ECG 3 chapters 1–7:Layout 1 10/09/2014 11:08 Page 29
Common arrhythmias
Figure 5.4 Atrial flutter diagram.
The rate of these flutter waves is about 300 per minute. If all these conduction waves were transferred through to the ventricles, we would see a heart rate of 300, which would certainly lead to loss of consciousness. Thankfully, the AV node does its best to control the heart rate, and does not allow every flutter wave to pass through to the ventricles. There is some variation in how often the AV node allows these conductions to pass into the ventricles. The ratio of flutter waves to each QRS complex may be 2:1, 3:1 or 4:1. REMEMBER: If this seems a little complicated, try thinking of these two arrhythmias as fireworks!
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Atrial fibrillation is like rockets going off at a firework party. At any one time, there may be many rockets exploding in the sky, sending out sparks in all directions. In the atria, many foci are going off like rockets, sending out waves of conduction in different, random and unpredictable directions. Atrial flutter is more like a catherine wheel, nailed to a post. It remains in one place but spins around very quickly, throwing out sparks in all directions each time it goes round. There is a more even pattern to the conduction in the atria because it is all coming from one place.
Supraventricular tachycardia Supraventricular tachycardia (SVT) is the name given to a whole range of arrhythmias, each with different causes. Their one common characteristic is that they all have their focus above the ventricles, as the name implies. SVT may be a result of the SA node conducting too quickly, or because of some genetic conduction disorder. Most commonly, SVT comes about as a result of some interruption or disturbance to some of the conduction tissue near (or sometimes inside) the AV node. This is referred to as AV nodal re-entry tachycardia (AVNRT). In this case, a small circuit develops in the conduction tissue that rapidly generates ventricular conductions (see fig 5.6). The circuit acts in a similar way to that which causes atrial flutter (see above).
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