The bio-potential generated by the muscles of the heart result in the
electrocardiogram, abbreviated ECG (sometime EKG, from the German electrokardiogram).
To understand the origin of the ECG, it is necessary
to have some familiarity with the
anatomy of the heart; a cross section of the interior of the
heart. The heart is divided into four chambers. The two upper chambers,
the left and atria, are synchronized
to act together. Similarly, the two lower chambers, the
ventricles, operate together. The right atrium receives blood through
the lungs, where it is oxygenated.
The oxygen-enriched blood then enters the left atrium, from
which it is pumped into the left ventricle. The left ventricle pumps
the blood into the arteries to circulated throughout
the body. Because the ventricles actually pump the blood
through the vessels ( and therefore do most of the work), the
ventricular muscles are much larger and more important than the muscles
of the atria. For the cardiovascular system to function properly, both
the atria and the ventricles must operate
in a proper time relationship.
Each action potential in the heart originates near the top
of the tight atrium at a point called the pacemaker
or sinoatrial node. The pacemaker is a group of specialized cells that
spontaneously generate action potentials at a regular rate,
although the rate is controlled by innervation,
to initiate the heartbeat, the action potentials
generated by the pacemaker propagate in all directions along
the surface of both atria. The wave-front of activation travel parallel
to the surface of the atria toward the junction of activation
travels parallel to the surface of the atria
toward the junction of the atria and the ventricles.
The wave terminates at a point near the center
of the heart, called the atrioventricular node.
At this point, some special fibers act as a "delay line" to provide proper
timing between the action of the atria and
the ventricles. Once the electrical
excitation has passed through the delay line, it is rapidly
spread to all parts of both ventricles by the bundle of His. The
fibers in this bundle, call purkinje fibers, divide into two branches
to initiate action potentials simultaneously in the powerful musculature
of the two ventricles. The wave-front in the ventricles does not
follows along the surface but is perpendicular
to it and moves from the inside to the outside of the ventricular
wall, terminating at the tip or apex of the
heart. As indicated earlier, a wave of repolarization,
follow the depolarization wave by about 0.2 to 0.4 second.
This repolarization, however, is not
initiated from neighboring muscle
cells but occurs as each cell returns to its resting
potential independently. A typical ECG as it appears
when recorded from the surface of the body. Alphabetic designations
have been given to each of the
prominent features. These can be identified with events related to
the action potential propagation pattern. To facilitate analysis, the horizontal
segment of this waveform preceding the P wave is designated
as the baseline or the isopotential line. The p wave represent depolarization
of the atrial musculature. The QRS complex is the combined
result of the repolarization of the atria and the depolarization
of the ventricles, which occur
almost simultaneously. The T wave is the wave is the
wave of ventricular repolarization, whereas the U wave,
if present is generally
believed to be the result
of after potentials in the ventricular muscle. The P-Q interval
represents the time during which the excitation
wave is delayed in the fibers near the Av node.
The shape and polarity of each of these features vary with
the location of the measuring electrodes with respect to the
heart. and a cardiologist normally bases his diagnosis on readings
taken from several electrode location.
The electrocardiogram is a graphic recording or display
of the time-variant voltages produced by the myocardium during
the cardiac cycle. Figure shows the basic waveform of the normal electrocardiogram.
The P, QRS and T waves reflect the rhythmic electrical depolarization
and repolarization of the myocardium associated with the contractions
of the atria and ventricles. The electrocardiogram
is used clinically in diagnosing
various disease and conditions associated with the heart.
It also serves as a timing reference for other
measurements.
To the clinician, the shape and duration of each feature
of the ECG are significant. The waveform, however, depends greatly upon
the lead configuration used, in
general, the cardiologist looks critically at the various
time intervals, polarities, and amplitudes to arrive at his diagnosis.
Some normal values for amplitudes and duration’s of important
ECG parameters are as follows:
Wave |
Amplitude |
P Wave |
0.25 mV |
R Wave | 1.60 mV |
Q Wave | 25 % of R wave |
T wave | 0.1 to 0.5 mV |
Interval | Duration |
P-R interval | 0.12 to 0.20 sec |
Q-T interval | 0.35 to 0.44 sec. |
S-T segment | 0.05 to 0.15 sec. |
P wave interval | 0.11 sec |
QRS interval | 0.09 sec. |
For his diagnosis a cardiologist would typically look first at the heart rate. The normal value lies in the range of 60 to 100 beats per minute. A slower rate than this is called bradycardia (slow heart) and a higher rate, tachycardia (fast heart). He would then see if the cycles are evenly spaced. If not, an arrhythmia may be indicated. If the P-R interval is greater than 0.2 second, it can suggest blockage of the AV node. If one or more of the basic features of the ECG should be missing, a heart block of some sort might be indicated.