Second degree AV block pathophysiology
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In Mobitz type I (Wenckebach) there is a progressive prolongation of the PR interval (AV conduction) until eventually an atrial impulse is completely blocked. When an atrial impulse is completely blocked there will be a P wave without a QRS complex. This pattern is often referred to as a “dropped beat.” Mobitz type I occurs because each depolarization results in the prolongation of the refractory period of the atrioventricular (AV) node. When an atrial impulse comes through the AV node during the relative refractory period, the impulse will be conducted more slowly, resulting in a prolongation of the PR interval. Eventually, an impulse comes when the AV node is in its absolute refractory period and will not be conducted. This will manifest on the ECG as a P wave that is not followed by a QRS complex. This non-conducted impulse allows time for the AV node to reset, and the cycle continues. This phenomenon leads to a grouped beating. In Mobitz type II there is a constant PR interval across the rhythm strip both before and after the non-conducted atrial beat. Each P wave is associated with a QRS complex until there is one atrial conduction or P wave that is not followed by a QRS. Mobitz type II is often a problem in the infra-nodal conduction system, and therefore, is associated with a widened QRS complex, bundle-branch block, or a fascicular block. When more than one P wave is not conducted this is no longer a Mobitz type II and is considered a high degree AV block.
Mobitz Type I
- The classic site of block in Mobitz type I second degree block is the AV node (70%-75%).
- In the remaining 25%-30% of the cases the site is infra-nodal (His bundle,bundle branches or fascicles).
- Mobitz type I is again composed of two variations which show Wenckebach periodicity: classic and atypical.
- Classic variety usually occurs within the AV node.
- It can be observed in antegrade AV conduction and also in retrograde VA conduction across the AV node.
- There is a gradually increasing PR interval and eventually a dropped beat.
- There is also usually a gradually decreasing R-R interval.
- The PR interval is usually shortest in the initial beat and gradually increases ending in a dropped beat and the cycle repeats.
- If the interval between the last conducted beat and the first beat of the next cycle is very long, the first beat may be a junctional escape rhythm rather than a conducted beat. This classic Wenckebach phenomenon occurs usually with ratios of 3:2, 4:3 or 5:4. This results in grouped beating.
- This variant of Wenckebach pattern is defined as long Wenckebach and is also called pseudo-Mobitz type II pattern as it simulates Mobitz type II block.
- In this pattern the conduction ratios usually exceed 6:5 or 7:6 and the last few beats of the cycle, before a dropped beat, show a relatively constant PR interval (maximum variation of 0.02 sec among them).
- The beats after the dropped beat again show gradually prolonging PR intervals.
Mobitz Type II
- Conduction delay in Mobitz type II second degree block is almost always infra-nodal (His bundle [20%], bundle branches or fascicles).
- Usually the morphology of the QRS complex is wide, except when the site of block is the His bundle.
- In this variant of second-degree heart block the PR interval is constant with occasional dropped beats as compared to the gradually prolonging PR interval in Mobitz type I.
- Bifascicular or trifascicular block is seen in two thirds of the patients with Mobitz type II.
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