Hydralazine is a medication used in the management of high blood pressure, or hypertension. By helping to lower blood pressure, the drug reduces the overall workload on the heart and decreases the risk of serious events like stroke or heart attack. It is often used when standard, first-line treatments are not fully effective or in specific emergency situations where blood pressure must be reduced quickly.
Defining the Drug Class
Hydralazine is a direct-acting peripheral vasodilator, which means it acts directly on the blood vessels to cause them to widen. This mechanism is fundamentally different from the action of a diuretic, or “water pill,” which is a common misconception about hydralazine. Diuretics work by increasing the excretion of salt and water from the body through the kidneys. This increased fluid output reduces the total blood volume, thereby lowering blood pressure.
By relaxing the muscular walls of arteries, hydralazine decreases the resistance blood encounters as it flows through the body. Interestingly, hydralazine can sometimes cause the body to retain fluid and sodium, which is precisely why it is frequently prescribed in combination with a diuretic.
How Hydralazine Lowers Blood Pressure
The primary action of hydralazine is the relaxation of the smooth muscle found in the walls of the small arteries, known as arterioles. This relaxation causes the arterioles to expand, which reduces the total peripheral resistance in the circulatory system. Since blood pressure is a product of cardiac output and peripheral resistance, lowering the resistance directly results in a decrease in blood pressure.
At the cellular level, the drug interferes with the movement of calcium within the smooth muscle cells. Muscle contraction requires a rise in intracellular calcium concentration, which binds to specific proteins to initiate the shortening of muscle fibers. Hydralazine is thought to prevent the release of calcium ions from the sarcoplasmic reticulum, which is the internal storage site for calcium inside the muscle cell.
By preventing this calcium release, the muscle cells cannot achieve the sustained contraction necessary to keep the blood vessel narrowed. This allows blood to flow more easily, decreasing the pressure against the vessel walls. Its effect is mainly on the arteries, which carry blood away from the heart, rather than the veins.
Primary Uses in Medical Treatment
Hydralazine is typically not considered a first-line therapy for routine, uncomplicated high blood pressure. It is most often used in the management of moderate to severe hypertension, frequently as an add-on medication when other drug classes have not achieved the target blood pressure. Because of its potent and rapid action, the drug can also be administered intravenously for a hypertensive crisis, which is a medical emergency involving severely elevated blood pressure.
The medication also has a significant role in treating hypertension that occurs during pregnancy. For patients with heart failure, hydralazine is utilized in a fixed combination with isosorbide dinitrate, particularly for individuals of African descent. In this context, the combination therapy helps to reduce the resistance the heart must pump against, improving its overall efficiency.
Monitoring and Safety Information
One of the well-known consequences of hydralazine’s action is reflex tachycardia, which is an increase in heart rate. As the drug causes a rapid drop in blood pressure, the body’s nervous system reflexively attempts to raise the heart rate and force of contraction to maintain adequate circulation. For this reason, hydralazine is often co-administered with a beta-blocker to help control the heart rate response.
Common, less severe side effects include headaches, flushing, dizziness, and gastrointestinal upset such as nausea or loss of appetite. A more serious, though rare, adverse effect associated with long-term use is drug-induced lupus erythematosus (DILE).
This condition can present with symptoms resembling systemic lupus, including joint pain, fever, and a rash. The risk of developing DILE is often dose-related and is higher in individuals who metabolize the drug slowly. Patients on long-term hydralazine therapy may require regular blood tests, such as an antinuclear antibody (ANA) titer, to monitor for early signs of this lupus-like syndrome.