ECG Review
AV Block or Blocked PACs?
By Ken Grauer, MD, Professor, Department of Community Health and Family Medicine, University of Florida. Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book. Dr. Grauer reports no other financial relationship to this field of study.
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Clinical Scenario: The ECG shown above was interpreted as showing sinus rhythm with blocked premature atrial contractions (PACs) as the reason for the slow rate. Do you agree?
Interpretation: The underlying rhythm in this tracing is sinus as regular and upright P waves of similar morphology are seen consistently throughout this lead II rhythm strip. The QRS complex is narrow, and the ventricular rhythm is irregular, at least in the beginning of the tracing. Some beats in the tracing are obviously conducting, as evidenced by the constant PR interval preceding beats #4-#7. Sandwiched in between these QRS complexes are non-conducting P waves. This defines the rhythm as some type of 2nd degree AV block, since not all P waves are being conducted.
Of the three types of 2nd degree AV block, Mobitz Type I (or AV Wenckebach) is by far the most common. Mobitz I is characterized by progressive lengthening of the PR interval within groups of beats until a beat is dropped. This is precisely what occurs for the sequence of beats #1-#3 in this tracing. Although subtle, the PR interval does increase until the P wave following beat #3 fails to conduct. Mobitz I usually occurs at the level of the AV node, so that the QRS complex typically will be narrow (unless the patient also has a bundle branch block).
In contrast to Mobitz I, with the Mobitz II type of 2nd degree AV block, the PR interval remains constant for consecutively conducting complexes until one or more beats are dropped. The importance of recognizing Mobitz II, and distinguishing it from the Mobitz I type of 2nd degree AV block, lies with the much more ominous prognosis of Mobitz II and its disturbing tendency to suddenly progress to profound conduction block. As a result, prompt implementation of a pacemaker is needed. Mobitz II usually occurs below the level of the AV node, and therefore typically is associated with a widened QRS complex.
The third type of 2nd degree AV block consistently manifests 2:1 AV conduction. Diagnosis with this third type of 2nd degree AV block is problematic since the lack of consecutively conducted complexes means you never see two conducted beats in a row, and therefore cannot determine if the PR interval is lengthening prior to the beat that is dropped. The latter part of this tracing illustrates the phenomenon of 2:1 AV conduction.
The reason we know that the rhythm is Mobitz I is the clear Wenckebach cycling in the first part of the tracing. Alternation between Mobitz I and Mobitz II block does not generally occur. Other reasons supporting the diagnosis of Mobitz I are the narrow QRS complex and the much greater prevalence of Mobitz I compared to Mobitz II. The regular atrial rhythm with P waves of similar morphology rules out PACs as a possibility.
The ECG shown was interpreted as showing sinus rhythm with blocked premature atrial contractions (PACs) as the reason for the slow rate.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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