ECG Review: Mobitz II in a Patient on Digoxin

By Ken Grauer, MD

Figure. Nonsequential lead II rhythm strips obtained from a 71-year-old man with heart failure.

Clinical Scenario: The nonsequential rhythm strips shown in the Figure were obtained from a 71-year-old man with a history of congestive cardiomyopathy and renal insufficiency. The patient was admitted for an exacerbation of heart failure. Digoxin was among the many medications he was taking. Assessment of the bottom rhythm strip was 2:1 AV block, Mobitz Type II. Do you agree?

Interpretation: Use of calipers will greatly facilitate interpretation of the 2 tracings shown in the Figure. Except for the very first T wave in the top tracing, virtual superposition of P waves on top of T waves masks atrial activity until the last beat (beat #10) in the top tracing. Inspection of the T wave of this beat #10 reveals the true shape of what T waves would look like without a superimposed P wave. Knowing this allows us to "walk out" with calipers—an essentially regular atrial rhythm (at a rate of 75-80/minute) throughout both rhythm strips.

With the exception of premature beat D in the lower tracing (which is a premature ventricular contraction [PVC]), the QRS complex is narrow in both rhythm strips. Upright P waves are present in these lead II rhythm strips and appear to be conducting, albeit with a PR interval that is not always constant. Second degree (2°) AV block is present because a number of P waves on these tracings do not conduct. Concern about the presence of 2° AV block, Mobitz Type II is raised because of the short run of 2:1 AV block with a constant PR interval (beats A through F in the bottom tracing). However, Mobitz II 2° AV block is not present. Instead, one can definitively diagnose Mobitz I (2° AV block of the Wenckebach type). Distinction between these 2 forms of AV block is important clinically because of the generally much more serious prognostic implications of Mobitz II, which in the acute setting is usually indication for immediate pacemaker placement.

Statistically, Mobitz Type I 2° AV block is a much more common conduction disturbance than Mobitz Type II. Because of the more proximal level of this conduction defect (which is usually at the level of the AV node), the QRS complex is usually narrow with Mobitz I AV block (as it is here). In contrast, the QRS complex with Mobitz II is usually (though not always) wide. Definitive diagnosis of Mobitz II AV block requires evidence of failed conduction (dropped beats) that occurs in association with the presence of consecutively conducted complexes that manifest a constant PR interval. This is why the short run of 2° AV block with 2:1 AV conduction seen in the lower tracing could represent either Mobitz I or Mobitz II (since you never see 2 conducted beats in a row, you cannot tell if the PR interval is increasing or not). That said, there is other clear evidence of Mobitz I 2° AV block on these 2 tracings. This virtually confirms Mobitz I as the true diagnosis because of the rarity of seeing rapid alternation between the Mobitz I and Mobitz II types of 2° AV block. Digoxin toxicity should be strongly suspected in this older patient with renal insufficiency who manifests Wenckebach type of 2° AV block. 

Dr. Grauer, Professor, Assistant Director, Family Practice Residency Program, University of Florida, ACLS Affiliate Faculty for Florida, is Associate Editor of Internal Medicine Alert.