How the 12 Lead ECG Works

THE ECG - sometimes called EKG, and short for electrocardiogram - is one of the most common and important tests done in hospital today. It works on the fact that heart muscle is stimulated to contract by electricity and that this electricity travels in a predictable way across your chest.

The ecg has so many connected pieces as each 'lead' acts like one camera angle capturing the path the electricity takes across your chest. By looking at all the printouts of the ecg a trained observer can put them together his/her mind into a three dimensional view of heart activity. Each event in the production of a heart beat makes a characteristic shape or 'signature' on the ecg printout.

Above left is a photograph of a portable ECG machine. And right, you can see how it's connected. There are four 'limb' straps (1) placed around each of your wrists and each ankles. The bulbs (2) are then connected across your chest in a special order and a 'trace' of electrical activity drawn by the machine as below.

Q-waves	Normal ECG	Inverted T-waves

Certain patterns are looked for by health practitioners to help diagnose a variety of abnormalities such as abnormal blood chemistry, arrhythmia misfiring, and of course, heart attacks. Q-waves and inverted T-waves are two diagnostic wave forms seen in old heart attacks and new heart attacks respectively.

Doctors learn to 'read' ecg's by learning basic patterns to look for and then fine tune their interpretation through experience viewing hundreds to thousands. Typically a doctor scans an ecg the way you'd scan a magazine article looking for things that jump off the page, then in more detail from the upper left corner down to the lower right corner.

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