Hydralazine
Hydralazine reduces afterload and is sometimes added for refractory congestive heart failure or hypertension.
Group
Arteriolar vasodilator
Mechanism of Action
Relaxes arteriolar smooth muscle, reducing vascular resistance; the precise mechanism is incompletely established.
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 | Instructions |
|---|---|---|---|---|
| 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
- Afterload reduction in refractory heart failure
- Hypertension
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.