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By Dr. Ken Grauer, Professor Emeritus in Family Medicine, College of Medicine, University of Florida
Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.
There are non-conducted P waves in the rhythm below. Is there AV block? If so, what type? Why is this not complete AV block?
Interpretation: At first glance, it appears that none of the P waves in this tracing are being conducted to the ventricles. That said, the best approach for assessing the AV blocks is to return to the basics. We summarize the five parameters to assess in the interpretation of any cardiac rhythm by the saying, “Watch your Ps and Qs — and the 3Rs.” We look for: i) Presence and nature of atrial activity (P waves); ii) QRS width; iii) and iv) Rate and regularity of both the atrial and ventricular rhythm; and v) If P waves are present, are they related to a neighboring QRS complex?
• The QRS complex is clearly narrow in this tracing. Therefore, the rhythm is supraventricular.
• Upright sinus P waves are present in this lead II rhythm strip. They are regular throughout the tracing at a rate of 75/minute.
• It is obvious that P waves for the initial part of this tracing are not related to their neighboring QRS (i.e., there is AV dissociation — at least for the first few beats). If all we had were the first 4 beats, one would think the degree of AV block was complete.
The key to interpreting this rhythm is to appreciate that the ventricular rhythm is not regular over the entire tracing! Instead, beat #5 comes early. Most of the time when there is complete (i.e., third-degree) AV block, the ventricular rhythm will be regular throughout. This is because with complete AV block, no atrial impulses are able to penetrate the AV node. As a result, an escape pacemaker arises from either the AV node or from the ventricles, and the rate of most escape pacemakers is at least fairly regular. The fact that beat #5 occurs early and is preceded by a P wave with a normal PR interval suggests that this beat is being conducted. Support for this theory is forthcoming from the fact that beat #6 follows with a similar R-to-R interval and with a similar preceding PR interval.
In summary, assessment of AV blocks is best accomplished by use of the same “Ps, Qs, 3R Approach” as for any other rhythm. AV dissociation may be transient (as it is here for the first three beats in this tracing). An important clue that despite transient AV dissociation, the degree of AV block is not complete — is when the ventricular rhythm is not regular. Attention to the beats that follow this change in rhythm regularity will usually be revealing. In this case, the rhythm is a form of high-grade second-degree AV block (since several of the regularly occurring P waves that should conduct do not conduct) — but the degree of AV block is clearly not complete.
NOTE: For review of the basics of AV Block, go to: http://www.avblockecg.com.
Figure: Lead II rhythm strip with non-conducted beats.