Understanding Hydralazine and Its Role
Hydralazine is a direct-acting vasodilator used in the management of hypertension [1.5.2]. Its mechanism involves relaxing the smooth muscles of arterioles, which leads to a decrease in peripheral resistance and a subsequent reduction in blood pressure [1.5.3]. While effective, it is often used as an add-on therapy when first-line medications do not sufficiently control blood pressure [1.2.4]. Its use is accompanied by a reflexive increase in heart rate and cardiac output, a key consideration in its clinical application [1.5.3].
Absolute Contraindications: When Hydralazine Is Strictly Avoided
There are specific conditions where the administration of hydralazine is strictly contraindicated due to the high risk of severe adverse outcomes [1.2.2].
Coronary Artery Disease (CAD)
The myocardial stimulation produced by hydralazine can provoke anginal attacks and ECG changes indicative of myocardial ischemia [1.5.5]. The drug has been implicated in causing myocardial infarction (heart attack) and should not be used in patients with known or suspected CAD [1.5.5, 1.2.2]. The reflex tachycardia increases myocardial oxygen demand, which can be dangerous for a heart with compromised blood flow [1.5.3].
Mitral Valvular Rheumatic Heart Disease
Hydralazine is contraindicated in patients with mitral valvular rheumatic heart disease [1.2.2, 1.2.3]. The "hyperdynamic" circulation it causes can increase pressure in the pulmonary artery, potentially worsening the patient's cardiovascular state [1.5.5].
Hypersensitivity
Patients with a known allergy or hypersensitivity to hydralazine or any of its components should not receive the medication [1.2.2]. Some formulations may contain tartrazine (FD&C Yellow No. 5), which can cause allergic-type reactions, including bronchial asthma, in susceptible individuals, particularly those with an aspirin hypersensitivity [1.5.5].
Important Warnings and Precautions
Beyond absolute contraindications, there are numerous situations where hydralazine must be used with extreme caution and close monitoring.
Drug-Induced Lupus Erythematosus (DILE)
Long-term use of hydralazine, especially at doses exceeding 200 mg per day, can induce a syndrome that mimics systemic lupus erythematosus (SLE) [1.2.2, 1.2.1]. This risk is higher in individuals who are "slow acetylators," a genetic trait affecting how the body metabolizes the drug [1.5.3]. Symptoms include joint pain (arthralgia), fever, and chest pain [1.2.2]. If a patient develops these symptoms or a positive antinuclear antibody (ANA) titer, the drug should be discontinued unless the benefits are determined to outweigh the risks [1.5.5].
Renal and Hepatic Impairment
Caution is advised for patients with severe renal or hepatic impairment [1.3.6]. Since hydralazine is metabolized by the liver and its metabolites are cleared by the kidneys, impaired function can lead to increased drug levels and a higher risk of toxicity [1.2.5, 1.3.5]. Dose adjustments may be necessary for these patients [1.3.6].
Cerebrovascular Accidents (Stroke) and Hypotension
Hydralazine should be used cautiously in patients with a history of cerebrovascular accidents (stroke) or pre-existing hypotension [1.2.2, 1.2.5]. The vasodilating effect can lead to postural hypotension, causing dizziness or fainting, which could be particularly dangerous in these individuals [1.5.5].
Drug Interactions
Hydralazine can interact with numerous other medications, potentiating its hypotensive effects. Co-administration with other antihypertensives, nitrates, alcohol, or MAO inhibitors can lead to a dangerous drop in blood pressure [1.3.6, 1.4.4]. Conversely, NSAIDs (like ibuprofen) and certain over-the-counter cold medicines may decrease hydralazine's effectiveness by increasing blood pressure [1.3.6, 1.2.5].
Comparison of Antihypertensive Classes
| Feature | Hydralazine (Vasodilator) | Beta-Blockers (e.g., Metoprolol) | ACE Inhibitors (e.g., Lisinopril) |
|---|---|---|---|
| Primary Mechanism | Directly relaxes arterial smooth muscle [1.5.3] | Blocks epinephrine effects, slowing heart rate [1.2.3] | Blocks angiotensin-converting enzyme, relaxing vessels [1.4.7] |
| Effect on Heart Rate | Increases (reflex tachycardia) [1.2.3] | Decreases [1.2.3] | Generally neutral |
| Common Side Effect | Headache, palpitations, flushing [1.2.1] | Fatigue, dizziness [1.2.4] | Dry cough, hyperkalemia |
| Key Contraindication | Coronary Artery Disease [1.2.2] | Severe asthma, bradycardia [1.3.3] | History of angioedema, pregnancy |
Conclusion: Prioritizing Patient Safety
While hydralazine can be a useful tool for managing hypertension, its powerful vasodilating effects and potential for significant side effects necessitate careful patient selection. It is crucial for healthcare providers to conduct a thorough medical history, paying close attention to cardiovascular conditions like coronary artery disease and mitral valvular disease, which are absolute contraindications [1.2.2]. Understanding the patient's renal function, other medications, and risk factors for drug-induced lupus is essential for safe administration [1.2.1, 1.2.2]. The decision to use hydralazine must always involve a careful weighing of its benefits against its substantial risks.
For further reading on hydralazine's pharmacology and clinical use, consider authoritative sources such as the NCBI StatPearls article on Hydralazine.
