ConditionHeart & circulation
Heart Arrhythmia: Harmless or Dangerous?
A heart arrhythmia is any heartbeat that is too fast, too slow or irregular because of a problem with the heart's electrical signals. Many, such as occasional skipped beats, are harmless. Others, like atrial fibrillation, raise stroke risk, and a few, such as ventricular tachycardia, can cause sudden cardiac arrest. Fainting, chest pain or breathlessness with a racing or irregular pulse needs emergency care.
Also called: Cardiac arrhythmia, irregular heartbeat, dysrhythmia, abnormal heart rhythm
At a glance
- What it is
- A heartbeat that is too fast (above 100 beats a minute at rest), too slow (below 60), or irregular.
- Usually harmless
- Occasional extra beats, sinus arrhythmia with breathing, and slow heart rates in fit people.
- Potentially dangerous
- Atrial fibrillation (stroke risk), ventricular tachycardia and fibrillation (cardiac arrest), and complete heart block.
- Diagnosis
- ECG, wearable or patch monitors, and an echocardiogram to check heart structure.
- Treatment
- Ranges from no treatment to medicines, ablation, pacemakers or implanted defibrillators.
When to get help
Call 911 or go to the ER
- Someone collapses and is not breathing normally: call 911, start CPR and use an AED if one is nearby
- Fainting, or nearly fainting, with palpitations or during exercise
- A racing or irregular heartbeat with chest pain, severe shortness of breath, confusion or a gray, sweaty look
- Face drooping, arm weakness or slurred speech, especially if you have atrial fibrillation
See a doctor within a day or two
- A fast heartbeat (over about 120 beats a minute at rest) that lasts more than a few minutes and does not stop
- A resting pulse below 40 with dizziness, tiredness or breathlessness
- A smartwatch alert for atrial fibrillation with any symptoms
- New palpitations in pregnancy, or with a family history of sudden death under age 50
Book a routine appointment
- Occasional skipped or extra beats that come and go
- An irregular pulse found by a watch or blood pressure monitor, without symptoms
- Palpitations triggered by caffeine, alcohol or stress that keep recurring
Usually safe to manage at home
- Brief flutters with no other symptoms: cut back on caffeine, alcohol and nicotine, sleep well and stay hydrated, and note when they happen
How a normal heartbeat works
Each beat starts in the sinus node, a cluster of cells in the right upper chamber (atrium). The signal spreads across both atria, pauses briefly at the AV node, then travels down to the lower chambers (ventricles), which pump blood to the body. A normal resting rate is 60 to 100 beats a minute and speeds up with exercise.
An arrhythmia happens when a signal starts in the wrong place, travels around a short circuit, or is blocked. For a full explanation of every type, see our main arrhythmia page. This page focuses on a practical question: which irregular heartbeats matter?
Harmless vs dangerous arrhythmias
| Arrhythmia | What it feels like | How serious |
|---|---|---|
| Premature atrial or ventricular contractions (extra beats) | A skip, thud or flip in the chest | Usually harmless; very frequent ones can weaken the heart |
| Sinus arrhythmia | Pulse speeds up on breathing in, slows on breathing out | Normal, especially in young people |
| Sinus bradycardia in athletes | Slow pulse, no symptoms | Normal adaptation |
| Supraventricular tachycardia (SVT) | Sudden racing heartbeat that starts and stops like a switch | Rarely dangerous but distressing; very treatable |
| Atrial fibrillation (AFib) | Irregular, often fast pulse, tiredness, breathlessness | Raises stroke risk about 5 times; needs assessment |
| Atrial flutter | Fast, regular or irregular pulse | Similar stroke risk to AFib |
| Heart block (second or third degree) | Dizziness, fainting, tiredness, very slow pulse | Can be serious; often needs a pacemaker |
| Ventricular tachycardia | Racing heart, dizziness, fainting | Can lead to cardiac arrest |
| Ventricular fibrillation | Collapse within seconds | Fatal without CPR and defibrillation |
Premature ventricular contractions are the most common cause of the "skipped beat" sensation, and most people have some every day without noticing.
Symptoms of an arrhythmia
- Heart palpitations: pounding, fluttering, skipping or racing
- Dizziness or lightheadedness
- Shortness of breath or reduced exercise tolerance
- Chest discomfort
- Tiredness and weakness
- Fainting or near-fainting
- Anxiety or a sense of something being wrong
Many arrhythmias, especially atrial fibrillation in older adults, cause no symptoms and are found by chance on a pulse check, ECG or smartwatch.
Warning signs that suggest a dangerous rhythm
- Fainting, especially during exercise or lying down
- Palpitations with chest pain or breathlessness
- Known heart disease, a past heart attack or heart failure
- A family history of unexplained sudden death, drowning or car crashes in young relatives
- Inherited conditions such as hypertrophic cardiomyopathy, long QT syndrome or Brugada syndrome
What causes heart arrhythmias?
