Heart disorders are the most common cause of an abnormal heart rhythm.
Sometimes people are aware of abnormal heart rhythms, but many times they feel only their consequences, such as weakness or fainting.
The diagnosis is based on electrocardiography (ECG).
Treatment involves restoring the heart to a normal rhythm and preventing further episodes.
The heart is a muscular organ with four chambers designed to work efficiently, reliably, and continuously over a lifetime. The muscular walls of each chamber contract in a regulated sequence, pumping blood as required by the body while expending as little energy as possible during each heartbeat.
Contraction of the muscle fibers in the heart is controlled by electricity that flows through the heart in a precise manner along distinct pathways at a controlled speed. The electrical current that begins each heartbeat originates in the heart’s pacemaker (called the sinus node or sinoatrial node), located in the top of the upper right heart chamber (right atrium). The rate at which the pacemaker discharges the electrical current determines the heart rate. This rate is influenced by nerve impulses and by levels of certain hormones in the bloodstream.
The heart rate is regulated automatically by the autonomic nervous system Autonomic nervous system , which consists of the sympathetic and parasympathetic divisions. The sympathetic division increases the heart rate through a network of nerves called the sympathetic plexus. The parasympathetic division decreases the heart rate through a single nerve, the vagus nerve.
Heart rate is also influenced by hormones released into the bloodstream by the sympathetic division:
Epinephrine and norepinephrine increase the heart rate. Thyroid hormone, which is released into the bloodstream by the thyroid gland, also increases the heart rate.
In an adult at rest, the normal heart rate is usually between 60 and 100 beats per minute. However, lower rates may be normal in young adults and those who are physically fit. A person’s heart rate varies normally in response to exercise and such stimuli as pain and anger. Heart rhythm is considered abnormal only when the heart rate is inappropriately fast (called tachycardia), slow (called bradycardia), or irregular or when electrical impulses travel along abnormal pathways.
Normal electrical pathway
The electrical current from the sinoatrial node flows first through the right atrium and then through the left atrium, causing the muscles of these chambers to contract and blood to be pumped from the atria into the lower heart chambers (ventricles). The electrical current then reaches the atrioventricular node, located in the lower part of the wall between the atria near the ventricles. The atrioventricular node provides the only electrical connection between the atria and ventricles. Otherwise, the atria are insulated from the ventricles by tissue that does not conduct electricity. The atrioventricular node delays transmission of the electrical current so that the atria can contract completely and the ventricles can fill with as much blood as possible before the ventricles are electrically signaled to contract.
Tracing the Heart’s Electrical Pathway
The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract.
After passing through the atrioventricular node, the electrical current travels down the bundle of His, a group of fibers that divide into a left bundle branch for the left ventricle and a right bundle branch for the right ventricle. The electrical current then spreads in a regulated manner over the surface of the ventricles, from the bottom up, initiating contraction of the ventricles, which eject blood from the heart.
Causes of Abnormal Heart Rhythms
The most common cause of arrhythmias is a heart disorder, particularly coronary artery disease Overview of Coronary Artery Disease (CAD) Coronary artery disease is a condition in which the blood supply to the heart muscle is partially or completely blocked. The heart muscle needs a constant supply of oxygen-rich blood. The coronary... read more , heart valve disorders Overview of Heart Valve Disorders Heart valves regulate the flow of blood through the heart's four chambers—two small, round upper chambers (atria) and two larger, cone-shaped lower chambers (ventricles). Each ventricle has... read more , and heart failure Heart Failure (HF) Heart failure is a disorder in which the heart is unable to keep up with the demands of the body, leading to reduced blood flow, back-up (congestion) of blood in the veins and lungs, and/or... read more . Many drugs, prescription or nonprescription, including those used to treat heart disorders, can lead to arrhythmias. Some arrhythmias are caused by anatomic abnormalities present at birth (congenital birth defects Overview of Heart Defects About one in 100 babies is born with a heart defect. Some are severe, but many are not. Defects may involve abnormal formation of the heart's walls or valves or of the blood vessels that enter... read more ). Age-related changes in the heart’s electrical system make some arrhythmias more likely.
Sometimes no cause for an arrhythmia can be identified.
Fast arrhythmias (tachyarrhythmias) may start on their own or be triggered by exercise, emotional stress, excessive alcohol consumption, smoking, or use of drugs that contain stimulants, such as cold and hay fever remedies.
An overactive thyroid gland (hyperthyroidism Hyperthyroidism Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Graves disease is the most common cause of hyperthyroidism... read more ), producing high levels of thyroid hormone, may cause fast arrhythmias.
