The difference between the three types of 2nd degree AV blocks and the first degree AV block is that in every single type of 2nd degree AV block, the electrical activity from the atrial passing through the AV node eventually becomes completely blocked so that it doesn’t pass into the ventricles and there is no QRS complex to follow in that instance. For the type 1, the waves are delayed but all are transmitted, none is ever completely blocked.
Mobitz type I second degree AV block
The PR interval progressively lengthens until a P wave fails to conduct and a QRS (i.e beat) is “dropped.” i.e. the PR interval progressively lengthens until the electrical activity from the atria is completely blocked in the AV node.
What that means is that in the AV node, the electrical activity from the atria is delayed for progressively longer and longer periods of time until the last one in that series is not allowed to pass down to the ventricles. That means on the ECG, the last cycle in the series of cycles with prolonged PR intervals will have a P wave without a QRS complex that follows it. Then the series begins again with increasing PR prolongation until another P wave is not conducted down to the ventricles. And the series starts over again.
Image from Wikipedia
Mobitz Type II second degree AV Block
Mobitz type II has a worse prognosis and is characterized by an abrupt AV conduction block without evidence of progressive conduction delay. That is, the unconducted p waves are blocked randomly without any progressive lengthening of the PR interval.
In other words, Mobitz type II is characterized by intermittently non-conducted P waves not preceded by PR interval prolongation ( as in Mobitz type I) and not followed by RR interval shortening.
- On an ECG, the PR intervals in the conducted beats remain the same (unchanged/constant) preceding the non-conducted P wave.
- The P wave rate remains constant. Atrial activity is unchanged.
- The RR interval surrounding the non-conducted P wave (s) is an exact multiple of the preceding RR interval (e.g. double the preceding RR interval for a single non-conducted p wave, tripled for two non-conducted p waves, etc).
- The site of the conduction block is usually located at the level of the His-Purkinje system (i.e. below the AV node).
- Having a Mobitz type II, especially if there is also a LBBB, usually is a sign that the patient is moving towards a complete heart block.
See LITFL and what they say about Mobitz type II.
AV 2:1 block
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