Overview of Abnormal Heart Rhythms
Abnormal heart rhythms (arrhythmias) are sequences of heartbeats that are irregular, too fast, too slow, or conducted via an abnormal electrical pathway through the heart.
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.
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 (sinus 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, 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, particularly 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.
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
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.
The most common cause of arrhythmias is a heart disorder, particularly coronary artery disease, heart valve disorders, and heart failure. 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). 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 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), 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. In most of these circumstances, the arrhythmia tends to resolve on its own.
An underactive thyroid gland (hypothyroidism), producing low levels of thyroid hormone, may cause slow arrhythmias.
Some people who have abnormal heartbeats may be aware of them. However, awareness of heartbeats (called palpitations) 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 (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.
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, or brief or prolonged and 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. However, certain diagnostic procedures are usually needed to determine the exact nature of the arrhythmia and its cause.
Electrocardiography (ECG) 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 (Holter monitor) may be used to record heart rhythm continuously or when the wearer senses an abnormal heart rhythm and activates the monitor. This monitor, usually worn for 24 or 48 hours, 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, which 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, called an event monitor, 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 exercise stress testing and blood pressure measurement during exercise, echocardiography to detect anatomic abnormalities, and electrophysiologic testing. 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
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, in the atria, 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. However, there are many exceptions.
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 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 proarrhythmic. Antiarrhythmic drugs may also cause other side effects.
Some Drugs Used to Treat Arrhythmias
Artificial pacemakers are electronic devices that act in place of the heart’s own pacemaker, the sinoatrial node. 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 low-energy circuitry and battery designs now in use allow these units to function about 10 to 15 years. New circuitry has almost completely eliminated the risk of interference from cell phones, automobile ignition systems, radar, microwaves, and airport security detectors. However, some equipment may interfere with pacemakers. Examples are electrocautery devices used to stop bleeding during surgery, diathermy (physical therapy treatments that use radiowaves to apply heat to muscles), and sometimes magnetic resonance imaging (MRI). MRI may be safe with certain types of pacemaker, depending on how they are constructed.
Keeping the Beat: Artificial Pacemakers
The most common use of pacemakers is to treat slow arrhythmias. When the heart slows below a set threshold, the artificial pacemaker begins to produce electrical impulses. Less commonly, pacemakers are used to treat fast arrhythmias by delivering a series of impulses to decrease the heart rate by converting the fast arrhythmia back to normal.
Cardiac resynchronization therapy (CRT) is another use for pacemakers. In some people with heart disorders, the four heart chambers do not follow their normal, orderly sequence of contractions. Special pacemakers with three leads can restore the normal sequence of contractions and improve outcome in some people with heart failure.
Sometimes an electrical shock to the heart can stop a fast arrhythmia and restore normal rhythm. Using an electrical shock for this purpose is called cardioversion, defibrillation, or electroversion, depending on the type of abnormal rhythm for which it is used. Cardioversion may be used for arrhythmias starting in the atria (such as atrial fibrillation) or the ventricles (such as ventricular fibrillation). However, an electrical shock cannot restart a heart that has no electrical activity at all (asystole). The machine that delivers the shock (a defibrillator) is used by a team of doctors and nurses, by paramedics, or by firefighters.
An implantable cardioverter-defibrillator (ICD), which is about one half the size of a deck of cards, can be placed. Most devices are implanted through the blood vessels just as a pacemaker is, thus eliminating the need for open chest surgery. ICDs continually monitor the rate and rhythm of the heart, automatically detect fast arrhythmias, and deliver a shock to convert the arrhythmia back to a normal rhythm. Most commonly, these devices are used in people who might otherwise die of the arrhythmia. An ICD can also act like a pacemaker, sending electrical impulses to overcome a slow arrhythmia. When an ICD delivers a shock, it can feel like a mild thump in the chest. When a stronger shock is given, people may feel as if they have been kicked.
People who have ICDs can safely be around most home electronic devices, including microwaves, and airport security detectors. However, some equipment with strong magnetic fields or strong electric fields may interfere with ICDs. Examples are electrocautery devices used to stop bleeding during surgery, diathermy (physical therapy treatments that use radiowaves to apply heat to muscles), and sometimes MRI.
Because ICDs do not prevent arrhythmias, drugs often must be taken as well. These devices last for about 5 years. People with an ICD who experience a single shock from the device and who otherwise feel well should contact their ICD clinic or specialist within the week. The device records the person's heart rhythm, thus allowing the doctor to see why a shock was given. People who had other symptoms, such as shortness of breath, chest discomfort, or palpitations, immediately before or after the shock, or who had multiple shocks may have a more serious problem. Such people should go to the emergency department right away.
An automated external defibrillator (AED), requires only minimal training for its use. For example, AEDs can be used by people who receive first-aid instruction in its use (see Figure: Automated External Defibrillator: Jump-Starting the Heart). AEDs can detect the presence of an arrhythmia, determine if a shock is advisable, and deliver the shock automatically. They are present in many public places, such as airports, sports arenas, hotels, and shopping malls.
Certain types of arrhythmias can be controlled by doing surgical and other invasive procedures. An arrhythmia due to a localized abnormal area in the heart’s electrical system can be controlled by destroying or removing that area (ablation). Most often, the abnormal area is destroyed by radiofrequency ablation (delivery of energy of a specific frequency through a catheter with a tiny electrode at its tip that is inserted into the heart). The success of the procedure is different for different arrhythmias ranging from 60 to 80% for more difficult arrhythmias (atrial fibrillation, atrial tachycardia, and ventricular tachycardia) to 90 to 95% for more responsive arrhythmias (supraventricular tachycardias). The procedure takes 2 to 4 hours and requires only 1 to 2 days in the hospital. Less commonly, the area is destroyed or removed during open heart surgery. Sometimes surgery is needed because catheter-based ablation was not effective. Other times, surgery is used because people are having heart surgery for another reason, such as to replace a heart valve.