Slow arrhythmias (bradyarrhythmias) may be triggered by pain, hunger, fatigue, digestive disorders (such as diarrhea and vomiting), or swallowing, which can stimulate the vagus nerve excessively. With enough stimulation, which is rare, the vagus nerve can cause the heart to stop for a moment. In most of these circumstances, the arrhythmia tends to resolve on its own.
An underactive thyroid gland (hypothyroidism Hypothyroidism Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions. Facial expressions become dull, the voice... read more ), producing low levels of thyroid hormone, may cause slow arrhythmias.
Symptoms of Abnormal Heart Rhythms
Some people who have abnormal heart rhythms may be aware of them. However, awareness of heartbeats (called palpitations Palpitations Palpitations are the awareness of heartbeats. The sensation may feel like pounding, fluttering, racing, or skipping beats. Other symptoms—for example, chest discomfort or shortness of breath—may... read more ) varies widely among people. Some people can feel normal heartbeats, and most people can feel heartbeats when they lie on their left side.
Arrhythmias have consequences that range from harmless to life threatening. The seriousness of an arrhythmia may not be closely linked with the severity of the symptoms it causes. Some life-threatening arrhythmias cause no symptoms, and some otherwise inconsequential arrhythmias may cause severe symptoms. The nature and severity of the underlying heart disorder are often more important than the arrhythmia itself.
When arrhythmias impair the heart’s ability to pump blood, they can cause weakness, a reduced capacity for exercise, shortness of breath, light-headedness, dizziness, fainting Fainting Light-headedness (near syncope) is a sense that one is about to faint. Fainting (syncope) is a sudden, brief loss of consciousness during which the person falls to the ground or slumps in a... read more (syncope), or death. Fainting occurs when the heart is pumping so inefficiently that it can no longer maintain adequate blood pressure. If such an arrhythmia persists, death may result. Arrhythmias may also aggravate the symptoms of an underlying heart disorder, including chest pain and shortness of breath. Arrhythmias that cause symptoms require prompt attention.
Did You Know...
Diagnosis of Abnormal Heart Rhythms
Often, a person’s description of symptoms can help doctors make a preliminary diagnosis and determine the severity of the arrhythmia. The most important considerations are whether the palpitations are
Fast or slow
Regular or irregular
Brief or prolonged
Another important consideration is whether the arrhythmia causes symptoms.
Doctors also need to know whether the palpitations occur at rest or only during strenuous or unusual activity and whether they start and stop suddenly or gradually.
Certain diagnostic procedures are usually needed to determine the exact nature of the arrhythmia and its cause.
Electrocardiography Electrocardiography Electrocardiography (ECG) is a quick, simple, painless procedure in which the heart’s electrical impulses are amplified and recorded. This record, the electrocardiogram (also known as an ECG)... read more (ECG or EKG) is the main diagnostic procedure for detecting arrhythmias and determining their cause. This procedure provides a graphic representation of the electrical current producing each heartbeat. Usually, ECG records the heart rhythm for only a very short time.
Because arrhythmias are often intermittent, a portable ECG monitor (called a Holter monitor Holter Monitor: Continuous ECG Readings or an event monitor Continuous Ambulatory Electrocardiography ) may be used to record heart rhythm continuously or when the wearer senses an abnormal heart rhythm and activates the monitor. These monitors may be worn for 24 or 48 hours or as long as 2 weeks, depending on the type of monitor and what disorders the doctor is looking for. All long-term monitors can record sporadic arrhythmias as the person engages in normal daily activities. During the recording period, the person also keeps a diary of symptoms and activities, so the doctor can see whether those symptoms are correlated with the arrhythmias.
To detect dangerous arrhythmias that occur very infrequently, doctors sometimes implant a recording device under the skin below the left collarbone (clavicle). The device can be left in place for long periods. It electronically transmits stored recordings of abnormal heart rhythms painlessly through the skin.
People with suspected life-threatening arrhythmias are usually hospitalized. Their heart rhythm is continuously recorded and displayed on a television-type monitor by the bedside or at the nursing station. Thus, any problems can be identified promptly.
Other diagnostic procedures include
Blood pressure measurement during exercise
Echocardiography Echocardiography and Other Ultrasound Procedures Ultrasonography uses high-frequency (ultrasound) waves bounced off internal structures to produce a moving image. It uses no x-rays. Ultrasonography of the heart (echocardiography) is one of... read more to detect anatomic abnormalities
During electrophysiologic testing, catheters with tiny electrodes at their tip are inserted through a vein and threaded into the heart. The electrodes are used to stimulate the heart, and the heart’s response is monitored, so that the type of arrhythmia and the preferred treatment options can be determined.
