Heart Electrical Activity (EKG)
heart contains special tissue that produces and sends electrical
impulses to the heart muscle. It is these impulses that trigger
the heart to contract. Each time the heart beats, it sends out
an electric-like signal. The heart's electrical signals can
be measured with a special machine called an electrocardiogram
(EKG or ECG).
record the ECG, small patches or stickers called electrodes
are placed on different parts of the body. One is put on each
arm and leg and six across the chest. Each of these leads monitors
distinct areas of the heart. Using combinations of these electrodes,
different tracings of the heart's electrical activity can be
made and permanently recorded on paper or in a computer. The
tracing at the top is how a given lead of an EKG tracing might
major waves of electric signals appear on the ECG. Each one
shows a different part of the heartbeat. The first wave is called
the P wave. It records the electrical activity of the heart's
two upper chambers (atria).The second and largest wave, the
QRS wave, records the electrical activity of the heart's two
lower chambers (ventricles).The third wave is the T wave. It
records the heart's return to the resting state.
studying the shape and size of the waves, the time between waves,
and the rate and regularity of beating, a doctor can learn a
lot about the heart and its rhythm.
sinoatrial node (SAN), located within the wall of the
right atrium (RA), normally generates electrical impulses that
are carried by special conducting tissue to the atrioventricular
reaching the AVN, located between the atria and ventricles,
the electrical impulse is relayed down conducting tissue (Bundle
of HIS) that branches into pathways that supply the right and
left ventricles. These paths are called the right bundle branch
(RBBB) and left bundle branch (LBBB) respectively.
The left bundle branch further divides into two sub branches
impulses generated in the SAN cause the right and left atria
to contract first. Depolarization (heart muscle contraction
caused by electrical stimulation) occurs nearly simultaneously
in the right and left ventricles 1-2 tenths of a second after
atrial depolarization. The entire sequence of depolarization,
from beginning to end (for one heart beat), takes 2-3 tenths
of a second.
heart cells, muscle and conducting tissue, are capable of generating
electrical impulses that can trigger the heart to beat. Under
normal circumstances all parts of the heart conducting system
can conduct over 140-200 signals (and corresponding heart beats)
SAN is known as the "heart's pacemaker" because electrical
impulses are normally generated here. At rest the SAN usually
produces 60-70 signals a minute. It is the SAN that increases
its' rate due to stimuli such as exercise, stimulant drugs,
the SAN fail to produce impulses the AVN can take
over. The resting rate of the AVN is slower, generating
40-60 beats a minute. The AVN and remaining parts of
the conducting system are less capable of increasing heart rate
due to stimuli previously mentioned than the SAN.
Bundle of HIS can generate 30-40 signals a minute. Ventricular
muscle cells may generate 20-30 signals a minute.
rates below 35-40 beats a minute for a prolonged period usually
cause problems due to not enough blood flow to vital organs.
with signal conduction, due to disease or abnormalities of the
conducting system, can occur anyplace along the heart's conduction
pathway. Abnormally conducted signals , resulting in alterations
of the heart's normal beating, are called arrhythmias
analyzing an EKG a doctor is often able to tell if there are
problems with specific parts of the conducting system or if
certain areas of heart muscle may be injured.