostly as metabolites; 2–14% as apparent hydralazine |
| Organ failure | Kidney or liver impairment raises levels: lengthen the interval |
Chemistry
- 1-Hydrazinophthalazine: a phthalazine ring with a hydrazine (–NH–NH₂) group; achiral
- Its reactive hydrazine forms hydrazones with pyruvate in blood (the main circulating form) and with pyridoxal, depleting vitamin B6
- pKa about 7.3: roughly half ionized at pH 7.4; water soluble, log P about 0.7
- Injection 20 mg/mL in propylene glycol with parabens, pH 3.4–4.4; use at once, never in infusion fluids; discard if metal contact discolors it
Adverse effects and cautions
In the one-pagers
- Aortic Dissection and Hypertensive EmergencyIV antihypertensives: Hydralazine: 10–20 mg IV: Unpredictable drop; used in pregnancy
- Obstetric EmergenciesPreeclampsia and eclampsia: Hydralazine IV: 5 mg over 2 min (or IM): 10 mg every 20 min; max 20 mg
- Spinal Motion RestrictionAutonomic dysreflexia: Systolic ≥150: a fast, short-acting drug, first line 2% nitroglycerin ointment on the chest or back, wiped off as BP falls (no nitrates within 24 h of sildenafil or vardenafil, 48 h of tadalafil); alternatives include nifedipine, hydralazine, captopril, clonidine.
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Sources
- FDA label: hydralazine injection (2018)
- Health Canada monograph: hydralazine (2021)
- Ludden 1980 (IV kinetics)
- Magee 2003 (BMJ meta-analysis)
For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow your local protocols and medical direction.