ECG: Reading the Waves
An electrocardiogram (ECG) represents the electrical current moving through the heart during a heartbeat. The current's movement is divided into parts, and each part is given an alphabetic designation in the ECG.
Each heartbeat begins with an impulse from the heart's pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers of the heart (atria). The P wave represents activation of the atria.
Next, the electrical current flows down to the lower chambers of the heart (ventricles). The QRS complex represents activation of the ventricles.
The electrical current then spreads back over the ventricles in the opposite direction. This activity is called the recovery wave, which is represented by the T wave.
Many kinds of abnormalities can often be seen on an ECG. They include a previous heart attack (myocardial infarction), an abnormal heart rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart (ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.
Certain abnormalities seen on an ECG can also suggest bulges (aneurysms) that develop in weak areas of the heart's walls. Aneurysms may result from a heart attack. If the rhythm is abnormal (too fast, too slow, or irregular), the ECG may also indicate where in the heart the abnormal rhythm starts. Such information helps doctors begin to determine the cause.
Treatment of Abnormal Heart Rhythms
Antiarrhythmic drugs, usually for a fast heart rate
An artificial pacemaker, usually for a slow heart rate
Delivering an electric shock, usually for a fast heart rate
Sometimes, destroying abnormal tissue responsible for the arrhythmia (ablation)
For people who have a harmless yet bothersome arrhythmia, reassurance that the arrhythmia is harmless may be treatment enough. Sometimes arrhythmias occur less often or even stop when doctors change a person’s drugs or adjust the dosages. Avoiding alcohol,caffeine (in beverages and foods), and smoking may also help. Avoiding strenuous exercise may help if palpitations occur only during exercise. Sometimes people need to stop driving until doctors can determine whether treatment is effective.
Antiarrhythmic drugs Drugs to Treat Abnormal Heart Rhythms There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more are useful for suppressing fast arrhythmias that cause intolerable symptoms or pose a risk. No single drug suppresses all arrhythmias in all people. Sometimes several drugs must be tried until the response is satisfactory. Sometimes antiarrhythmic drugs can worsen or even cause arrhythmias. This effect is called proarrhythmia. Antiarrhythmic drugs may also cause other side effects.
Artificial pacemakers Artificial Pacemakers There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more are electronic devices that act in place of the heart’s own pacemaker, the sinoatrial node Regulation of the Heart The heart and blood vessels constitute the cardiovascular (circulatory) system. The heart pumps the blood to the lungs so it can pick up oxygen and then pumps oxygen-rich blood to the body.... read more . These devices are implanted surgically under the skin, usually below the left or right collarbone. They are connected to the heart by wires (leads) running inside a vein. The tips of the wires are implanted in the wall of one or more heart chambers. The pacemaker sends an electrical signal that makes the heart muscle contract.
Electrical shocks to the heart (cardioversion and defibrillation Cardioversion-Defibrillation There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more ) can sometimes stop a fast arrhythmia and restore normal rhythm. The shock briefly stops the heart from beating and, after a second or two, it starts beating again on its own. Often, it starts back in a normal rhythm, but sometimes the arrhythmia starts again. Electrical shocks cannot restart a heart that has no electrical activity at all (asystole). Cardioversion and defibrillation are usually done using an external device, but people with dangerous arrhythmias may have a device, called an implantable cardioverter-defibrillator Implantable Cardioverter-Defibrillator (ICD) There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more (ICD), permanently placed in their body.
Certain types of arrhythmias are due to a localized abnormal area in the heart’s electrical system. Destroying or removing that area (ablation Destroying Abnormal Heart Tissue (Ablation) There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more ) can sometimes eliminate the arrhythmia.
Prognosis for Abnormal Heart Rhythms
Most arrhythmias neither cause symptoms nor interfere with the heart’s ability to pump blood. Thus, they usually pose little or no risk, although they can cause considerable anxiety if a person becomes aware of them. However, some arrhythmias, harmless in themselves, can lead to more serious arrhythmias.
Any arrhythmia that impairs the heart’s ability to pump blood adequately is serious. How serious depends in part on whether the arrhythmia originates in the sinoatrial node Regulation of the Heart The heart and blood vessels constitute the cardiovascular (circulatory) system. The heart pumps the blood to the lungs so it can pick up oxygen and then pumps oxygen-rich blood to the body.... read more , in the atria, in the atrioventricular node, or in the ventricles. Generally, arrhythmias that originate in the ventricles are more serious than those that originate in the atria, which are more serious than those that originate in the sinoatrial node or atrioventricular node. However, there are many exceptions.
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
American Heart Association: Arrhythmia: Information to help people understand their risks of arrhythmias as well as information on diagnosis and treatment