![]() Third-degree AV nodal block - conduction through the AV node is completely blocked so that no impulses can be transmitted from the atria to the ventricles. Also note that the QRS complexes appear normal in shape and duration because they are still being triggered by impulses from the atria passing through the AV node. In either type of second-degree block, the ventricular rate will be less than the normal sinus rhythm (indicated by green arrows in figures). This is an example of a 2:1 rhythm because there are two P waves for each QRS. Mobitz II occurs is when the P-R interval is fixed in duration, but some P waves are not followed by a QRS, as illustrated in the second tracing below. The fifth beat starts this cycle over again. In the fourth beat, the P wave is not followed by a QRS therefore, the ventricular beat is dropped. In the first tracing below, the PR interval for the first beat is 0.16 sec and increases to 0.24 sec by the third beat. In Mobitz I (also called "Wenckebach"), the PR interval gradually increases over several beats until it is sufficiently prolonged (that is, AV conduction is sufficiently impaired) that the impulse cannot pass into the ventricles (i.e., a QRS will not follow the P wave). There are two subtypes of second-degree AV blocks: Mobitz Type I and Mobitz Type II. This results in P waves that are not followed by QRS complexes. Second-degree AV nodal block - the conduction velocity is slowed to where some impulses from the atria cannot pass through the AV node. This type of block can be caused by enhanced vagal tone, digitalis, beta-blockers, calcium channel blockers, or ischemic damage to the nodal tissue. Rate is not altered by the prolonged PR interval because it is still being controlled by the SA node. In the tracing below, the PR interval is 0.24 sec.
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