Irregularities in your heart rhythms can be described by the effect they have on the speed of your heartbeat (acceleration or deceleration) and where they occur in your heart (in the atria or in the ventricles). Another type of arrhythmia, called heart block, is a partial or complete interruption in the transmission of the electrical impulses between the upper and lower chambers of your heart.
Bradycardia and Tachycardia
An irregular heartbeat can be either too slow (bradycardia) or too fast (tachycardia). A healthy person generally has a resting heart rate of 60 to 100 beats per minute.
Bradycardia, a heart rate of less than 60 beats per minute, may not be a medical problem. A physically active person whose heart pumps very ef?ciently may have a lower heart rate that is not at all abnormal. But a very slow heart rate can become a problem if the brain does not receive enough blood, causing symptoms such as light-headedness or fainting.
Bradycardia most commonly affects older people because with age- related damage of the heart’s electrical system, all the impulses from the atrium may not get to the ventricle. It may be caused by damage to the sinoatrial node (where the electrical pulse begins) or to the biological “wires” that connect the upper chambers (atria) to the lower chambers (ventricles). This damage may be brought on by heart disease, aging, a genetic defect, or some drugs or medications. Medications or a tempo- rary pacemaker can speed up the heart’s contractions temporarily. A pacemaker is also a long-term treatment.
Tachycardia, a very rapid heart rate of more
than 100 beats per minute, can take many forms, depending on where in the heart it occurs. Fibrillation, perhaps the most serious form of tachycardia, causes the heart muscle to quiver instead of contracting rhythmically. (For symptoms, see page 258.) The heartbeat is not only too fast but uncoordinated as well. Both tachycardia and ?brillation, in various forms, can be treated with medications, surgery, or mechanical devices.
Heart Block
Heart block is a condition in which the sinoatrial node sends a normal electrical impulse, but the signal does not travel through the atrioventricular node and into the ventricles as it should. Therefore, there may be inef?cient contraction of the ventri- cles. It usually occurs as a result of aging, or because the heart is scarred from chronic heart disease such as coronary artery disease or from valvular heart disease (which a per- son may be born with). Prior heart surgery may also cause scarring. Certain medications that slow the electrical conduction through the heart—for example, digitalis, beta-blockers, or some calcium blockers—can worsen heart block.
Heart block is classi?ed into three groups, according to how severe it is. In ?rst-degree heart block, the electrical impulse moves too slowly through the atrioventricular node. Your doctor may refer to the PR interval, which is a part of an ECG recording that measures the amount of time it takes for an impulse to get from the atria into the ventricles (see page 265). If your PR interval is longer than 0.2 seconds, you have ?rst-degree heart block. If your heart rate and rhythm are normal, there may be nothing wrong with your heart. In fact, some highly conditioned athletes also have ?rst-degree heart block. Usually, you will not require treatment for a ?rst-degree heart block. If you are taking medications such as digitalis or beta-blockers , the drug may be causing the condition.
If you have second-degree heart block, some signals from your sinoatrial node do not reach your ventricles. In most people with second-degree block, impulses are progressively delayed in the atri- oventricular node with each heartbeat until a full beat is skipped. This is called a Mobitz type of block. You may have no symptoms, or you may experience some dizziness, but the condition is not serious. On an ECG, the skipped beat will show up as a P wave that is not followed by a QRS wave—a tracing of a contraction in the atria that did not activate the ventricles (see page 265). In a Mobitz type II heart block, the inter- val between the P wave and the QRS wave remains constant, but the atrioventricular node intermittently blocks the electrical impulses. A Mobitz type I block may pass on its own, but a Mobitz type II block is generally more serious and requires that you have a pacemaker implanted .
In a person who develops third-degree or complete heart block, no signals at all are passing from the atria into the ventricles. To compen- sate, the ventricles use their own secondary pacemaker to contract and keep blood moving. But the heartbeats generated this way are slow and cannot maintain full heart function. On an ECG, the relationship between the P wave and the QRS wave is completely abnormal (see page 266). A person with third-degree heart block may lose conscious- ness, may develop heart failure, and is at risk of cardiac arrest. A mechanical pacemaker must be implanted on an emergency basis. If it is not possible to put one in right away, a temporary pacemaker device can be used to keep the person alive until surgery can be done.
