The McGill Physiology Virtual Lab

Cardiovascular Laboratory

ECG> Introduction
  The electrocardiograph (abbreviated ECG or EKG) is a device that picks up electrical activity originating in the heart from the surface of the body.  In the clinic, the ECG is one of the most commonly used diagnostic machines.   The recording produced by the electrocardiograph is called an electrocardiogram, also abbreviated ECG or EKG.

Electrocardiogram Waveform



The figure to the right shows a typical ECG.   Three characteristic features of the waveform are easily identified: the P wave, the QRS complex, and the T wave.  The P wave is associated with the activation of the atria, the QRS complex with the activation of the ventricles, and the T wave with repolarization of the ventricles.

Electrocardiogram Intervals

  • The P-R interval is the time from the beginning of the P wave to the start of the QRS complex.
  • The QRS interval or duration or width is the time from the beginning to the end of the QRS complex.
  • The QT interval is the time from the beginning of the QRS complex to the end of the T  wave.
  • The RR interval is the time from the peak of one R wave to that of the following R wave.
Electrocardiograph:   Technical Principles
The electrocardiograph is essentially an electronic device that amplifies the very small potentials present at the surface of the body, so that they can be displayed on a video screen or recorded permanently on paper. The signal is picked up by electrodes placed at certain well-defined anatomical positions on the body surface.

The heart of the ECG is an electronic amplifier with two input terminals, a non-inverting input terminal (+) and an inverting input terminal (-).  The output voltage Vo is simply proportional to the difference between the voltages V+ and V- appearing at the two input terminals:

  Vo = G( V+ - V- ),

where G is the gain of the amplifier.  The amplifier is thus termed a differential amplifier, since it measures the difference between two voltages.

You will remember from your basic physics courses that electrical voltage or potential, unlike length or mass, is not an absolute quantity, but rather a relative quantity, in that potential itself cannot be measured, only differences in potential.  Thus V+ and V- are each measured with respect to some third reference point that is arbitrarily taken to be at zero potential.  In electrocardiography, this point is the right leg.
The differential amplifier has the advantage that any component of the signal appearing simultaneously at both inputs is cancelled out and so does not appear at the output.  This "common-mode rejection" is important, since electrical 115-volt power wiring in a building can induce signals at 60 Hz (the power line frequency) on the body surface that are many times larger than the ECG signal itself.  Use of a differential amplifier prevents this large spurious signal from swamping out the ECG signal.
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