Diagnostic Considerations
Mobitz I and Mobitz II atrioventricular (AV) block must be differentiated from sinus node dysfunction and increased vagal tone. AV block generally occurs with a constant PP interval. Blocked premature atrial beats must be differentiated from Mobitz II AV block. Blocked atrial beats will occur earlier than expected compared with the fixed PP interval in Mobitz II AV block. Atrial fibrillation with a slow heart rate may indicate second-degree AV block. If the ventricular rate is slow and regular during atrial fibrillation, third-degree AV block is likely present.
Important considerations in the differential diagnosis
AV dissociation due to sinus bradycardia with a junctional escape rhythm is not AV block. Severely prolonged first-degree AV block may appear to be a junctional rhythm. The P waves may be buried in the preceding QRS complex and suggest retrograde activation. However, in junctional rhythm, the retrograde P waves are inverted and they are upright in sinus rhythm with first-degree AV block. Variations in rate start with changes in timing of the PP interval rather than the RR interval.
The irregularity of the RR intervals in Mobitz I AV block may appear to be sinus rhythm with premature atrial complexes or multifocal atrial tachycardia. The presence of distinct, similar P waves and the grouped beating of the QRS complexes can help to distinguish Mobitz type I AV block from the aforementioned arrhythmias.
Differential Diagnoses
-
Atrioventricular Block. This rhythm strip shows first-degree atrioventricular block with a PR interval of 0.360 sec. Note the fixed prolonged PR interval.
-
Atrioventricular Block. First-degree atrioventricular block. PR interval is constant and is 0.280 sec.
-
Atrioventricular Block. Second-degree Mobitz type I atrioventricular block. Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval following the dropped beat.
-
Atrioventricular Block. Second-degree atrioventricular block, Mobitz type I (Wenckebach). Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval following the dropped beat.
-
Atrioventricular Block. Second-degree Mobitz II atrioventricular block. Note the fixed PR interval, but after the third beat, an atrial impulse fails to conduct to the ventricle. Courtesy of Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Second_degree_heart_block.png).
-
Atrioventricular Block. Second-degree atrioventricular block, Mobitz type II. A constant PR interval in conducted beats is present. Intraventricular conduction delay is also present.
-
Atrioventricular Block. A constant PP interval and normal PR interval in conducted beats is present. This progresses to 2:1 atrioventricular (AV) block. A 2:1 AV block can be present with conduction delay in the AV node or His-Purkinje system, but it is more likely to be in the AV node for all patients (with a greater chance of AV block in the His-Purkinje system if there is a bundle branch block). Review extended monitoring strips because Mobitz I or Mobitz II may be present at other times, and this might help to determine the level of the block.
-
Atrioventricular Block. High-degree atrioventricular block is demonstrated with a 4:1 atrial-to-ventricular conduction ratio. Note the P wave prior to the QRS conducts whereas the others do not. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/high-grade-block/), Edward J Burns, MD, Sydney, Australia.
-
Atrioventricular Block. This rhythm strip shows third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no association exists between the atrial and ventricular activity. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/complete-heart-block/), Edward J Burns, MD, Sydney, Australia.
-
Atrioventricular Block. Third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no association exists between the atrial and ventricular activity.
