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Understanding Rhythms

The Normal Heart

The heart is a muscle, about the size of a clenched fist. It is responsible for pumping blood throughout the body, every second of every day. The circulating blood delivers oxygen and nutrients to the body’s organs and tissues, and drops off waste products to be filtered out by the kidneys, liver and lungs. 

The heart performs its task through the collaborative efforts of three components:

  • The heart structure (the muscle's chambers and valves)

  • The electrical system (the signals that tell the heart to beat)

  • The circulatory system (the blood pathways) 



The Heart Structure


The heart muscle is divided into four chambers — the left and right atria (the upper chambers) and the left and right ventricles (the lower chambers). With each heartbeat, the atria draw blood into the heart and send it on to the ventricles, which push the blood out of the heart. Valves located between the chambers and at the ends of the ventricles, open and close like doorways to make sure the blood continues to move forward in the right direction.

The Electrical System

What keeps the heart pumping is a built-in electrical system. An electrical impulse generator, called the “sinus node,” sends signals from the right atrium to trigger the heartbeat, like a natural pacemaker. The electrical current follows a web of pathways through the heart, causing the chambers to squeeze and release in a steady, rhythmic sequence that draws blood into the heart and pushes it out.

The Circulatory System 

Arteries are the vessels that carry “fresh” blood, rich in oxygen, from the left side of the heart to smaller vessels, called capillaries, throughout the body. Veins return “used” blood from the capillaries to the right side of the heart, which routes it to the lungs to pick up fresh oxygen. The fresh blood then returns to the left side of the heart, and the cycle repeats.


When Things Don't Work Together 

Trouble in any part of the heart’s components can disrupt the entire system and lead to problems elsewhere. For instance, a clogged artery can cause a heart attack, which can damage a part of the heart muscle, which can disrupt the electrical pathway in the damaged area, which can cause a disturbance in the heart rhythm (an arrhythmia).


Keeping Things Running Smoothly


Keeping this interrelated system healthy is partially under your control. Following the commonly prescribed advice to eat well, exercise, avoid excess alcohol and refrain from smoking can go a long way toward maintaining good heart health. However, some heart problems can occur despite the best efforts to stay healthy. Ongoing research continues to lead to new medicines, procedures and interventions to help minimize heart problems and keep the whole system running smoothly.

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Heart Rhythm Disorders


Heart rhythm disorders, or arrhythmias, can be electrical, circulatory or structural in origin.

Learn more about acquired and inherited rhythm disorders such as atrial fibrillation, sudden cardiac arrest and Long QT Syndrome below; information is organized in three major categories: electrical, circulatory and structural diseases and disorders.



The following arrhythmias originate in the heart’s upper chambers, or atria:

  • Atrial Fibrillation (AF)
    More than 2 million people in the United States have atrial fibrillation (AF), making it a very common heart rhythm disorder. In AF, the top chambers (atria) quiver in a very rapid, irregular pattern. The heartbeat (determined by the bottom pumping chambers, the ventricles) is irregular and rapid, sometimes beating even faster than 200 times a minute (a normal resting heart rate is typically between 60-100 beats a minute). Although it isn't life threatening, AF can lead to other heart problems, fatigue, congestive heart failure, or stroke. The chances of having a stroke are five times higher for those with AF. Learn more »


  • Atrial Flutter (AFL)
    Atrial flutter is similar to AF, characterized by a rapid heartbeat. Instead of rapid disorganized signals in the atria, however, AFL is caused by a single electrical wave that circulates very rapidly in the atrium (usually the right atrium), about 300 times a minute, leading to a very fast, steady heartbeat. Learn more » 


  • Sick Sinus Syndrome (SSS)
    Also called “sinus node dysfunction,” SSS is not a specific disease, but a group of signs or symptoms that indicate the heart’s natural electrical pacemaker, the sinoatrial node, is not functioning properly. In SSS, the heart rate may be abnormally slow at rest (bradycardia), or it may fail to speed up with activity or in response to adrenaline (chronotropic incompetence). If symptoms develop from SSS, or if the heartbeat is extremely slow even without symptoms, a permanent pacemaker is the primary treatment. Learn more »


  • Sinus Tachycardia
    A harmless rhythm, sinus tachycardia is a normal increase in heart rate that happens with fever, excitement and exercise. It rarely requires treatment aside from treating the underlying problem, such as anemia, infection, or hyperthyroidism. Rarely, the sinus node can cause the heart to beat faster than it should without any underlying cause. If symptoms result, the condition is known as “inappropriate sinus tachycardia,” and some treatment options are available. 


