Complete Heart Block or Not?

Vignette:

A 70 yo patient with history of hypertension and hyperlipidemia is admitted due to dizziness. What is your interpretation of this ECG strip?



Image 1 - ECG case in full disclosure



Image 2 - ECG case labeled

The rhythm is sinus at an atrial rate of about 60 bpm. The P waves are marked with red asterisk and some P waves are partly hidden from view or are merged with the QRS (red arrows). The R to R is irregular. R1R2, R3R4 and R4R5 have the same RR interval (1800 ms or 1.8 sec) while R2R3 and R5R6 have the same RR interval (1400 ms or 1.4 sec). 

At a quick glance, you will immediately notice that there seemed to be no association between the P and QRS. So, you will jump into the conclusion that this is complete heart block (CHB) or third degree AV block. However, for CHB the RR interval MUST BE REGULAR. It is because supraventricular impulses are blocked and the ventricles are depolarized by either a junctional or ventricular pacemaker and they generate a regular RR to interval. In the case presented, the RR interval is irregular. Thus, we can conclude that SOME OF THE IMPULSES were able to conduct to ventricles. R3 and R6 are captured beats or sinus beats that were conducted to the ventricles (PRI of about 0.40 sec). Hence, this is not complete heart block. This is called HIGH-GRADE or ADVANCED SECOND-DEGREE AV BLOCK.
Second Degree AV Block Syndromes

Traditionally, you are taught of 2 types of second degree AV block: Type I (Mobitz I or Wenckebach) and Type II (Mobitz II). There is a subset of second degree AV block called HIGH-GRADE or ADVANCED AV BLOCK. This manifest on the surface ECG as multiple consecutive P waves that are not conducted (P-QRST ratio of 3:1 or 4:1). This often implies advanced conduction disease and may progress to complete heart block. In the case presented, some of the P waves are not conducted and there is a junctional escape beat. This confuses an ECG reader of interpreting the strip as intermittent complete heart block. In it full sense, it is not CHB because some of the impulses are conducted which manifested as shortening of RR interval. 

There is a fourth subtype of second degree AV block which is called 2:1 AV block which this ECG case also manifested.



Image 3 - ECG case presenting as 2:1 AV Block

Every other P wave is conducted to the ventricles in a 2:1 pattern. ECG patterns like this is to be interpreted as 2:1 AV block and NOT MOBITZ II. An ECG presenting as 2:1 AV block can either be Mobitz I or Mobitz II. You need a longer strip to document whether there is prolongation of the PRI (Mobitz I) before the drop or there is none (Mobitz II). 2:1 AV block is a subtype of second degree AV block.

Final Interpretation: Sinus rhythm with high-grade or advanced second-degree AV block.

What happened to the case?

The ventricular rate stayed around mid-30's and a permanent pacemaker was eventually implanted.

References:
Bonnow et al. 2014. Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 10th Edition. PA.Saunders 
Goldberger A. 2013. Goldberger’s Clinical Electrocardiography : A Simplified Approach 8Ed. Ph Elsevier
Ufberg JW and Clark JS. 2006. Bradyarrhythmias and AV Conduction Blocks. Emergency Clinics of North America ; 24:1-9

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