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Interpretation: The underlying rhythm in the Figure is sinus, as suggested by the initial three beats in the lower tracing. There is a 1° AV block (the PR interval of these first 3 beats clearly exceeds 0.20 second). Significant bradycardia and AV block are present on the remainder of these rhythm strips. The QRS complex is narrow. P wave morphology is consistent throughout, and demonstrates a fairly regular atrial rate of 80/minute in the top tracing (some P waves being hidden by/deforming several of the T waves). Despite the presence of AV dissociation in the top tracing, the top rhythm strip does not represent complete (3°) AV block. The key clue to this conclusion is that the last beat in the top tracing occurs early. Most of the time, the ventricular rhythm will be regular when the degree of AV block is complete (due to the regular rate of the escape pacemaker). The occurrence of an early beat in the top tracing suggests that at least some sinus impulses are being conducted. Further support that this last beat is in fact being conducted is forthcoming from the observation that the PR interval preceding it is identical to the PR interval of the three sinus conducted beats.
Significant block is definitely present in the lower tracing, with nonconduction of four consecutive atrial impulses. However, further support against a diagnosis of complete AV block is forthcoming from the presence of three sinus conducted beats at the beginning of this lower tracing, and 2:1 AV conduction (constant PR interval) at the end of the tracing. Slight slowing of the atrial rate (to 75/minute) at the beginning of the lower strip may be the reason that 1:1 conduction was transiently able to occur (the atrial rate had been 80/min in the top tracing). Nevertheless, high grade (albeit not complete) AV block is clearly present, and permanent pacing was required for treatment of this elderly man with syncope.