Section 3. Heart and Blood Vessel Disorders

Chapter 27. Abnormal Heart Rhythms

Topics: Introduction | Atrial Premature Beats | Atrial Fibrillation and Atrial Flutter | Paroxysmal Supraventricular Tachycardia | Wolff-Parkinson-White Syndrome | Ventricular Premature Beats | Ventricular Tachycardia | Ventricular Fibrillation | Pacemaker Dysfunction | Heart Block | Bundle Branch Block

Introduction

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.

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 and 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 (see Section 6, Chapter 76), 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 (adrenaline) and norepinephrine (noradrenaline), which 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) or slow (called bradycardia), or is irregular or when electrical impulses travel along abnormal pathways.

Normal Electrical Pathway

See the figure Tracing the Heart's Electrical Pathway.

The electrical current from the pacemaker 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.

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 ejects blood from the heart.

See the animation ECG: Reading the Waves.

Causes

The most common cause of arrhythmias is heart disease, particularly coronary artery disease, heart valve disorders, and heart failure. Many drugs, prescription or nonprescription, 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. An overactive thyroid gland (hyperthyroidism), producing high levels of thyroid hormone, may cause fast arrhythmias. An underactive thyroid gland (hypothyroidism), producing low levels of thyroid hormone, may cause slow arrhythmias. 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. 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.

Symptoms

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 cause severe symptoms. Often, the nature and severity of the underlying heart disease are more important than the arrhythmia itself.

When arrhythmias impair the heart's ability to pump blood, they can produce weakness, a reduced capacity for exercise, light-headedness, dizziness, and fainting (syncope (see Section 3, Chapter 23)). 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 underlying heart disease, including chest pain and shortness of breath. Arrhythmias that produce symptoms require prompt attention.

Diagnosis

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 produces 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 often needed to determine the exact nature of the arrhythmia and its cause.

Electrocardiography (ECG) (see Section 3, Chapter 21) 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 (see Section 3, Chapter 21)) 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 hours, can record sporadic arrhythmias as the person engages in normal daily activities. During the 24-hour period, the person also keeps a diary of symptoms and activities, which are correlated with the arrhythmias.

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 nursing station. Thus, any problems can be identified promptly.

Other diagnostic procedures include exercise stress testing (ECG and blood pressure measurement during exercise (see Section 3, Chapter 21)) and electrophysiologic testing (see Section 3, Chapter 21). 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.

Prognosis and Treatment

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 heart's normal pacemaker, 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 pacemaker. However, there are many exceptions.

For people who have a harmless yet worrisome 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), smoking, or strenuous exercise may also help.

Antiarrhythmic drugs are useful for suppressing fast arrhythmias that cause intolerable symptoms or pose a risk. No single drug cures 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 also produce other side effects.

Artificial pacemakers are electronic devices that act in place of the heart's own pacemaker. These devices are implanted surgically under the skin, usually below the left or right collarbone. They are connected to the heart by wires running inside a vein. Because of new low-energy circuitry and battery designs, these units now last about 10 to 15 years. New circuitry has almost completely eliminated the risk of interference from automobile distributors, radar, microwaves, and airport security detectors. However, some equipment may interfere with pacemakers. Examples are machines used in magnetic resonance imaging (MRI) and in diathermy (physical therapy in which heat is applied to muscles).

See the figure 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 fire off electrical impulses. Less commonly, pacemakers are used to treat fast arrhythmias by delivering a series of impulses to decrease the heart rate.

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. Cardioversion may be used for arrhythmias starting in the atria or the ventricles. The machine that delivers the shock (a defibrillator) is used by a team of doctors and nurses, by paramedics, or by firefighters. Also, an implantable defibrillator, which is about one half the size of a deck of cards, can be surgically implanted just as a pacemaker is. The implantable defibrillator automatically senses fast arrhythmias and delivers a shock to convert the arrhythmia back to a normal rhythm. Most commonly, these devices are used in people who would otherwise die of the arrhythmia. Because implantable defibrillators do not prevent arrhythmias, drugs often must be taken as well.

A new type of defibrillator, called 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 Section 24, Chapter 299). AEDs can detect the presence of an arrhythmia, determine if a shock is advisable, and deliver the shock automatically. They are being placed in many public places, such as airports, sports arenas, hotels, and shopping malls.

Certain types of arrhythmias can be controlled by performing surgical and other invasive procedures. For example, performing angioplasty or coronary artery bypass surgery (see Section 3, Chapter 33) may control arrhythmias due to coronary artery disease. An arrhythmia due to a localized abnormal area in the heart's electrical system can be controlled by destroying or removing that area. Most often, the abnormal area is destroyed by radiofrequency ablation (delivery of energy of a specific frequency through an electrode catheter inserted in the heart). This procedure is successful in 90 to 95% of people, 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.

See the drug table Some Drugs Used to Treat Arrhythmias.