Why An Irregular Heartbeat Could Make You Pass Out

Learn why people faint—and when it might be cause for concern.

Medically reviewed in March 2022

If you’ve ever fainted, with or without warning, you probably wanted to know why it happened and if it signaled a more serious health issue. One common reason people faint is that their brain isn’t getting an adequate supply of blood and oxygen. In many cases, this shortage tracks back to what’s going on in their heart.

Why fainting happens
A fainting episode, also known as syncope, may be brief—ranging from just three seconds to a couple of minutes—but it can look and feel dramatic.

Some but not all people may experience warning signs, such as shortness of breath, dizziness or lightheadedness, feeling like your heart is racing, fluttering or pounding, or pressure or discomfort in your chest. A black dot may appear in your field of vision and expand, or your visual field may become all white as the episode progresses. You may not have control over your muscles or posture and may fall to the floor or slump over, if you’re sitting.

In short, without essential blood and the oxygen it carries, your brain calls “lights out” pretty quickly.

Fainting is not rare. It strikes about 1 in 5 people over a lifetime and might be more common among women. After an episode occurs, most people regain consciousness once sufficient blood is flowing to the brain again.

How your heart could cause you to faint
The most common cause of syncope is a sudden drop in blood pressure caused by a reflex, known as vasovagal syncope. This occurs when the vagus nerve—one of the nerves that connects the brain to the body and that also regulates heart rate—signals inappropriately. This kind of fainting, which isn’t connected to heart problems, may be triggered by things like standing up too quickly, overheating, or stress.

But certain heart conditions can also cause fainting, so you should always consult your healthcare provider if you have an episode of syncope. The heart problems that could trigger these events can be divided into two broad categories: a problem with the structure of the heart and issues with the electric signaling that drives heart rate.

People with existing heart disease, for example, such as hardening heart valves or enlarged heart muscle, may experience related syncope. In some cases involving structural heart problems, there is even a risk for sudden death.

Another possible factor is a heart that’s beating with an abnormal rhythm, or an arrhythmia. “One of the biggest things we get consulted for is how arrythmias lead to this low blood volume to the brain and passing out,” says Saumil R. Shah, a cardiologist who specializes in abnormal heart rhythms with Virginia Arrhythmia Consultants at Chippenham Hospital in Richmond, Virginia.

Abnormal heartbeats and fainting
A too-slow or too-fast heart rate can underlie syncope. If the heart beats too fast (a condition called tachycardia), it can’t fill up with enough blood to pump. The opposite can also cause fainting: the heart beats so slowly (bradycardia) that routing of blood to the brain is too slow. In short, when this relay is disrupted—like an electrical wire that fritzes in and out—the heart rate is, too.

So, what triggers an abnormal heart rate? The heart relays electrical signals from regions called nodes, which control your heartbeat, along to the heart muscle. Changes in these electrical signals due to damage from disease, injury, or genetics can all trigger an arrhythmia. Other factors, such as alcohol intake, smoking, using illegal drugs, an electrolyte or hormone imbalance, and taking certain medications may also play a role.  

When to seek help
The first thing to do after fainting or even having a near-fainting episode, says Shah, is to look at what happened before and after it.

“You might get a little lightheaded, not enough to faint, so that is a warning sign we look for: people feeling like they’re going to faint but don’t,” he explains. Was there nausea, vomiting, dizziness, or lightheadedness? If so, he says, talk to your healthcare provider.

In order to confirm or rule out an arrhythmia, your healthcare provider will first ask you about your symptoms and discuss your medical history, including any family history of arrhythmia. In addition to a physical exam, certain tests may be ordered, including an EKG, which is used to assess if the heart is beating with a steady rhythm or if it’s beating too fast or too slow.

“If we can’t get symptoms and a pattern,” Shah says, “we call it ‘unexplained syncope’ and recommend longer monitoring with a mobile heart monitor to look for abnormal heart rhythm.”

Although this approach can often capture an arrhythmia, it’s not foolproof, he notes. Sometimes, arrythmias can happen a month apart and just after someone stops using the monitor, thus going uncaptured. One solution to that problem is using an implanted monitor. This device is placed just under the skin and can monitor the heart for between 18 to 36 months. Using such a monitor over that length of time should certainly detect abnormal heart rhythms, Shah says.

If you’re diagnosed with an abnormal heartbeat, your treatment, which may include medications or other interventions, will depend on the specific type of arrythmia you have. For example, a disruption in the electrical signals in the heart’s lower heart chambers, or ventricles, which can be particularly dangerous, may be treated with a device called an implantable cardioverter-defibrillator (ICD). This device will use a shock to reset signaling in a ventricle that’s contracting in an out of control way.

For people who have a too-slow heart rate—or problems that block the electrical signal’s movement through the heart—the usual therapy will be implantation of a permanent pacemaker. As its name implies, this device takes over pacing of the heart rate when the heart’s own electrical system isn’t doing the job right.

Reduce your risk
The risk for heart-related syncope increases with age. People who have existing coronary artery disease, chest pain (called angina), a history of signaling or structural abnormalities of the heart, or hereditary or congenital heart conditions may have increased risk for syncope.

Although most of these factors are not controllable, it is possible to take lifestyle-related steps to protect your heart health and reduce your risk for coronary artery disease and arrhythmias. These steps include eating a heart-healthy diet and keeping your cholesterol levels and blood pressure in check, limiting your alcohol intake, maintaining a healthy weight, not smoking, and getting regular physical activity.


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