What is ECG?
The heart is a muscular bag that pumps blood by contracting and relaxing. Messages for the muscle to contract are carried electrically from one part of the heart to another. An electrocardiograph is a machine that measures the electrical activity of the heart to produce a tracing (electrocardiogram – ECG) that can sometimes be used to diagnose heart disease.
How does ECG work?
Early ECG recordings were made by putting both the patient’s arms and a leg in a bucket of salty water (which conducts electricity), and putting a wire from an instrument for measuring changes in electrical current (a galvanometer) into each bucket. Modern electrocardiographs are more sophisticated but work on the same principle, using electrodes (metal plates or plastic patches covered in a gel) and leads instead of wires and buckets of water. A present day ECG machine is relatively small – about the size of a shoebox – but still quite expensive. An electrocardiogram is the single most important test for coronary heart disease due to narrowing of the arteries.
To have an ECG the patient is asked to strip to the waist (including removal of the bra in women) so that ten electrodes can be attached to the chest, arms and legs. They are held in place by sticky pads. There is no discomfort or pain of any kind. While lying quietly, the doctor will to operate the machine to measure the electrical activity being picked up from the body by the ten different leads. If the patient moves to any significant degree, the electrical activity generated in the muscles may interfere with the reading.
The machine will produce a graph consisting of a continuous wriggly line that looks like a landscape of steep mountains and deep valleys and represents the activity of the heart. The graph may appear on either a screen or on a long strip of paper. A normal healthy heart has a characteristic pattern. Any irregularity in the heart rhythm or damage to the heart muscle will show up as being different from the normal pattern.
The different bumps and spikes in the printout are identified by letters of the alphabet. The small bump marked P shows the activity of the atrium (the upper chamber of the heart). The QRS zigzag is the contraction of the ventricles (lower heart chamber), and the T wave is the recharging of the heart ready for the next beat.
The ECG may show enlargement of the heart, irregular beats, damage to the heart (eg. by a heart attack), poor blood supply to the heart, abnormal position of the heart, high blood pressure, abnormal nerve pathways, and even an imbalance in the blood chemicals that control heart activity.
The interpretation of an ECG reading is a very complex task, and may take up a considerable time. Doctors receive special training in this art during their course. Twelve different lines are recorded in an ECG one after another (or simultaneously on very sophisticated machines) and each line has its own normal and abnormal characteristics. As an example, the figure above is characteristic of lead IV, but if the Q wave was much larger and there was a smoothing of the angle between the S and T waves, the ECG would be indicating that a heart attack may have recently occurred. Others of the twelve leads would also have to show abnormal characteristics to confirm the diagnosis.
The letters P, Q, R, S and T used to identify the main waves of an ECG tracing were chosen arbitrarily by William Einthoven, the Dutch physician who invented the ECG in 1903.
Can ecg detect heart attack?
Unfortunately the cardiograph only shows what is happening to the heart at the moment the reading is taken. It cannot always predict what will happen to the heart in the future. It is not unknown for a patient to have a heart attack only minutes after a normal ECG. For this reason, if there is significant suspicion (not just for routine investigations), an exercise ECG (heart stress test) will be performed.
Electrocardiograms are sometimes performed as a routine part of a medical examination, but more commonly if a doctor is suspicious about a patient’s health.