The Electrocardiogram or ECG
The first publication on information about the heart occurred in 1749 authored by Jean Baptiste de Sénac. In 1761, Leopold Auenbrugger published and taught that you could determine the condition of the heart by tapping on the chest and listening to the returning sounds. About 190 years later cardiac ultrasound was demonstrated.
In 1816, René Laënnec developed the stethoscope. It was not until 1903 when Willem Einthoven developed an electrocardiogram (ECG) amplifier that physicians were able to view the electrical activity of the heart. Now 260 years after the first information was published on the heart we still cannot agree on all the terms, standards and color coding of leads. This makes our jobs a little harder than they need to be.
For purposes of this article the term ECG will be used exclusively.
Einthovens leads continue to lead
The ECG amplifier may have started off as a simple design, but over time many improvements were added. The amplifier generally has two frequency responses: monitoring and diagnostic. Depending on which standard the manufacturer follows, the monitoring frequency response is 0.5 to 35 or 40 hertz, while the diagnostic frequency response is from 0.05 to 100 hertz with notch filters at 50 and 60 hertz to eliminate interference from power lines. The standard gain is 1,000, meaning that an ECG signal from the patient that is 1mV in strength has an output of one volt. On most of the modern ECG amplifiers the gains will range from 250 to 5,000. If you need a gain of 5,000 to get that one volt out, the output of the heart is only 0.05 mV. If a patients output is that low, many heart rate detection systems incorporated in ECG amplifiers may not be able to determine the heart rate.
To this day the manufacturers have kept to the leads that Einthoven published, with the right leg as the ground and right arm, left arm and left leg as the active electrodes.
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