the doctor detects during examination. An ELEC-
TROCARDIOGRAM (ECG) confirms the diagnosis.
ECHOCARDIOGRAMmay reveal the underlying cause
of the arrhythmia, especially when VALVULAR HEART
DISEASEis to blame. Antiarrhythmia medications
such as beta blockers or calcium channel blockers
restore a normal heart rhythm (normal sinus
rhythm) in most people who have atrial fibrilla-
tion. These medications can have mild to signifi-
cant side effects and can slow the heart too much,
causing bradycardia, another arrhythmia.
For atrial fibrillation that does not respond to
these medical measures, the cardiologist may sug-
gest CARDIOVERSION, which uses electrical shock to
jolt the heart back into normal rhythm, or
RADIOFREQUENCY ABLATION, which destroys a small
portion of heart tissue to permanently disrupt the
flow of electrical impulses in the heart. Cardiolo-
gists typically prescribe ANTICOAGULATION THERAPY,
usually aspirin or warfarin and sometimes both, in
addition to antiarrhythmia medications for people
who have atrial fibrillation, to reduce the risk for
clot formation and resulting stroke.
Common causes of atrial fibrillation include
HYPERTENSION (high blood pressure), CORONARY
ARTERY DISEASE (CAD), congestive HEART FAILURE,
PERICARDITIS, and RHEUMATIC HEART DISEASE. Atrial
fibrillation may follow MYOCARDIAL INFARCTIONand
is also more common among people who have
DIABETESor HYPERTHYROIDISM. There are no known
measures for preventing atrial fibrillation beyond
lifestyle behaviors to maintain overall cardiovas-
cular health.
See also GALLOP; LIFESTYLE AND CARDIOVASCULAR
HEALTH; LONG QT SYNDROME(LQTS); MEDICATIONS TO
TREAT CARDIOVASCULAR DISEASE; WOLFF-PARKINSON-
WHITE SYNDROME.
atrioventricular (AV) node A cluster of electrical
fibers in the HEARTthat focuses and intensifies the
electrical impulses the SINOATRIAL(SA) NODEiniti-
ates. The SA node, a cluster of specialized NERVE
fibers at the top of the right atrium near the supe-
riorVENA CAVA, is the heart’s natural PACEMAKER. It
generates a rhythmic electrical impulse that
spreads through the myocardial cells of the right
atrium. The AV node, located at the base of the
right atrium at its juncture with the right ventri-
cle, gathers the impulse and amplifies it, sending it
through the BUNDLE OFHISand into the right and
left bundle branches. The electrical impulse gath-
ers STRENGTHas it travels these electrical conduits,
culminating in the Purkinje fibers near the base of
each ventricle. The impulse then cascades through
the myocardial cells of the ventricles, bringing the
cells and the ventricles to synchronized contrac-
tion. The AV node also can generate a pacing
impulse when disease or damage to the heart
blocks the SA node’s impulse, though the AV node
impulse is considerably weaker and can sustain
limited cardiac function for only a short time.
For further discussion of the AV node within
the context of cardiovascular structure and func-
tion, please see the overview section “The Cardio-
vascular System.”
See also BUNDLE BRANCH; BUNDLE BRANCH BLOCK;
CARDIAC CYCLE; ELECTROCARDIOGRAM(ECG); SICK SINUS
SYNDROME.automated external defibrillator (AED) A
portable, computerized device to shock a HEARTin
fibrillation (rapid, useless contractions) into a
functional rhythm. AEDs debuted in the 1990s
and now are available in many public locations
and workplaces. Older models are the size of a
small briefcase; newer models are smaller and
lighter, with some designed for transport by res-
cuers on bicycles or on foot. Though manufactur-
ers and emergency medical response experts
recommend people obtain training in their use,
the devices are simple enough for anyone to use
without training. Most models use a computerized
voice to provide step-by-step instructions. Once
the rescuer applies the pads to the chest of the
person having the HEART ATTACK, the AED auto-
matically reads the electrical activity of the heart
and determines whether there is sufficient activity
for an electrical shock to be therapeutic. An elec-
trical shock cannot help a heart that has no elec-
trical activity. The AED is preset to deliver a
precise level and length of shock. AEDs are also
available for home use by people at high risk for
life-threatening arrhythmias. Many emergency
response courses, including the basic life support
curriculum, routinely teach AED use.
See also ARRHYTHMIA; CARDIAC ARREST; CARDIOPUL-
MONARY RESUSCITATION(CPR); CARDIOVERSION; ELEC-
TROCARDIOGRAM(ECG); SUDDEN CARDIAC DEATH.automated external defibrillator (AED) 23