Hydralazine

Hydralazine is a direct arteriolar vasodilator used in selected veterinary cardiovascular patients. Its principal role is adjunctive afterload reduction in dogs with refractory congestive heart failure, particularly severe valvular regurgitation. It also has an antihypertensive role. Clinical use is less common than that of several other cardiac vasodilators.

Group

Arteriolar vasodilator

Mechanism of Action

Hydralazine relaxes arteriolar smooth muscle through incompletely defined intracellular signaling effects. The reduction in systemic vascular resistance lowers left-ventricular afterload and may improve forward blood flow when excessive resistance contributes to cardiac dysfunction. In severe mitral regurgitation, reduced outflow resistance can reduce the regurgitant fraction. Arteriolar dilation also lowers systemic arterial pressure.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Afterload reduction — Initial	0.5 mg/kg	PO
Afterload reduction — Maintenance	0.5–2 mg/kg	PO	Every 12 h
Hypertensive emergency	0.5–3 mg/kg	IV	Every 12 h	Bolus alternative; assess pressure response before repeating.
Hypertensive emergency	1.5–5 mcg/kg/min	IV	Continuous	Infusion option after an initial bolus.
Afterload reduction	0.5–3 mg/kg	PO	Every 12 h	Titrate from a low dose using pressure and clinical response.
Resistant systemic hypertension	0.5 mg/kg	PO	Every 12 h	Adjunct after other antihypertensives; evaluate blood pressure before escalation.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Afterload reduction	2.5 mg/animal	PO	Every 12–24 h
Resistant systemic hypertension	0.5 mg/kg	PO	Every 12 h	Adjunct after other antihypertensives; evaluate blood pressure before escalation.

Horses

Use / stage	Dose	Route	Schedule
Hypertension	1 mg/kg	PO	Every 12 h
Hypertension	0.5 mg/kg	IV	As needed
Heart-failure afterload reduction	0.5–1.5 mg/kg	PO	Every 12 h
Heart-failure afterload reduction	0.5 mg/kg	IV

Indications

Adjunctive afterload reduction in selected dogs with refractory congestive heart failure, especially severe mitral regurgitation.
Specialist reduction of excessive systemic vascular resistance in selected patients with other regurgitant valvular or structural cardiac disorders.
Treatment of selected systemic hypertension in dogs or cats when additional arteriolar vasodilation is required.

Contraindications

Hypotension

Interactions

Other vasodilators, antihypertensives and diuretics can intensify hypotension.

Adverse effects

Hypotension and reflex tachycardia
Reduced cardiac output with excessive vasodilation
Activation of the renin-angiotensin-aldosterone system during repeated use

Toxicity

Excess vasodilation requires withdrawal and haemodynamic support.

Preparations

Preparation	Form	Strength	Route
Hydralazine 10 mg tablet	Tablet	10 mg / tablet	PO
Hydralazine 25 mg tablet	Tablet	25 mg / tablet	PO
Hydralazine 50 mg tablet	Tablet	50 mg / tablet	PO
Hydralazine 100 mg tablet	Tablet	100 mg / tablet	PO
Hydralazine 20 mg/mL injection	Injection	20 mg / mL	IV

Monitoring and Administration

Measure blood pressure while titrating, starting at the lowest appropriate dose.

Protect from light. Mixing with juices, syrups or flavouring vehicles may cause decomposition within 24 hours.

Canine oral titration protocols reassess 1-2 hours after each increment with blood pressure monitoring, or more slowly over 12-48 hours clinically; cumulative 3 mg/kg within 12 hours is the specified ceiling. Start at 0.5 mg/kg with concurrent ACE inhibition. Feline heart-failure titration begins at 2.5 mg/cat and may reach 10 mg/cat; do not start at the upper amount.

Follow perfusion, thoracic imaging, electrolytes and occasional CBC. Avoid refrigeration/freezing of injection. Hydralazine is incompatible with 10% dextrose and with hydrocortisone succinate, aminophylline and barbiturate admixtures.