For all types of heart block, the decision of whether to implant a pacemaker is based on the severity of the bradycardia symptoms. In some cases, the deciding factor is how slow your heart rate has become.
Ventricular Arrhythmias
Generally, an arrhythmia in the ventricles is a more serious condition than one in the atria, because the ventricles perform the heart’s essen- tial pumping functions. Most serious ventricular arrhythmias occur in association with other forms of heart disease, rather than as an isolated problem. A healthy person may have numerous isolated extra heart- beats originating in the ventricle, and a person with normal heart func- tion usually does not require treatment. Ventricular tachycardia is made up of several of these irregular heartbeats in a row.
Premature Ventricular Contraction
Premature ventricular contractions occur when your ventricles contract too soon and interrupt the normal heartbeat. They may happen without warning, and often occur after you have consumed caffeine or taken over-the-counter medications that contain ephedra or ephedrine. By themselves, premature contractions may be harmless and often do not require treatment. But if you have another heart condition such as car- diomyopathy or heart failure, premature ventricular contractions can be a warning of more serious or prolonged rhythm disturbances such as ventricular tachycardia or ventricular ?brillation.
Ventricular Tachycardia
In a person with ventricular tachycardia, a series of ventricular contrac- tions originates from a spot within the ventricles, and the heartbeat quickens—from 100 to 250 beats per minute. The initial concern with this form of tachycardia is that the arrhythmia may interfere with the ability to pump blood, and the person may become dizzy or faint. But ventricular tachycardia may deteriorate without warning into ventricu- lar ?brillation, which is life-threatening.
Therefore, ventricular tachycardia is considered a medical emer- gency. The goal of treatment is to stop the rapid heartbeat, with elec- trical shock (de?brillation) if necessary, and then to prevent it from recurring. If the heart cannot return to a normal rhythm, it may go into ventricular ?brillation, which can be fatal in minutes.
Ventricular Fibrillation
Ventricular fibrillation is the most dangerous form of arrhythmia, requiring immediate emergency attention. In this form of extreme tachycardia, several impulses may be ?ring from different locations in the heart, and the heart contractions are in chaos. Although the heart rate may be as high as 300 beats per minute, the heartbeats are com- pletely ineffective and very little blood leaves the heart. Since the brain is the organ most sensitive to the loss of oxygenated blood, ven- tricular ?brillation causes unconsciousness. Someone should call 911 or emergency medical services immediately and begin cardiopul- monary resuscitation (CPR) immediately if you are not breathing properly. Electric shock (de?brillation) is usually essential to restore heart rhythm, to prevent severe damage to the brain and other organs. Cardioversion (see page 271) may be used to deliver the necessary shocks. As many as 250,000 people die suddenly each year from ven- tricular ?brillation.
A de?brillator (sometimes called an automated external de?brilla- tor, or AED) is an electronic device that emergency medical services personnel or other trained “?rst responders” use to deliver shock to someone whose heart is fibrillating. These defibrillators are now available in many public places such as health clubs and airports.
Supraventricular Arrhythmias
An arrhythmia that occurs in either of the two atria of your heart, located above your ventricles, is considered a supraventricular (or atrial) arrhythmia.
Supraventricular Tachycardia
Supraventricular (or atrial) tachycardia is a regular but very rapid heart- beat (more than 100 beats per minute) involving the upper chambers of the heart. It can occur in several different forms, when regions of the atria other than the sinoatrial node (the natural pacemaker) develop the ability to ?re electrical impulses repetitively. The path that these “extra” impulses take determines what type of tachycardia you have.
In one type (atrioventricular nodal reentrant tachycardia), electrical impulses travel in an abnormal circular path around the atrioventricu- lar node between the atria and the ventricles, causing the heart to beat with each circle. Another form, called Wolff-Parkinson-White syn- drome, occurs when there is an extra electrical pathway between the atria and ventricles that causes electrical impulses to arrive at the ven- tricles too soon, resulting in a rapid heart rate. Some are caused by short circuits or extra electrically active tissue in the heart. It turns out that these “reentry circuits” are the most common mechanism.