  • Supraventricular Tachycardia (SVT)
    This term describes several other types of fast heart rhythms that are not typically dangerous, but can cause symptoms of palpitations, fatigue, or shortness of breath. They typically start suddenly and stop suddenly, and may last for minutes or hours, with a rapid but steady pulse during the episode. The fast rhythm is usually caused by either an irritable spot that fires rapidly (atrial tachycardia, or AT), or by an electrical “short circuit” that involves an extra electrical connection between the top and bottom chambers of the heart (atrioventricular node reentry, AVNRT, or atrioventricular reentry with an accessory pathway, AVRT). Treatment options include medications or a catheter ablation procedure.

The following arrhythmias originate in the heart’s lower chambers,or ventricles

  • Ventricular Tachycardia (VT)
    Characterized by a very fast heart rate, VT usually is seen in the setting of other serious heart disease, and may be life-threatening. Occasionally, it occurs in people with normal hearts. If it does not stop on its own, VT usually requires prompt treatment with either medication or an electrical jolt to the heart (electrical cardioversion). Further treatment of VT may involve antiarrhythmic medications, a catheter ablation procedure, or rarely surgery. Often, people with VT and heart disease are protected by implantation of a defibrillator (ICD). Because VT may lead to ventricular fibrillation (VF - next item), it is considered a serious condition that warrants aggressive monitoring and treatment. 


  • Ventricular Fibrillation (VF)
    Ventricular fibrillation causes sudden cardiac arrest (SCA), which poses the greatest threat of all arrhythmias and accounts for half of all cardiac deaths. In VF, the heartbeat is rapid and chaotic, which prevents the lower heart chambers, or ventricles, from pumping blood to the brain or body. During VF, the blood pressure falls to zero, and the person falls unconscious. A prompt, life-saving shock (defibrillation) must be delivered to the heart to restore a normal rhythm.

               Sometimes, VF can happen during a heart attack (myocardial infarction), because the heart muscle is irritated by the sudden blockage of an artery. VF can also happen at other times, and be caused by previous heart damage or an inherited (genetic) heart condition. It is important to realize that VF is an electrical disorder of the heart (not the same thing as a “heart attack”) and may or may not be related to a problem with clogged arteries that supply the heart with blood.

                VF is sudden and happens without any warning: it stops all heart functioning. The lack of blood and oxygen throughout the body, and especially to the brain, is deadly within a few minutes if not treated promptly with defibrillation. Although CPR can provide temporary benefit, the only truly effective treatment for VF is defibrillation, which relies on paddles or electrodes to "shock" the heart back to normal rhythm. Without treatment, loss of consciousness comes in seconds, and death is inevitable within minutes.


There are additional disorders or symptoms related to electrical problems:

  • Premature Contractions
    Extra, early, or "skipped" beats are the most frequent cause of irregular heart rhythms. These early, extra beats are weak and may not be felt in the chest or in the pulse at the wrist, which is why symptoms are frequently described as “skipped” beats. Extra beats can come from either the upper chambers (premature atrial complexes, PACs), or the lower chambers (premature ventricular complexes, PVCs) of the heart. 


  • Long QT Syndrome (LQTS)
    Long QT Syndrome is a disorder of the electrical system in which the heart cells take longer than normal to recover electrically after each heartbeat. It can be inherited, acquired after taking certain medications, or caused by a combination of heredity and medications. People with LQTS may be susceptible to ventricular fibrillation.


