We should start the examination of the ECG by looking at the individual components of the tracing. There are 12 leads that are placed on the body, 6 on the chest and 6 on the arms and legs. Each lead is represented on the standard 12 lead ECG printout. The precordial (chest) leads are labeled from V1 - V6. The limb leads are labeled I, II, III, aVR, AVL and aVF. Each lead serves to give a different view of the heart.

As each electrical impulse travels from the SA node through the conduction system, the ECG can record this progress. The initial impulse travelling from the SA node is represented by the P wave. The QRS complex represents the impulse as it travels through the ventricle (depolarization). The T wave represents the repolarization of the ventricle as it prepares for the next impulse being conducted from the SA node.

ECG 1 - normal

Note that the rhythm is regular - each beat is an equal distance from the next beat which means that the time between each beat is the same. The rhythm strip is the long tracing on the bottom of the ECG which shows several continuous seconds of the heart. This allows us to look at the rhythm of the heart, and determine the heart rate.

ECG 2 - atrial fibrillation
The ECG in atrial fibrillation reflects many of the changes we have discussed previously that take place in the heart. The most obvious difference is the absence of the P wave. Leads I, II and III show the classical appearance of AF, the "undulating baseline". As you can see, the tracing never really "sits still". The other major change is that the heart rhythm is no longer regular. If you look at the distance between each QRS complex in the rhythm strip, you can see that the distance is variable meaning that the timing of each beat is irregular. There is no pattern to the irregularity, so the rhythm of AF is called "irregularly irregular".

ECG 3 - atrial flutter

The ECG in atrial flutter also has a very distinct appearance. The "flutter waves" that we expect to see in AFL are noticeable throughout the ECG, but are very easy to see in the rhythm strip. The rhythm is still regular, although this is not always the case in AFL.