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Continuing Problems with the Diagnosis of Mobitz Type II Second-Degree Atrioventricular Block

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Continuing Problems with the Diagnosis of Mobitz Type II Second-Degree Atrioventricular Block Continuing Problems with the Diagnosis of Mobitz Type II Second-Degree Atrioventricular Block S. Serge BAROLD M.D.* Florida Heart Rhythm Institute, Tampa, Florida, USA ABSTRACT Second degree atrioventicular block remains poorly understood despite the major advances in cardiac elec- trophysiology. In this paper, we summarized the continuing problems with the diagnosis of Mobitz Type II second degree atrioventricular block. KEYWORDS Atrioventricular block, Second degree Möbitz type II atrioventricular block, Second degree type I atrioventricular block İkinci Derece Mobitz Tip II Atriyoventriküler Bloğun Tanısında Devam Eden Sorunlar ÖZET Kardiyak elektrofizyolojide büyük gelişmelere rağmen ikinci derece atriyoventriküler blok tam olarak anla- şılamamıştır. Bu yazıda ikinci derece Mobitz tip II atriyoventriküler bloğun tanısında devam eden sorunlar özetlenmiştir. ANAHTAR KELİMELER Atriyoventriküler blok, ikinci derece Mobitz tip II atriyoventriküler blok, ikinci derece tip I atriyoventriküler blok. İLETİŞİM ADRESİ S. Serge BAROLD, M.D. S. Serge Barold MD. 5806 Mariner’s Watch Drive, Tampa FL 33615, USA. Continuing Problems with the Diagnosis of Mobitz Type II Second-Degree Atrioventricular Block 55 econd-degree AV block remains poorly un- conducted P waves (i.e., 3:2 AV block) to deter- Sderstood despite the major advances in car- mine behavior of the PR interval (Figure 1). The diac electrophysiology in the last four deca- PR interval after the blocked impulse always des. The literature is replete with varying de- shortens if the P wave is conducted to the vent- finitions of second-degree AV block especially ricle. The term “inconstant” PR or AV intervals Mobitz type II block. It should therefore not be is important because many sequences of type I surprising that during formal testing, physici- second-degree AV block are atypical and do not ans score more poorly with second-degree AV conform to the traditional mathematical behavi- block ECGs than with those of other arrhythmi- or of the PR intervals described in standard text- as. Several workers have indicated that Mobitz books. For example, the second PR interval (after type II block is misunderstood more than any a blocked impulse) often does not show the grea- other abnormality of rhythm or conduction. The test increment, whereas the PR interval may ac- reasons for these difficulties and the pitfalls in tually shorten in the middle of a type I sequen- the diagnosis of second-degree AV block form ce. The term “progressive” is misleading beca- the basis of this review. use PR intervals may stabilize and show no dis- cernible change in the middle or at the end of a Contemporary Definitions type I sequence (Figure 1 and 2). This arrange- Type I block and type II second-degree AV ment at the end of a type I structure simulates block are electrocardiographic patterns that re- type II block (Figure 2). fer to the behavior of the PR intervals (in sinus rhythm) in sequences (with at least 2 consecuti- Type II Second-Degree AV Block ve conducted PR intervals) where a single P wa- Type II describes what appears to be all- ve fails to conduct to the ventricles. The ana- or-none AV conduction without visible incre- tomic site of block should not be characterized ments in conduction time in the standard ECG. in terms of either type I or type II because the According to the definitions codified by the type I and II designations refer only to electro- World Health Organization (WHO) and the cardiographic patterns. The definitions of type American College of Cardiology (both in 1978), I and type II second-degree AV block are purely type II second-degree AV block is defined as descriptive, do not refer to the site of the block. the occurrence of a single non-conducted P wa- They are predicated on the absence of an escape ve associated with constant PR intervals before Q& RS complex after the blocked impulse. and after the blocked impulse, provided the si- nus rate or the P-P interval is constant (no slo- Type I Second-Degree AV Block wing) and there are at least two consecutive Type I second-degree AV block describes conducted P waves (i.e., 3:2 AV block) to reve- visible decremental or varying AV conduction al the behavior of the PR interval (Figure 2 and