  • Heart Block
    When some or all of the electrical impulses from the upper chambers of the heart are not properly transmitted to the lower chambers, the condition is known as “Heart Block.” Different types of heart block may require different treatments, depending on exactly which part of the conduction system is faulty. If the bottom chambers beat too slowly, symptoms such as fatigue, lightheadedness, or fainting could develop. If the type of heart block is dangerous, or if symptoms result, a permanent pacemaker is the primary treatment. Learn more »


  • Syncope (Fainting)
    Fainting, or feeling as if one might faint, can be caused by different conditions, ranging from dehydration to serious heart rhythm disorders, so it needs to be evaluated carefully. Sometimes the cause is not heart-related but still can be dangerous, such as when low blood sugar is to blame. Other conditions can cause a drop in blood pressure and lead to syncope, including medications or certain nerve reflexes to the heart and blood vessels. No matter what the cause, fainting can be dangerous simply because of the potential for injuries from falling. Learn more »



The following are circulatory disorders of the heart:

  • Heart Attack (Myocardial Infarction)
    When arteries are clogged to the point of decreasing or stopping blood flow to the heart muscle, a lack of oxygen damages or kills heart muscle causing a heart attack. Recognizing symptoms and getting prompt emergency treatment can eliminate, prevent or limit the amount of heart muscle damage. If you think of the heart as a house, a heart attack is a "plumbing" problem, where the arteries that supply the heart with blood are clogged.

  • Stroke
    Although not true heart disorders, strokes are a related condition. While some strokes occur when a blood vessel bursts and causes bleeding in the brain, most strokes happen for the same reasons as a heart attack — clogged or blocked vessels. A blocked blood vessel in the brain can result either from a blood clot forming right in the spot of an abnormal artery, or a blood clot can develop elsewhere (such as in the heart), and travel in the bloodstream until it gets stuck in a small blood vessel in the brain. All strokes pose serious health threats.



Structural anomalies of the heart can be congenital or acquired:

  • Heart Failure
    When the heart muscle is too weak to effectively pump blood through the body, pressure can back up into the lungs, causing shortness of breath, or into the rest of the body, causing swelling. This backup of blood and pressure is known as “congestive heart failure.” Any condition that results in weakening of the pumping strength of the ventricles, or that causes areas of scar tissue to develop in the ventricles, is known as a ”cardiomyopathy.” Early diagnosis and treatment of cardiomyopathy and heart failure is important to stop or slow progression of heart damage and symptoms. Learn more »


  • Heart Valve Problems
    Heart valve problems can occasionally be inherited, but usually develop from other heart conditions or “wear and tear” during life. Abnormally tight or leaky heart valves can interfere with the heart’s ability to push blood forward from chamber to chamber, and then out to the lungs and body. Medications and valve surgery are treatment options. Some valve problems can only be fixed with surgery. 

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Symptoms & Diagnosis

While some heart rhythm disorders may go unnoticed, others may cause symptoms ranging from a mild fluttering in the chest, to a feeling of breathlessness, to even blacking out.  Several different heart rhythm disorders, including fast arrhythmias (tachycardia), abnormally slow rhythms (bradycardia), and extra beats, may cause similar symptoms in different people.  Therefore, heart rhythm specialists can use a variety of different heart tests to look at the heart’s structure and circulation, as well as the heart’s electrical system, in order to pinpoint the exact cause of arrhythmia symptoms and decide on a treatment plan.

Every heart has an electrical system running throughout its muscle tissue. The sinus node, often called the heart’s natural pacemaker, contains the most active electrical cells, and it initiates heartbeats. Other nodes are responsible for transmitting signals to different sections of the heart. For instance, the atrioventricular node’s function is to transmit signals from the upper chambers, called atria, to the lower chambers, called ventricles. A problem in a node or anywhere along the electrical path can disrupt the regular beating of the heart and cause an arrhythmia. The heartbeat can become too fast or too slow and can be chaotic or steady. Symptoms vary based on the rhythm change.

  • Palpitation or a Skipped Beat - Although it may seem as if the heart missed a beat, really the heart has a premature heartbeat, almost an extra beat happening too soon, which may result in a pause – the “skipped beat.” Learn more »

  • Rapid Heartbeat – Tachycardias - Tachycardia can cause the heart to beat faster than 100 beats per minute, in some cases racing up to 400 beats per minute. Learn more »

  • Fainting – Syncope - Fainting is a sudden loss of consciousness. It most often occurs when the blood pressure is too low (hypotension) and the heart does not pump a normal supply of oxygen to the brain. Learn more »

  • Fluttering - A fluttering sensation is usually due to a series of “skipped beats” that occur in quick succession.  Learn more »

  • Slow Heartbeat – Bradycardias - A diagnosis of bradycardia means that your heart rate is slower than 60 beats per minute, either occasionally or all the time. Learn more »

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