Drug Monograph
Full clinical overview, indications, dosage references & safety notes.
Available forms2 forms · 5 strengths documentedShow all ↓
Tablet 10 mgTablet 25 mgTablet 50 mgTablet 100 mg
20 mg/mL
Overview
Hydralazine is a direct systemic arterial vasodilator used primarily in dogs and cats as an afterload reducer for congestive heart failure (CHF) and for the management of systemic hypertension.
In patients with CHF, hydralazine increases cardiac output by reducing systemic vascular resistance and promoting forward left ventricular stroke volume while reducing the mitral regurgitant fraction. It may also be useful in dogs and cats with large septal defects or severe aortic regurgitation.
Mechanism of Action (MOA): Hydralazine acts directly on vascular smooth muscle, producing arterial vasodilation with a greater effect on arterioles than veins. It reduces peripheral vascular resistance and blood pressure, likely by altering calcium metabolism in vascular smooth muscle, thereby interfering with calcium movement required for contraction.
Indications
Hydralazine is used as an arterial vasodilator in dogs and cats.
- Congestive heart failure (CHF): Adjunctive afterload reduction, particularly in dogs with heart failure secondary to mitral valve disease that has become refractory to ACE inhibitors and pimobendan.
- Systemic hypertension: Management of primary or secondary systemic hypertension; it is not typically a first-line drug.
- Aortic valve disease: Afterload reduction.
- Large septal defects or severe aortic regurgitation: Adjunctive therapy.
- Sick sinus syndrome: It may be tried in dogs with bradycardia and sinus arrest.
Dosage
Dogs
The following dosages have been reported for hydralazine in dogs.
| Clinical use | Route | Dose | Frequency / timing | Notes |
|---|---|---|---|---|
| Heart failure (adjunctive) or systemic hypertension | PO | 0.5–3 mg/kg | q8–12h | Start at 0.5–1 mg/kg q12h, or at 0.5 mg/kg in dogs already receiving an ACE inhibitor. Monitor blood pressure regularly and increase to 2 mg/kg or, if necessary, up to 3 mg/kg q12h. |
| Severe acute hypertension (hypertensive emergency) | IV | 0.5–3 mg/kg | q12h | – |
| Severe acute hypertension (constant rate infusion) | IV | 0.1 mg/kg loading dose, then 1.5–5 µg/kg/minute | Loading dose over 2 minutes, then constant rate infusion | Continuous or frequent blood pressure monitoring is required to confirm an adequate reduction and avoid hypotension. |
| Severe acute hypertension (hypertensive emergency) | SC or IM | 0.5–2 mg/kg | q8–12h | Continuous direct blood pressure monitoring is necessary. |
| Sick sinus syndrome (bradycardia and sinus arrest) | PO | 1–2 mg/kg | q8–12h | Only if anticholinergic drugs are ineffective. |
| Hypertension in the pregnant bitch (pregnancy edema) | IM | 0.2 mg/kg | q2h as needed | Hospitalize until normotensive. |
Cats
The following dosages have been reported for hydralazine in cats.
| Clinical use | Route | Dose | Frequency / timing | Notes |
|---|---|---|---|---|
| Heart failure (adjunctive) or systemic hypertension | PO | 2.5–10 mg/cat | q12h | Start at the low dose and titrate upwards cautiously while monitoring blood pressure. |
| Systemic hypertension (long-term) | PO | 2.5 mg/cat | q12–24h | Rarely required in cats. |
| Severe acute hypertension (hypertensive emergency) | SC | 1–2.5 mg/cat | Repeat in 15–30 minutes if systolic pressure has not fallen | Has been used to treat acute hypertension after feline renal transplantation. |
| Severe acute hypertension (constant rate infusion) | IV | 0.1 mg/kg loading dose, then 1.5–5 µg/kg/minute | Loading dose, then constant rate infusion | Evidence for parenteral vasodilators in cats is sparse; continuous or frequent blood pressure monitoring is required. |
Warnings & Precautions
Hydralazine should be initiated cautiously with careful dose titration and blood pressure monitoring to minimize the risk of severe hypotension.
- Contraindications: Do not use in patients with known hypersensitivity, hypovolemia, or pre-existing hypotension, and avoid or use with caution in patients with intracranial bleeding.
- Dose titration: Increase the dose gradually, as severe hypotension may occur if doses are escalated too rapidly.
- Renal disease: Use with caution in patients with renal disease. Reduced renal blood flow may activate the renin-angiotensin-aldosterone system (RAAS) and exacerbate renal injury.
- Autoimmune disease: Use cautiously in patients with pre-existing autoimmune diseases.
- Pregnancy and lactation: Safe use has not been established in animals. Use only when the maternal benefits outweigh the potential risks to the offspring.
- Medication error: Do not confuse hydrALAzine with hydrOXYzine.
- Obstructive heart disease and shunts: Avoid in heart failure from pure mitral stenosis, in pulmonic stenosis and in right-to-left patent ductus arteriosus, and, as for other vasodilators, in pericardial effusion or fixed outflow obstruction.
Drug Interactions
The following drug interactions have been reported or are considered clinically relevant.
- ACE inhibitors (e.g., enalapril): May produce an additive hypotensive effect; this combination may be used therapeutically.
- Diazoxide: May cause profound hypotension.
- Diuretics: May produce an additive hypotensive effect; this combination is usually used for therapeutic advantage.
- Furosemide: Hydralazine may increase the renal effects of furosemide.
- Monoamine oxidase inhibitors (e.g., amitraz, linezolid, selegiline): May produce an additive hypotensive effect.
- Propranolol: May increase the risk of propranolol adverse effects, including bradycardia, fatigue, and bronchospasm.
- Sildenafil: May produce an additive hypotensive effect.
- Sympathomimetics (e.g., epinephrine): May decrease the pressor effect of sympathomimetics and increase tachycardia.
Side Effects & Overdose
Side Effects
Adverse effects are most commonly related to excessive vasodilation or reduced renal perfusion.
- Cardiovascular: Hypotension, reflex tachycardia, weakness, lethargy, syncope, and sodium/water retention.
- Gastrointestinal: Vomiting and diarrhea.
- Renal: Increased creatinine concentrations due to decreased renal perfusion.
- Other reported effects: Systemic lupus erythematosus-like syndrome, lacrimation, conjunctivitis, blood dyscrasias, urinary retention, constipation, hypersensitivity reactions, and peripheral neuritis (associated with pyridoxine deficiency) have been documented in humans and could occur in animals.
Overdose
Hydralazine overdose primarily produces excessive cardiovascular effects.
- Clinical signs: Severe hypotension, tachycardia or other arrhythmias, skin flushing, and myocardial ischemia.
- Management: Provide cardiovascular support, evacuate gastric contents and administer activated charcoal if ingestion was recent and the patient is stable. Treat shock with volume expanders whenever possible, avoiding pressor agents; if required, phenylephrine is the preferred vasopressor. Monitor blood pressure and renal function closely.
Key Notes
Practical clinical points for the use of hydralazine in dogs and cats.
- Heart failure: Hydralazine is most beneficial when afterload reduction is required and is not particularly useful for heart failure caused primarily by myocardial disease.
- Monitoring: Regular assessment of therapeutic response and adverse effects is recommended throughout treatment.
