What is an ECG? A brief introduction

What is an ECG: a brief introduction. Electrocardiogram, also called an electrocardiograph, 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 reflections called waves and complexes. When no current is being detected, you can see a flat line, called a baseline.

We 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 by five small squares and are identified by darker outlines.

Being 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 forces, the taller or deeper the deflections. On the vertical axis, 10 millimetres– or 10 small squares in height– are equivalent to 1 millivolt of electrical current.

On the horizontal axis, we measure time in seconds, which means that 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.

On the screen, you can now see an example of a 12 lead ECG, with the standard voltage and time settings written along the bottom.

The ECG shows 12 leads, or viewpoints, of the heart. There are six limb leads which are called one, two, three, AVR, AVL, and AVF, and six chest leads, called C-1 to C-6.

Although the leads are looking at the same electrical events, they’re viewing these events from different angles, so the waveforms 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.

Some of the 12 leads have similar views, so they can be grouped together into specific areas or territories. For example, leads One, AVL, C-5, and C-6 all view the left side of the heart, which is called the lateral territory. Leads Two, Three, and AVF view the bottom of the heart, known as the inferior territory. Finally, leads C-1 through C-4 all broadly view the front, or anterior, territory of the heart.

Along the bottom of the ECG, you will usually see a continuous strip of lead two. This makes it easier to assess the heart rhythm without the distraction of the intermittently changing leads above.