Atrial Fibrillation (AFib) & Normal Sinus Rhythm: Are They Interconnected?

Medically Reviewed by Zilpah Sheikh, MD on January 14, 2024
Written by Mary Jo DiLonardo, Marijke Vroomen Durning, RN
6 min read

Normal sinus rhythm is when your heart beats in a regular, organized way. Your body has a built-in pacemaker called the sinoatrial (SA) node, or sinus node. It uses electrical signals to make sure that the different chambers of your heart are contracting together at a steady pace. This makes your heart beat in a regular pattern. 

 

 

 

Each heartbeat starts with a signal from the SA node, in the right atrium of your heart. The SA node is called the heart's pacemaker because it adjusts your heart rate based on what you're doing. It raises your heart rate (the same thing as your pulse) when you exercise and slows it down when you sleep.

The SA node sends signals that spread across the right and left atria. That makes them contract and forces blood into channels called ventricles. Then, the signal moves to the atrioventricular node (AV) node, which is near the middle of your heart.

From there, the signal travels through the ventricle walls. That makes them contract and squeeze blood out of the heart. The right ventricle sends blood to your lungs, while the left pumps it to the rest of your body. The walls then relax and wait for the next signal.

When everything is working smoothly, you have a normal sinus rhythm and your heart beats between 60 and 100 times per minute.

When something causes problems with your sinus rhythm, you'll have an irregular heartbeat.

Sinus tachycardia

Tachycardia is a fast heart rate above 100 beats per minute. It’s called sinus tachycardia when your SA node is causing your heart to beat that quickly. Sinus tachycardia is quite common and has many causes, such as:

  • Being scared, anxious, or distressed
  • Not drinking enough fluids
  • Exercising
  • Having a fever
  • Consuming caffeine
  • Pain

Many medications and illicit drugs can also cause sinus tachycardia, such as:

  • Albuterol
  • Antihistamines
  • Cocaine
  • Dopamine
  • Epinephrine
  • Scopolamine
  • Tricyclic antidepressants

Sinus tachycardia can also be the result of a serious medical problem such as:

  • Sepsis
  • Shock
  • Hypoxia (lack of oxygen)

If you have one of these, you need immediate medical treatment.

Sinus bradycardia

Bradycardia is the opposite of tachycardia; your heart beats slowly, under 60 beats per minute. Like sinus tachycardia, sinus bradycardia is caused by the SA node. It’s also common, although many people don’t realize they have it unless they have symptoms, such as feeling faint or short of breath. Several things can cause sinus bradycardia, such as:

  • Being very physically active or an athlete
  • Being a young adult
  • Being older, especially while sleeping
  • Imbalance of calcium, magnesium, potassium, and other electrolytes in your body
  • Infections
  • Sleep apnea
  • Low levels of thyroid hormone (hypothyroidism)
  • Anorexia nervosa, an eating disorder
  • Lupus, an autoimmune condition
  • Hypothermia
  • Inflammation in the heart lining (endocarditis), muscle (myocarditis), or the sac around the heart (pericarditis)
  • Rheumatic fever
  • Rheumatic heart disease
  • Lyme disease
  • Heart attack
  • Heart disease

Some drugs can also cause sinus bradycardia, such as:

  • Narcotics (opioids)
  • Cannabis
  • Beta-blockers
  • Calcium channel blockers
  • Anti-arrhythmias 
  • Lithium

Bradycardia can be dangerous if it causes you to faint while driving or operating machinery. You could also hurt yourself if you fall. If you think your pulse is too low, speak with your doctor to rule out heart disease or any other medical problem that could cause it.

Sick sinus syndrome is a problem with the SA node. It is sometimes called sinus node dysfunction. It causes heart rhythm problems, such as sinus tachycardia and sinus bradycardia, or a combination of both. Anyone can have sick sinus syndrome, but it affects older people more often.

Sick sinus syndrome causes

Sometimes, doctors can’t find out what causes sick sinus syndrome in someone, but the most common causes are:

  • Wear and tear on the heart due to age
  • Scarring
  • Low levels of minerals called electrolytes (sodium, potassium, and chloride)
  • A problem in the heart’s electrical system
  • Injury to the SA node
  • Heart disease
  • Previous heart attacks
  • Obstructive sleep apnea
  • Hypothyroidism
  • Muscular dystrophy and other neuromuscular disorders
  • Medications, such as calcium channel blockers, beta-blockers, anti-arrhythmics, digoxin, and lithium

Sometimes it can be caused by genetics, passed down in the family. But that is rare.

When the electrical signals in your heart get mixed up, your heartbeat can get out of rhythm, or your heart can beat too fast or too slow. This is called atrial fibrillation, or AFib. Abnormal electrical signals overpower your SA node's normal signals, leading to strange heartbeat patterns.

The most common causes of AFib are coronary artery disease (narrowing or blocked arteries) and hypertension (high blood pressure). Other causes include:

  • A birth defect called congenital heart defect
  • Sick sinus syndrome
  • A sleep disorder known as obstructive sleep apnea
  • Heart valve disease or heart attack
  • Pneumonia and other respiratory diseases
  • Thyroid problems
  • Viral infections

AFib symptoms

Some people have no symptoms. If you have them, you might feel:

Some people have AFib every once in a while. Symptoms might last a few minutes or a few hours, after which things return to normal. Other people have AFib often or even all the time and need treatment to get their heartbeat to return to a normal sinus rhythm.

AFib vs. sick sinus syndrome

AFib can cause sick sinus syndrome and the syndrome can cause AFib, but the two are not the same thing. AFib is a problem with abnormal electrical signals overpowering your SA node's normal signals. Sick sinus syndrome happens when your SA node does not work as it should. Both can cause an irregular heartbeat but for different reasons.

 

 

 

 

Usually, sinus rhythm problems are caused by something that isn’t serious or that can be managed. If you feel that your heart is racing too often or it is slow and making you feel lightheaded or faint, speak with your doctor as soon as you can. The problem could be something easily treated, such as changing medication. If something more serious is causing your fast or slow heartbeat, your doctor can come up with a treatment plan so it doesn't get worse or cause other problems.

Is AFib a life-threatening condition?

Most of the time, occasional episodes of AFib are not life-threatening, although they can be worrisome. But, AFib isn’t something you should ignore as it could be a sign of a problem with your heart or lead to complications, such as a stroke. Speak with your doctor if you think you are having AFib so you can be checked out.

How do I stop worrying about AFib?

The best way to not worry about having AFib is to see your doctor. Don’t ignore it. Your doctor will do tests to find out what is causing your abnormal heartbeat and come up with a treatment plan.

What are AFib warning signs?

If you have AFib, you may feel some of these symptoms:

  • Fast, irregular heartbeat
  • Fluttering, thumping, or flip-flopping in your chest
  • Shortness of breath
  • Dizziness
  • Lack of energy when you exercise
  • Weakness
  • Fatigue
  • Lightheadedness
  • Confusion
  • Chest pain

What triggers AFib?

For some people, AFib just happens. Others may have triggers, which can include:

  • Poor sleep, often because of sleep apnea
  • Drinking alcohol
  • Consuming caffeine
  • Eating large meals
  • Exercising too much
  • Dehydration
  • Stress

Is normal sinus rhythm a good thing?

For most people, normal sinus rhythm means their heart is beating well. It usually means that the heart muscles are healthy and the electrical signals are passing through as they should.