Second degree heart block which is also called Mobitz 1 or Wenckebach is a disease of the electrical conduction system of the heart in which the has progressive prolongation until finally the atrial impulse is completely blocked and does not produce a QRS electrical impulse.
Once the p-wave is blocked and no QRS is generated, the cycle begins again with the prolongation of the PR interval.
One of the main identifying characteristics of second-degree AV block (Type 1) is that the atrial rhythm will be regular.
In the above image, notice that the p-waves are regular, the PR-interval progressively gets longer until a QRS is dropped and only the p-wave is present.
Although second-degree AV block (Type 1) is not typically clinically significant for ACLS, recognition of the major AV blocks is important because treatment decisions are based on the type of block present.
Second-degree AV block (Type I) may be a normal variant in the conduction system of the heart and is usually due to a reversible conduction block at the level of the AV node. There are several disorders that can cause Second-degree AV block (Type I). The most common causes are listed below:
Causes of Second-degree AV block (Type I):
- Increased vagal tone (well trained athlete)
- Medications that inhibit AV node conduction (amiodarone, beta-blockers, calcium channel blockers, digoxin)
- Myocarditis caused by infections
- Hypoxemia (see more in infants and children)
- Cardiac surgery
- Myocardial infarction
- Any condition that stimulates vagal tone
Below is a short video which will help you quickly identify second-degree AV block (Type 1) on a monitor. Please allow several seconds for the video to load. (3.69 mb)
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