Each beat of the heart is associated with an electrical impulse which passes over the heart muscle.
This electrical impulse originates in the right auricle (sino-auricular node) and travels over both the auricles in a wave-like fashion to reach a place in the wall between the ventricles (auriculo-ventricular node) and then spreads over both the ventricles.
This electrical impulse is conducted throughout the body tissues to the body surface. Here it can be recorded by placing electrodes at strategic points. The record of this electrical impulse of the heart is called an Electrocardiogram (E.C.G.) and such a test is called an ECG test
Normal ECG reading
A typical E.C.G. record of a heart beat shows a sequence of waves which have been labeled P, QRS, and T. P is due to electrical activity in the upper chambers, the auricles, and QRS, and T wave due to activity in the lower chambers, the ventricles.
Since the ventricles constitute the major muscle mass, they are correspondingly responsible for the major and more important share of the final electrocardiogram.
In taking an E.C.G., metallic electrodes are placed on the strategic points, 1.e. the forearms, legs and over the chest of the patient. The electrodes with their connections act as leads to convey the electrical impulses of the heart from skin to the electrocardiograph, the machine.
The electrodes on the limbs and the chest, convey the electrical impulse of the heart as it presents itself there. Inference based upon impulses taken at different places represents a comprehensive picture of the state of the health of the heart.
ECG test and heart attack
In case of injury to the heart muscle, as happens in a heart attack, the configuration of the waves, particularly of the QRS and T waves which represent the ventricles and which are often damaged in a heart attack, are changed.
This is due both to the absence and/or diminished blood supply to the muscle fibres leading to their death or injury.
The nature of the changes in the electrical waves depends upon the site and extent of injury as well as on the time interval after which an E.C.G. has been taken after a heart attack. An E.C.G. taken immediately after a heart attack may not show any change.
The earliest changes effect the RS-T segments and are usually observed within 24 hours, rarely within a few hours, after the attack (in some instances, the RS-T changes may appear only after 2 or 3 days or even longer).
The first significant abnormality is an elevation of the RS-T segment above the base line in lead I, III and a VF or in the chest leads.
Characteristically, this segment presents a “high take-off” from the descending limb of the transient, lasting only a few hours or days, but it may persist for 2 to 3 weeks.
Subsequent changes produce a progressive regression of the S-T segment to the base-line and a concomitant lowering and eventual inversion of the T wave in the same lead Q wave in the same lead.
The E.C.G. changes which are regarded as characteristic of a heart attack are observed only when the infarct is large and extends the major thickness of the left ventricle.
There are innumerable wave patterns which appear in an E.C.G. after a heart attack. Only a competent heart specialist is in a position to draw inference from them, and even to differentiate an abnormal from a normal one.
Usually E.C.G. discloses changes in the impulse patterns of the heart which are not only indicative of the disease but also tell the area involved in the process.
In some cases, for various reasons, an E.C.G. may not do so.
This may happen:
When the patient has had attacks before and the picture of the previous one superimposed upon the fresh one, changes it drastically or even neutralizes it.
The area of the heart involved in the process is too small to influence the main QRS complex and only S-T segment or T wave changes are observed.
Infarcts at some situation, e.g. the lateral wall, either because of the small size or because of their peculiar situation with respect to the placement of electrodes, fail to influence the recorded impulse in the E.C.G.
Any previous abnormality of the heart, not due to a previous heart attack, obscures the electrocardiographic pattern of a heart attack.
When alongwith a heart attack, some complication such as inflammation of the covering of the heart (pericarditis) occurs, so that E.C.G. changes are dominated by and are diagnostic of the pericarditis only.
The number of E.C.G. taken is insufficient, or the time of which these are taken in relation to the occurrence of the infarct, is not adequate to disclose the change.
Previous treatment by digitalis for an already existing heart disease, conceals changes due to heart attack.
If the chest is deformed due to any previous disease, the E.C.G. does not indicate changes due to heart attack.
An electrocardiogram is only a help in arriving at the diagnosis. It can, by no means, replace a careful history taking and physical examination of the patient. E.C.G. taken at frequent intervals and the evolving pattern of changes observed, are helpful in proper assessment of the recovery.
A single E.C.G. may mislead; and E.C.G. read without knowing the history of the patient, quiet often, is misleading.
In general, if the symptoms and signs of heart attack in a patient are decidedly characteristic, there is less need for characteristic E.C.G. changes to establish the diagnosis.
Long after a heart attack has healed, an E.C.G. may continue to show changes due to the occurrence of the heart attack; these changes, however, are not as conclusive or striking as they are soon after the heart attack.
An E.C.G. in cases of through and through heart infarct returns to normal in about 10 per cent of cases, usually within a year after the attack. In the remainder, Q waves, inverted T waves or T waves of low voltage or slurred, notched and prolonged QRS complexes, persist without necessarily denoting an unfavourable prognosis.
There are other laboratory investigations, besides an E.C.G., which help in arriving at the diagnosis and in assessing the severity of the condition.