- Heart disease: coronary artery disease, scarring from a heart attack, heart failure, valve disease and cardiomyopathy
- High blood pressure, which stretches and stiffens the atria
- Aging, which affects the conduction system
- Thyroid problems, especially an overactive thyroid
- Sleep apnea
- Electrolyte imbalances: low or high potassium or magnesium
- Stimulants: caffeine, nicotine, alcohol binges ("holiday heart"), cocaine, amphetamines and some decongestants
- Medicines that prolong the QT interval, including some antibiotics, antidepressants and antipsychotics
- Genetic conditions affecting heart muscle or ion channels
- Fever, dehydration, anemia and stress, which speed the sinus rate
How doctors find the cause
- ECG (electrocardiogram): a 10-second tracing. It diagnoses the rhythm if you are having it at the time.
- Holter or patch monitor: records every beat for 1 to 14 days.
- Event monitor or implantable loop recorder: for symptoms that happen rarely; a loop recorder can record for up to 3 years.
- Smartwatch ECG: useful for capturing episodes, but needs confirmation by a doctor.
- Echocardiogram: ultrasound of the heart to look for structural disease.
- Blood tests: thyroid function, electrolytes, blood count and kidney function.
- Exercise stress test: for symptoms during exertion.
- Electrophysiology study: catheters inside the heart map and trigger the arrhythmia, often followed by ablation in the same session.
Treatment
Treatment depends on the type, your symptoms and your risk:
- Reassurance and trigger reduction for harmless extra beats.
- Vagal maneuvers for SVT, such as bearing down as if straining for 15 seconds, which your doctor can teach you.
- Rate-control medicines (beta-blockers, calcium channel blockers) to slow a fast heart.
- Rhythm-control medicines (antiarrhythmics) to keep a normal rhythm.
- Blood thinners to prevent stroke in atrial fibrillation and flutter, based on a risk score that includes age, blood pressure, diabetes and prior stroke.
- Cardioversion: a timed electric shock under sedation to reset the rhythm.
- Catheter ablation: burning or freezing the tissue that triggers or sustains the arrhythmia. It cures most SVT and is a first-line option for many with AFib.
- Pacemaker for slow rhythms and heart block.
- Implantable cardioverter-defibrillator (ICD) for people at high risk of ventricular arrhythmias. It shocks the heart back to normal automatically.
Complications
- Stroke from clots forming in the atria during atrial fibrillation
- Heart failure from a heart that beats too fast for weeks or months
- Sudden cardiac arrest from ventricular arrhythmias. See ventricular fibrillation
- Falls and injuries from fainting
Living with an arrhythmia
Most people with an arrhythmia lead normal lives. Helpful steps include keeping blood pressure under control, treating sleep apnea, limiting alcohol, losing excess weight (which reduces AFib episodes), staying active within your doctor's advice, and checking with a pharmacist before taking cold remedies or new medicines. If you have a device, carry its ID card.
Frequently asked questions
Is a heart arrhythmia dangerous?
It depends on the type. Occasional extra beats and a slightly irregular pulse with breathing are normal. Atrial fibrillation is not immediately life-threatening but raises stroke risk and needs treatment. Ventricular tachycardia, ventricular fibrillation and complete heart block can be life-threatening and need urgent care.
What does an irregular heartbeat feel like?
People describe a flutter, a skip followed by a hard thump, a racing or pounding heart, or a chaotic, irregular pulse. Some feel dizzy, breathless or tired. Others feel nothing at all and only find out from a pulse check, a smartwatch or an ECG.
Can anxiety cause a heart arrhythmia?
Anxiety releases adrenaline, which speeds the normal heart rhythm and can trigger extra beats and occasionally episodes of SVT or atrial fibrillation in people prone to them. Anxiety does not cause dangerous arrhythmias in a healthy heart, but palpitations should still be checked once with an ECG.
Can a heart arrhythmia go away on its own?
Some do. Extra beats come and go, and arrhythmias triggered by fever, dehydration, alcohol or an overactive thyroid often settle once the trigger is treated. Paroxysmal atrial fibrillation stops by itself but tends to return. Structural or inherited rhythm problems need ongoing management.
Should I trust a smartwatch arrhythmia alert?
Treat it as a prompt, not a diagnosis. Smartwatch alerts for atrial fibrillation are fairly accurate but can give false alarms and miss other arrhythmias. Save the tracing and show it to your doctor, who will confirm with a standard ECG or monitor. Seek emergency care if the alert comes with symptoms.
Sources
This guide was written from current patient and clinical guidance, including:
- American Heart Association: About Arrhythmia
- NHLBI: Arrhythmias
- ACC/AHA/ACCP/HRS: 2023 Guideline for the Diagnosis and Management of Atrial Fibrillation
- NHS: Arrhythmia
- Heart Rhythm Society: Heart rhythm disorders