Warning : Use the following information at your own risk. While accuracy is one my goals, there is always the possibility that some of the information could be wrong. There could be typos. I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within. Remember, this is just some page on the internet. (If you do find errors, please notify me by feedback.)
Atrioventricular heart blocks (referred to simply as heart blocks from here on) is the name given to conditions in which electrical conduction at the AV node is somehow affected. We generally speak of three broad types (or degrees) of heart blocks. In a nutshell, they are :
 | 1st degree heart block : AV conduction is (excessively) slowed |
| 2nd degree heart block : AV conduction is incompletely (i.e. occasionally) blocked |
| 3rd degree heart block : AV conduction is completely blocked |
Before you continue, stop and predict what each type of heart block will look like. Use what you already know about P waves, QRS complexes, and the space between them. If AV conduction is slowed, how will this appear on an ECG? If conduction is completely blocked, will QRS complexes follow P waves?
1st degree heart blocks :
This occurs when conduction at the AV node is slowed beyond the normal amount. This is manifested as a PRI that is longer than 0.20 seconds. This PRI will generally remain constant. (If the PRI is changing from beat to beat, you may a second degree heart block.) When you see a rhythm with a first degree heart block, you generally name the rhythm according to this pattern : <underlying rhythm> with a first degree heart block. For example, if you were to see a case of sinus bradycardia but the PRI consistently measures 0.22, you would call the rhythm : sinus bradycardia with a first degree heart block.
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Figure 11-1 : Sinus rhythm with a first degree heart block |
2nd degree heart blocks :
The most confusing thing about second degree heart blocks is that there are two subtypes and they are called a variety of names. Whenever you see Mobitz, think 2nd degree heart block. Therefore, Mobitz I is the same thing as 2nd degree heart block type I.
| 2nd degree heart block type I : This rhythm is also called Mobitz I. It is also called Wenckebach. It consists of the PRI getting longer with each electric beat until eventually a P wave occurs but the QRS never shows (essentially skipping a beat). The process is then repeated. This is like showing up for work Monday an hour late, on Tuesday two hours late, and so on until Friday comes around and you don't even show up at all. The following Monday, you start the same cycle again. You could call this behavior "pulling a Wenckebach" but nobody would get it and people would just think you're weird. |
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Figure 11-2 : Second degree heart block type I (Wenckebach) |
| 2nd degree heart block type II : This rhythm is also called Mobitz II. This occurs when a QRS suddenly fails to show up after a P wave. It usually makes an appearance the next wave. This rhythm lacks the increasing PRI that is seen with the Wenckebach type. It would be as if you showed up 30 minutes late Monday through Thursday, but failed to show up Friday. I have always considered this the "duck duck goose" rhythm because it maintains a relatively constant PRI until it skips. |
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Figure 11-3 : Second degree heart block type II |
It is important to recognize the two subtypes of 2nd degree heartblocks. The second subtype tends to be much worse than the first subtype (Wenckebach).
Note : For second degree heart blocks, it is common to specify the ratio of P waves to QRS complexes. This is the conductance ratio. In a second degree heart block with a 2:1 conductance, there will be only one PRI. It will be impossible to distinguish between the two subtypes of 2nd degree heart blocks using only the ECG.
3rd degree heart blocks :
These are also called complete heart blocks. This is when the atria and the ventricles are essentially divorced. If no electricity travels through the AV node for a little while, the ventricle's backup pacemaker starts calling the shots. The atria are being controlled by one pacemaker, the ventricles by another. This often manifests itself on an ECG as P waves occuring at regular intervals with QRS complexes occuring at regular intervals, but no apparent relationship between any P wave or QRS complex. Sometimes it may look like a P wave follows a QRS, sometimes vice-versa, but they don't seem to affect each other.
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Figure 11-4 : Third degree heart block |
In figure x-x, you may have to play "Where's Waldo?" with the P waves. The first two are clearly visible. The last two are hiding in QRS complexes. Compare the shape of the QRS complexes. You should notice a slight difference where you expect the P wave to be. In the middle QRS, the P wave is evident at the very end. In the last QRS complex, it is at the very beginning.
To be considered a true third degree heart block, the ventricles should be in an escape rhythm. Why? There are many situations in which the atria and ventricles can be completely independent when there is no "true block" between the atria and ventricles. If a ventricular ectopic pacemaker were firing at such a rate that the sinus and ectopic impulses meet head-on somewhere in the junction, you would see these two impulses cancel each other out; the atria and ventricles would be doing their own thing despite no real problem with AV conduction.
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