What Are Vasodilators?

Medically reviewed by Anthony Pearson, MD, FACC

Vasodilators are a class of medication that dilate arteries and/or veins. This results in increased blood flow and lowered blood pressure. Vasodilators are commonly used to treat high blood pressure (hypertension) and heart conditions.

This article discusses vasodilators and their uses, administration, and side effects.

How Do Vasodilators Work?

Vasodilators relax the muscles within blood vessel walls so they remain more widely open. The various classes of vasodilators target different pathways to dilate the blood vessel walls but have the same overall effect: lowering blood pressure and improving blood flow.

Types of Vasodilators

Vasodilators may target the arteries, veins, or both. They may act directly on the blood vessel wall or indirectly through other pathways.

Nitrovasodilators

Nitrovasodilators include nitrates (chemical compounds containing nitrogen and oxygen), like nitroglycerine, isosorbide dinitrate and isosorbide, and sodium nitroprusside.

Nitrates work on veins and are used to treat heart conditions like angina, heart attacks, and heart failure. Sodium nitroprusside acts on both veins and arteries and is used to treat hypertensive emergencies and heart failure.

Direct Acting Arterial Vasodilators

Hydralazine and Minoxidil are direct-acting vasodilators that preferentially dilate arteries. They are not the first-choice treatment for hypertension but may be used with other medications to treat resistant hypertension.

Indirect Arterial Vasodilators

Some examples of indirect arterial vasodilators include:

• Calcium channel blockers, like amlodipine and nifedipine, have vasodilating properties by blocking calcium channels in the smooth muscle of blood vessels.

• Some beta-blockers, like carvedilol and labetalol, have vasodilating effects through their action on the alpha receptor on blood vessel walls. Nebivolol, the newest beta-blocker, exhibits vasodilation through its effects on nitric oxide generation in the arterial lining. These beta-blockers are not first-line therapy for high blood pressure but may be given in specific cases with the added benefit of lowering blood pressure.

• Renin angiotensin aldosterone system (RAAS) blockers including angiotensin converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), and aldosterone antagonists cause vasodilation through their effects on the RAAS system.

• Central acting adrenergic blockers like clonidine and methyldopa work indirectly to dilate blood vessels by acting on the brain. Neither of these is used first-line to treat blood pressure. Methyldopa is one of the few blood pressure medications that has improved safety in pregnancy.

• Alpha-1 receptor blockers like doxazosin and tamsulosin block the action of norepinephrine on blood vessel walls, resulting in vasodilation. These medications are commonly used to treat benign prostate hypertrophy (BPH) and can cause dizziness due to their effect on blood pressure.

• Phosphodiesterase inhibitors include medications for pulmonary hypertension and erectile dysfunction. These block an enzyme that indirectly results in the dilation of blood vessels.
  

• Endothelin receptor antagonists are a type of vasodilator used to treat pulmonary hypertension.

Indications for Using Vasodilators

Based on their specific properties, individual vasodilators have different uses. Vasodilators are most commonly prescribed to treat the following conditions.

High Blood Pressure

The vasodilators that are more specific to arteries are used to treat high blood pressure. These include calcium channel blockers, RAAS blockers, hydralazine, and minoxidil.

Hydralazine and minoxidil are not first-line therapies, but they are used in combination with other medications to treat high blood pressure that has not been responsive to other treatments.

Angina and Heart Attack

Nitroglycerin and the long-acting nitrate medication isosorbide mononitrate are used in the treatment of angina, which is chest pain related to narrowing of the coronary arteries. Nitrates dilate veins and coronary arteries to improve blood flow to the heart.

Pulmonary Hypertension

Phosphodiesterase inhibitors and endothelin receptor antagonists are types of vasodilators used in the treatment of pulmonary hypertension (PH). They dilate blood vessels in the lungs to lower the elevated blood pressure in PH.

Administration

Vasodilators can be given in various forms:

• Sublingual (under the tongue) and oral spray formulations are rapidly absorbed and commonly used for the treatment of angina and heart attack

• Oral medication taken as a pill

• Intravenous infusion is required in some drug formulations to treat pulmonary hypertension, hypertensive emergency, and preeclampsia

• Transdermal patches and ointment formulations of nitrates may be used for treating angina

Tachyphylaxis in Nitrates

One important thing to note about nitrovasodilators is the concept of tachyphylaxis. Over time, the body develops tolerance to nitrates, and higher and higher doses are required to achieve the same effect. To avoid this, a nitrate-free period is built into the administration. For example, if you are prescribed a nitrate patch, you may be told to remove it overnight to avoid developing tachyphylaxis.

Side Effects

As a drug class, vasodilators can cause several side effects, some of which may be serious. These can include:

• Low blood pressure

• Headache

• Fast heart rate (tachycardia)

• Dizziness or lightheadedness

• Weakness

• Flushing

• Nausea and vomiting

• Anaphylaxis or allergic reaction

• Skin reaction, especially with transdermal formulations

Additional Side Effects

Different vasodilators each have their own specific side effects beyond those mentioned. If you are experiencing new symptoms after starting a vasodilator, be sure to discuss them with your healthcare provider.

Interactions

Vasodilators have a few important drug interactions.

• Any other blood pressure-lowering medication in combination with a vasodilator can cause further lowering of blood pressure. Sometimes this is done intentionally, such as when hydralazine is given in combination with other medications to treat resistant hypertension. In other circumstances, this can lead to unwanted side effects, such as dizziness, when using an alpha blocker for enlarged prostate in combination with blood pressure medications.

• Combining vasodilators can be dangerous. For example, a phosphodiesterase inhibitor for erectile dysfunction should never be taken in combination with a nitrate due to the risk of severe drop in blood pressure.

Related: High Blood Pressure and Erectile Dysfunction: What to Know

Summary

Vasodilators are an important class of medications that lower blood pressure and improve blood flow by relaxing the muscles in blood vessel walls. They are commonly used to treat high blood pressure, heart conditions like heart failure and angina, and pulmonary hypertension.

Common side effects of vasodilators include dizziness and headache. Severe side effects can include anaphylaxis and fainting. Combining vasodilators can result in severe blood pressure drops.

A Word From Verywell

Heart disease is a serious condition that requires dedicated treatment. It's important to take all prescribed medications for your heart issues as directed by a healthcare provider. If you are prescribed a vasodilator, take it as recommended and watch for any side effects. You can bring up any concerns you have with your healthcare provider.

Read the original article on Verywell Health.