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TOOLS TO DIAGNOSE HEART DISEASE: ELECTROCARDIOGRAPH

In many cases the family doctor will want a second opinion from a cardiologist, who has the added advantage of being able to draw on a wider range of medical technology. One of the most frequently used tools of the cardiologist's trade is the electrocardiograph - the ECG, sometimes known in the USA as the EKG. The ECG is in fact a highly sensitive galvanometer, an instrument which measures differences in electrical potential. Coupled to the galvanometer is a time-marker and a recorder.

When the heart beats, there are minute changes in the electrical currents transmitted through the heart muscle, and the size and direction of these currents may be measured and recorded to help in the identification of any normal or abnormal functioning.

The tiny currents are measured by recording differences in electrical potential between pairs of electrodes attached to various parts of the body. The standard method has three pairs of electrodes wired, respectively, to the right and left arm, the right arm and left leg, and the left arm and left leg. The idea is that these provide a good general impression of the electrical activity of the heart as a whole, since they are located to kick up very characteristic signals. It is customary to supplement these three pairs with a number of other electrodes placed directly on

Exercise ECGs being taken.

the wall of the chest (usually in six positions); being closer to the heart, these can pick out small defects which the standard three pairs might miss, and give a more precise location to any aberration.

An ECG is often taken under controlled-exercise conditions (the exercise ECG) because this will reveal abnormalities not apparent on the resting ECG tracing. The demands on the heart and the pressure it develops are greater during exercise, so latent as well as existing CHD - or stress-related heart-rhythm disturbances may be detected.

What do the readings mean? A' normal electrocardiograph will produce a printout (an electrocardiogram) such as you see in the diagram. Tall peaks interspersed with a regular lower one represent the muscular contractions of the heart under the control of the special pacemaker cells.

Look now at the next trace, where you can see the visual results of an over-rapid beat -tachycardia. Here the 'atrial flutter' shows as a series of fast small peaks.

In the third trace we see a similar sort of irregularity, called 'atrial fibrillation': there are sharp bursts of lower peaks, interspersed this time with fast and irregular responses from the ventricles. The atria seem to be twitching rapidly in an uncoordinated way instead of setting a steady pace for the ventricles to follow.

Should the ventricles follow suit and begin to fibrillate this is very bad news, because it usually spells the death of the individual. The ECG here looks like the fifth diagram; disorganized peaks of ventricular activity that effectively block the action of these pumping chambers.

The electrocardiogram means as much to the cardiologist as your native tongue does to you. He can look at the peaks, troughs, rhythms and all sorts of subtleties that most of us might not notice, and see in them the 'finger-printings' of particular kinds of disease. He may, though, want to supplement this data with information from X-rays to see whether the heart has been enlarged by disease, or he may want to go further and use more invasive methods to penetrate into the body itself. Usually this last step is not necessary, but if, for instance, it looks as if surgery may be needed, the technology is available for doctors to take a journey to the heart before an incision is made.

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