1. Clinical Overview
A fixed-dose combination of a nitrate vasodilator (Isosorbide Dinitrate) and a direct-acting arteriolar vasodilator (Hydralazine). This combination is primarily used in the management of chronic heart failure (HF), specifically for patients of African descent or those intolerant to ACE inhibitors/ARBs, as per landmark trials. It reduces preload and afterload, improving cardiac output and symptoms.
| Onset | Duration | Bioavailability |
|---|---|---|
| Isosorbide Dinitrate: Sublingual: 2-5 minutes; Oral: 45-60 minutes. Hydralazine: Oral: 45-60 minutes. | Isosorbide Dinitrate: Sublingual: 1-2 hours; Oral: 4-6 hours. Hydralazine: Oral: 6-8 hours. | Isosorbide Dinitrate: Oral: ~25% (extensive first-pass metabolism). Hydralazine: Oral: ~30% (extensive first-pass metabolism). |
2. Mechanism of Action
The combination works synergistically to reduce cardiac workload and improve hemodynamics in heart failure. Isosorbide Dinitrate primarily acts as a venodilator, reducing preload by pooling blood in the venous capacitance vessels. Hydralazine acts as a direct arteriolar vasodilator, reducing afterload by decreasing systemic vascular resistance. Together, they lower ventricular filling pressures and increase cardiac output without a significant reflex tachycardia (partly due to nitrate-mediated counteraction).
3. Indications & Uses
- Adjunct in the management of moderate to severe Chronic Heart Failure with reduced ejection fraction (HFrEF)
- Treatment of heart failure in self-identified Black/African-American patients (as per A-HeFT trial evidence)
- Alternative for patients with HFrEF intolerant to ACE inhibitors or ARBs due to cough/angioedema
4. Dosage & Administration
Adult Dosage: Heart Failure: Typically, one tablet (Isosorbide Dinitrate 20mg + Hydralazine 37.5mg) three times daily. Initiation often starts with half a tablet TID and titrated upward over weeks to the target dose to improve tolerability. Maximum studied dose in trials: Isosorbide Dinitrate 40mg + Hydralazine 75mg TID.
Administration: Take orally with or without food. To prevent nitrate tolerance, the dosing schedule should provide a daily nitrate-free interval of 10-12 hours (e.g., doses at 8 AM, 2 PM, and 6 PM, avoiding bedtime dose). Do not crush or chew. Swallow whole with a glass of water.
5. Side Effects
Common side effects may include:
- Headache (nitrate-induced, often diminishes with continued use)
- Dizziness, lightheadedness
- Postural hypotension
- Nausea, vomiting
- Flushing
- Tachycardia or palpitations
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Phosphodiesterase-5 Inhibitors (Sildenafil, Tadalafil, Vardenafil) | Profound, life-threatening hypotension due to synergistic vasodilation | Contraindicated |
| Other Antihypertensives (Beta-blockers, ACEi, ARBs, Diuretics, Calcium channel blockers) | Additive hypotensive effect; risk of severe hypotension | Major |
| Alcohol | Enhanced vasodilation and hypotensive effect | Major |
| Diazepam, Midazolam | Hydralazine may increase plasma levels of these drugs | Moderate |
| MAO Inhibitors (e.g., Phenelzine) | May potentiate hypotensive effect | Moderate |
| Sympathomimetics (e.g., Epinephrine) | May decrease antihypertensive effect of hydralazine | Moderate |
| NSAIDs (e.g., Ibuprofen, Diclofenac) | May attenuate the hypotensive effect | Moderate |
| Dihydroergotamine | Nitrates may reduce its vasoconstrictive effect | Moderate |
7. Patient Counselling
- DO take the medication exactly as prescribed, adhering to the specific timing to avoid nitrate tolerance.
- DO rise slowly from sitting or lying position to minimize dizziness.
- DO inform all healthcare providers (including dentists) you are taking this medication.
- DO keep a regular check on your blood pressure if advised.
- DONT stop taking this medicine suddenly, especially if used for angina, as it may cause rebound worsening.
- DONT take any erectile dysfunction drugs (like Viagra, Cialis) while on this medication.
- DONT consume alcohol.
- DONT take over-the-counter NSAIDs (like Ibuprofen) without consulting your doctor.
8. Toxicology & Storage
Overdose: Primarily vasodilation leading to: Severe hypotension with reflex tachycardia, throbbing headache, dizziness, syncope, palpitations, visual disturbances, nausea/vomiting, diaphoresis, flushing, cold/clammy skin, methemoglobinemia (cyanosis, dyspnea) with nitrates, and eventual cardiovascular collapse, coma, and death.
Storage: Store below 30°C. Protect from light and moisture. Keep in the original blister pack or container. Keep out of reach of children. Do not use after the expiry date printed on the pack.