An electrocardiogram (ECG
or EKG) is a recording of the electrical activity of
the heart made from electrodes (electrical conductors) placed on the surface
of the skin, usually on each arm and leg and across the chest.
The function of the heart in expanding and contracting to pump blood to the
body is controlled by small electrical impulses within the heart.
These impulses can be detected by electrodes on the skin and transmitted
to the electrocardiogram machine by wires.
The impulses are then translated into peaks and valleys or squiggles by the ECG machine,
which includes a pen that moves up and down over a long strip of paper passing
through the machine at a steady rate. Each cardiac cycle,
or heartbeat, is recorded as a particular series of peaks and valleys.
The April 23/30, 2003, issue of JAMA includes 2 articles about
prolongation of an ECG interval, the QT interval. Some drugs and genetic
conditions can prolong the QT interval; in a few people this can lead to
dangerous abnormalities of heart rhythm.
Electrocardiograms are useful in indicating
If there are any irregular heartbeats or arrhythmias (abnormal rhythms)
If there is a decreased supply of blood or oxygen to the heart
Whether a heart attack has occurred or is likely to be occurring
What part of the heart may have been damaged
If myocarditis (inflammation of the heart) is present
Pain with nursing (can also be caused by sore or cracked nipples
The ECG records the electrical activity of the heart that triggers the
heart muscle to contract. A normal ECG pattern is made up of several peaks
and valleys representing a single cardiac cycle. A cardiac cycle consists of a
single heartbeat when the heart fills with blood and then contracts to push
the blood throughout the body. A specialized part of the heart called the
sinoatrial (S-A) node signals the beginning of a cycle. In the heart 's
atria (chambers where blood is temporarily held before passing into
ventricles where it is pumped out), an electrical change causes the ECG pen to move and
then return to the neutral position. The wave produced on the ECG is called
a P wave.
Next, the electrical impulse travels through
the atrioventricular (AV) node to reach the muscle cells of the ventricles, causing them to
depolarize (change their electrical charge). Because the walls of the ventricles are
much larger than those of the atria, the amount of electrical change is
greater, and the pen draws a larger wave. When the electrical change
finishes, the pen returns to the neutral position, leaving a mark called
the QRS complex, which usually consists of a
Q wave, an
R wave, and an
S wave. The electrical changes occurring as the ventricular muscle fibers
repolarize (recover their electrical charge) produces a
T wave as the pen
moves again, ending the pattern created by a single cardiac cycle or heartbeat. The ECG peaks and valleys thus indicate
whether the electrical impulses are traveling through the heart at the right speed in the right order.
American Heart Association 800/242-8721http://www.americanheart.org
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Sources: American Heart Association, American Medical Association
Encyclopedia of Medicine
Topic: HEART DISEASE
Parmet S, Lynm C, Glass RM. Electrocardiograms. JAMA. 2003;289(16):2166. doi:10.1001/jama.289.16.2166