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:
|R Wave||1.60 mV|
|Q Wave||25 % of R wave|
|T wave||0.1 to 0.5 mV|
|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.