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.