If you have supraventricular tachycardia, you may experience palpi- tations or a sense that the heart is ?uttering or racing. Often these symptoms occur abruptly with little or no warning. Some people have shortness of breath, chest pressure or pain, or light-headedness. These sensations may last for a few seconds or several hours. The symp- toms can be alarming, but usually supraventricular tachycardia is not life-threatening. Of course, if you have these symptoms, you should have your doctor diag- nose and treat your condition. Treatment with drugs (see page 268) can relieve symptoms, or a cardiac ablation procedure (see page 269) can cure the condition.
If you have severe symptoms and go to the emergency room, doctors may give medica- tions that can stop the supraventricular tachy- cardia and thus relieve your symptoms rapidly. Also, if the type of tachycardia you have has not yet been diagnosed, an ECG performed while you are experiencing the symptoms is very helpful in determining the best long-term treatment.
Atrial Fibrillation
Atrial fibrillation (AF) is the most common type of arrhythmia in the United States, occurring in 5 to 10 percent of all people over 65. People over the age of 80 are especially vulnerable, too, although it can occur in some people who are 40 or younger. In a person with AF, the electrical impulse from the sinoatrial node accelerates as it spreads across the atria, causing these upper chambers of the heart to quiver, contracting rapidly and irregu- larly—at rates of 400 to 600 beats per minute. A specialized structure between the atria and the ventricles, the atrioventricular node, acts as a safeguard, stopping one or two of every three signals from the atria before they reach the ventricles. But the ventricles still beat too rapidly and irregularly.
AF may occur without any associated heart disease. However, it is commonly linked with hypertension (high blood pressure), coronary artery disease, mitral valve disease, pericardial disease, lung disease, cardiomyopathy, or thyroid disease. When AF occurs, it is important to slow the ventricular rate and then look for the cause and treat that.
Several different forms of AF can occur, and the symptoms can vary widely. Some people experience AF only occasionally, with symptoms such as palpitations that last from a few seconds to a few days before subsiding spontaneously; this form is called paroxysmal atrial ?brillation. In a person with persistent AF, episodes do not stop by themselves, and drugs or other treatments—such as ablation or cardioversion are required to restore normal heart rhythm. Permanent AF is constant and does not respond to treatment. In these situations treatment focuses on heart rate control and prevention of blood clots. AF can cause symptoms of fatigue or short- ness of breath and lead to ?uid buildup. Over time the heart rate may slow to the point of causing bradycardia .
For many people, the experience of AF is unpleasant—causing a sensation of palpitation and unwellness—but not necessarily harmful. Treatment can relieve the symptoms, and AF is generally unlikely to advance to a more serious condition. But having palpitations can be frightening and worrisome. If you experience palpitations for the ?rst time, you should always get medical attention to diagnose the problem.
AF can cause blood to pool in the atria, which can lead to blood clots. If a clot travels from the heart into a smaller artery in the brain, it can cause a stroke. About 15 percent of strokes occur in people with AF, and among those with AF, the rate of strokes is about 5 percent per year. Once AF is diagnosed, your doctor may prescribe warfarin, a blood thinner, which prevents blood clots from forming and reduces the risk of stroke by two thirds. Risk factors for blood clots associated with AF include advanced age, diabetes, high blood pressure, previous heart damage, and a history of stroke.
Left untreated, AF can cause a chronic increase in heart rate, which can weaken the ventricles over time and cause heart failure. But most people seek treatment before this occurs.
Atrial Flutter
Atrial ?utter is another common form of arrhythmia in which the atria beats rapidly but relatively regularly. It usually occurs when electrical impulses are trapped in an endless loop, typically in the lower right atrium. Although the atria may be contracting as quickly as 300 times per minute, the atrioventricular node allows only some of those beats to pass into the ventricles. Still, the ventricles are contracting too quickly and the heart is not pumping as ef?ciently as it needs to. Atrial ?utter or atrial ?brillation often occurs as a consequence of a heart attack or surgery on the heart or lungs.