When your heart beats irregularly, it can be scary. However, arrhythmia (abnormal heartbeat) is actually very common. Arrhythmia translates to when the heart beats too fast, too slowly, too early, or irregularly.

There are more than three million cases of arrhythmia in the U.S. every year, and the disorder is especially prevalent in older adults.

We spoke with Dr. Maria Vila, DO, a family medicine specialist and the medical advisor for eMediHealth to flesh out common types of arrhythmias and their causes.

Note: Though most people have experienced arrhythmias that aren’t cause for concern, seek medical attention if you have additional symptoms with your irregular heartbeat or you’ve had a heart attack or other heart stress.

1. Atrial fibrillation (AF or AFib)

Atrial fibrillation (also called AFib or AF) is a type of arrhythmia that feels like a “quivering” of the heart. AF can lead to blood clots, stroke, heart failure and other complications.

According to Dr. Vila, atrial fibrillation is “most commonly due to longstanding hypertension, but also due to CAD (coronary artery disease), hyperthyroidism, myocardial infarction, and binge drinking alcohol.”

2. Supraventricular tachycardia (SVT)

“SVT happens in patients [who have] two electrical pathways near the AV node,” Vila tells Considerable. Though SVT can occur in patients without cardiac problems, Vila says that it is more common in patients with heart disease (coronary artery disease, heart failure, etc).

“SVT is generally triggered by stimulants, alcohol, nicotine or exercise,” says the doctor.

3. Ventricular Tachycardia

Ventricular tachycardia (VT) is a fast, abnormal heart rate that starts in the ventricles (your heart’s lower chambers). According to Cedars Sinai, VT is defined as 3 or more heartbeats in a row, at a rate of more than 100 beats a minute.

“[VT is] more common in patients with heart disease and structural heart disease (hypertrophic cardiomyopathy or congenital heart disease),” Vila says. “It can be brought on by exercise, but can also be due to electrolyte abnormalities, medication side effects, anemia and excessively low blood pressure.”

4. Ventricular fibrillation

Ventricular fibrillation (V-fib or VF) is a rare and life-threatening type of rapid heart rhythm that starts in the bottom chambers of the heart.

“[VF] leads to death and requires defibrillation,” Vila says. “It is common in patients with cardiomyopathies and structural heart disease, but also can be due to electrolyte abnormalities, medication side effects, drugs, and a heart attack.”

5. Sinus bradycardia

Sinus bradycardia is an arrhythmia that involves a slow heart beat. It is defined as a resting heart rate of 60 beats per minute (BPM) or less, however, symptoms are usually only experienced when heart rate drops to less than 50 BPM.

“Sinus bradycardia can be common in patients who are well-trained athletes. It’s also common during sleep when your parasympathetic nervous system is more active and can happen in [older adults] due to dysfunction in their conduction system,” says Vila.

Moreover, Vila says that medication side effects (e.g. opiates, antiarrhythmic medications, chemotherapeutic drugs, blood pressure medications) can cause sinus bradycardia.

6. Premature Ventricular Beats (PVCs)

Premature ventricular contractions (PVCs) are a very common type of arrhythmia that involve extra heartbeats that begin in one of the heart’s two lower pumping chambers. PVCs typically cause a fluttering sensation or a skipped heart beat in the chest.

According to Vila, PVCs can happen in patients with heart disease, structural heart disease, electrolyte abnormalities, myocardial infarction and cardiomyopathy. “[PVCs] can also be caused by medication side effect or in patients with very high calcium levels,” says Vila.

See also: What your resting heart rate can reveal about your longevity