Does the AV Block Get Worse?
ECG REVIEW
Does the AV Block Get Worse?
Figure — Sequential tracings from a patient with syncope.
Scenario: Imagine the ECG tracings shown above were sequentially obtained from a patient with syncope. What kind of atrioventricular (AV) block is present in each tracing? Does the degree of AV block worsen from tracing A to tracing B?
Interpretation: Beginning with tracing A, there is group beating. The QRS complex is narrow. Underlying sinus arrhythmia is present. The PR interval progressively increases within each group until a beat is dropped. This is second-degree AV block, Mobitz Type I (AV Wenckebach).
In tracing B obtained a little while later on this same patient, the overall ventricular rate has slowed. The QRS complex is again narrow. Now, only one out of every two P waves is conducted to the ventricles. Every other P wave does conduct — as evidenced by the constant (albeit slightly prolonged) PR interval preceding each QRS complex. This is second-degree AV block with 2:1 AV conduction.
Although at first glance one might think that the degree of AV block in tracing B has become "worse," this is not the case. Note that the underlying atrial rate is faster in tracing B than it was in tracing A. Thus, only one out of every two P waves is able to penetrate the AV node when the atrial rate is 80-85/minute — whereas a greater percentage of P waves penetrate in Tracing A when the atrial rate is slower (in the 60-70/minute range).
Distinction between Mobitz I vs Mobitz II second-degree AV block is not possible from inspection of tracing B alone. Because there is never conduction of two consecutive P waves in a row, we simply cannot tell if the PR interval would prolong if given a chance to do so. This distinction is important because Mobitz II usually requires treatment with pacing, whereas Mobitz I does not. That said, it is probable that tracings A and B both represent Mobitz I because: 1) Mobitz I AV block is far more common than Mobitz II; 2) the QRS complex is narrow (whereas Mobitz II usually manifests QRS widening; 3) the PR interval for beats that conduct in Tracing B appears to be long (more common with Mobitz I); and 4) it is rare for a patient to switch back-and-forth between Mobitz I and Mobitz II conduction defects, and we know that Tracing A clearly manifests Mobitz I AV block.
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