Under normal circumstances the heart beats 60 to 80 times per minute, with coordinated electrical surges that keep the rhythm steady. But sometimes the heart beats irregularly—either too fast, too slow or in an erratic manner.
These abnormalities, called arrhythmias, can cause disruptive and frightening symptoms. In the worst-case scenario, they can trigger a potentially fatal [cardiac arrest.].
A normal heartbeat begins with an electrical signal generated by cells in the upper right part of the heart—a region called the sinus node. The sinus node is the heart’s natural pacemaker, initiating the heartbeat. Arrhythmias occur when these signals or their transmission go awry.
The various types of arrhythmias are categorized according to two features: their site of origin and their effect on heart rate.
Arrhythmias can originate in the atria (the two upper chambers of the heart); the atrioventricular (AV) node (a cluster of cells in the center of the heart that conducts electrical impulses from the atria to the ventricles); or the ventricles (the pumping chambers in the lower part of the heart).
Arrhythmias can also produce a heart rate that is too slow (bradycardia—under 60 beats a minute) or too fast (tachycardia—over 100 beats a minute at rest).
Most common arrhythmias
Atrial fibrillation, ventricular tachycardia, and ventricular fibrillation are common arrhythmias.
• Atrial fibrillation. About one in four people age 40 and older develop atrial fibrillation, a form of tachycardia in which the atria quiver (fibrillate) chaotically instead of contracting normally. This causes the ventricles to contract irregularly and often rapidly, and blood is not circulated efficiently throughout the body.
The [risk of atrial fibrillation] increases with age. People with coronary heart disease, heart valve abnormalities, or an overactive thyroid (hyperthyroidism) are also at higher risk than the general population.
Common symptoms of atrial fibrillation include weakness, shortness of breath, lightheadedness, palpitations, and chest pain, although some people have no symptoms.
Atrial fibrillation can lead to blood clots in the atria that can loosen and travel through the circulatory system. If one of these clots blocks an artery supplying blood to the brain, a stroke can occur.
People with atrial fibrillation have a fivefold [increased risk of suffering a stroke].
• Ventricular tachycardia and ventricular fibrillation.
Both of these arrhythmias, which are caused by abnormal electrical signals that originate within the ventricles, are much more serious than atrial fibrillation.
They can develop during a heart attack or in the weeks or months afterward, as damaged heart muscle releases substances that interfere with the normal conduction of electrical impulses in the heart.
In ventricular tachycardia, the heart beats at a rate of more than 100 beats a minute. This rapid heartbeat can cause palpitations, lightheadedness, dizziness, weakness, and fainting, as well as a dangerous drop in blood pressure because too little blood is pumped with each beat.
Depending on how fast the heart rate is, individuals with ventricular tachycardia can survive for hours without treatment. But ventricular tachycardia can progress to ventricular fibrillation, which is fatal if not treated promptly.
During ventricular fibrillation the ventricles twitch rapidly and chaotically, rendering them unable to pump blood throughout the body. As a result, ventricular fibrillation leads to unconsciousness within seconds.
Collapse and cardiac arrest can occur quickly and lead to death within a few minutes without treatment. More than 240,000 Americans die each year of cardiac arrest associated with ventricular fibrillation.
Diagnosing an irregular heartbeat
If your doctor suspects a non-life-threatening arrhythmia, he or she will first take a medical history to find out whether you have any medical conditions or are taking any medications that could be causing the problem. The doctor will also listen to your heart with a stethoscope.
An [electrocardiogram (ECG) at the time of the arrhythmia is the definitive diagnostic test]. The ECG records the electrical pattern of the heart while you are lying down on an examination table at your physician’s office.
Your doctor might also check your heart rhythms with a Holter monitor, which records your ECG for 24 hours as you go about your daily activities.
Electrophysiological studies also are used to assess arrhythmias. Performed in a cardiac catheterization lab, the tests provide information about the electrical activity of the heart and allow the doctor to evaluate where the arrhythmia originates and its severity.
In the procedure, a catheter is inserted into a vein in the groin and threaded into the heart. Local anesthesia is used to numb the area where the catheter is inserted.
Electrodes in the catheter record the heart’s electrical activity and can be used to transmit electrical signals to the heart to reproduce arrhythmias. Medications are then administered intravenously and orally to determine which ones are most effective at preventing or suppressing the arrhythmias.
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