Skip to 0 minutes and 9 secondsWhat is an ECG? A brief introduction. Electrocardiogram, also called an electrocardio graph or ECG, is a graphic representation of two electrical events called depolarisation and repolarisation. Depolarisation is the spread of the electrical impulse across the heart, and repolarisation is the recovery stage following this. Current is detected by electrodes or conductors which are placed on the skin, and is represented on the ECG graph paper as positive and negative deflections called waves and complexes. When no current is being detected, you can see a flat line, called a baseline.

Skip to 0 minutes and 59 secondsWe now need to look in more detail at the ECG graph paper itself. On the screen, you can see a magnified image of the paper. But on an ECG printout, one small square measures one millimetre in width and one millimetre in height. Larger squares comprise five x five small squares, and are identified by dark outlines.

Skip to 1 minute and 26 secondsIn a graph we have a vertical and a horizontal axis. On the vertical axis, we measure amplitude, which is the amount of electrical force being generated. This means that the greater the force is, the taller or deeper the deflections. On the vertical axis, 10 millimetres, or 10 small squares in height are equivalent to one millivolt of electrical current.

Skip to 2 minutes and 0 secondsOn the horizontal axis, we measure time in seconds, which means the longer it takes for the electrical forces to move across the heart tissue, the wider the wave forms will appear. One small square or one millimetre on the horizontal axis is equivalent to 0.04 of a second, and this corresponds to a standard paper speed of 25 millimetres a second.

Skip to 2 minutes and 30 secondsOn the screen, you can now see an example of the 12-lead ECG, with the standard voltage and time settings written along the bottom.

Skip to 2 minutes and 41 secondsThe ECG shows 12 leads, or viewpoints, of the heart. There are six limb leads, which are called I, II, III, aVR, aVL, and aVF, and six chest leads, called C1 to C6.

Skip to 3 minutes and 1 secondAlthough the leads are looking at the same electrical events, they are viewing these events from different angles, so the wave forms in each lead will look slightly different. Having multiple viewpoints is useful because it provides more detailed information about what may be happening in different areas of the heart. You might compare this to watching a football match, where in order to get to good overall picture of the action, you would need to have cameras positioned in different locations around the pitch.

Skip to 3 minutes and 37 secondsSome of the 12 leads have similar views, so can be grouped together into specific areas or territories. For example, leads I, aVL, C5, and C6 all view the left side of the heart, which is called the lateral territory. Leads II, III, and aVF view the bottom of the heart, known as the inferior territory. Finally, leads C1 to C4 all broadly view the front or anterior territory of the heart.

Skip to 4 minutes and 18 secondsAlong the bottom of the ECG, you will usually see a continuous strip of lead II. This makes it easier to assess the heart rhythm, without the distraction of the intermittently changing leads above.

What is an ECG? A brief introduction

An electrocardiogram - or ECG - is a simple and useful test which records the rhythm and electrical activity of your heart.

Watch the presentation to see what is measured on the ECG recording and how the leads relate to different viewpoints of the heart.

As this is a powerpoint presentation, we’ve also created an illustrated transcript for you to download in the download section.

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This video is from the free online course:

ECG Assessment: an Introduction for Healthcare Providers

St George's, University of London