with large or small increments in AV conduction 3). The pause encompassing the blocked P wa- time and is defined as the occurrence of a sing- ves should equal two (P-P) cycles. Stability of le non-conducted P wave associated with incons- the sinus rate with absence of slowing is an im- tant PR intervals before and after the blocked im- portant criterion because a vagal surge can ca- pulse provided there are at least two consecutive use simultaneous sinus slowing and AV nodal CİLT 7, SAYI 3, Ekim 2009 56 Türk Aritmi, Pacemaker ve Elektrofizyoloji Dergisi FIGURE 1 FIGURE 2 Diagramatic representation of various forms of second-degree AV block with the same format as in Fig 1. (C): Relatively Diagrammatic representation of various forms of second- long and atypical type I sequence with several constant PR degree AV block. The 3 levels represent activation of the intervals before a dropped beat. Note the shorter PR interval atria, AV junction (PR intervals are shown between the after the blocked P wave. This pattern should not be called lines), and ventricles respectively. All values are in msec. (A) type II AV block. It is essential to examine all the PR intervals Classic type I AV block. (B) Relatively long and atypical type in long rhythm strips and not merely several PR intervals I sequence. Note the irregular fluctuations of the PR intervals preceding a blocked impulse. (D): True type II AV block. Every before the dropped beat. (From Barold SS, Friedberg HD. atrial impulse successfully traverses the AV node which is not Second-degree AV block. A matter of definition. Am J Cardiol afforded a long recovery time as occurs in type I AV block. 1974;33:311–313, with permission.) Note that the PR interval after the blocked beat is unchanged. (From Barold SS, Friedberg HD. Second-degree AV block. A matter of definition. Am J Cardiol 1974;33:311–313, with permission.) FIGURE 3 Sinus rhythm with second-degree type II AV block in the presence of right bundle branch block and left anterior hemiblock.There are tiny q waves in V2 and V3 probably related to left anterior hemiblock rather than old anterior myocardial infarction. Note that the sinus rate is constant and the PR interval after the blocked beat remains unchanged. block, generally a benign that can superficially The literature of type II block is replete with resemble type II second-degree AV block (Fi- errors because the diagnostic importance of the gure 4). The 2008 ACC/AHA/HRS guidelines rate criterion (no slowing) and need for an unc- do not state that the diagnosis of type II block hanged PR interval after a single blocked im- requires the absence of heart rate slowing. pulse are often ignored. A constant PR after the CİLT 7, SAYI 3, Ekim 2009 Continuing Problems with the Diagnosis of Mobitz Type II Second-Degree Atrioventricular Block 57 A B C FIGURE 4 Representative Holter recordings from an asymptomatic patient with a type I block variant that was misdiagnosed as type II block by several physicians. A: Type I block with constant PR intervals before the blocked beat. Note that there is a slight increase in the sinus rate in the sequence before the blocked beat. However, the sinus rate then slows and the blocked P wave occurs in association with sinus slowing a combination consistent with a vagal phenomenon. The PR intervals after the blocked beat are inconstant. B: Type I variant simulating type II block. The PR intervals are constant before and after the blocked beat. However, there is obvious sinus slowing simultaneously with the nonconducted P wave. C: Type I block. Note that in the presence of a narrow QRS complex, the occurrence of type I (with fairly large increments of the PR intervals) and what appears to be type II block basically rules out the presence of a true type II block. blocked beat is a sine qua non of type II block. Site of AV Block The diagnosis of type II cannot be made if the P Type II block according to the strict defini- wave after a blocked impulse is not conducted tion occurs in the His-Purkinje system and ra- with the same PR interval as all the other con- rely above the site of recording of the His bund- ducted P waves. In other words, type II second- le potential in the proximal His bundle or the degree AV block cannot be diagnosed whene- nodo–Hisian junction. Type II second-degree ver a shortened AV interval occurs after the AV block has not been convincingly demonstra- blocked P wave regardless of a string of cons- ted in the N zone of the AV node. Type II block tant PR intervals before the block (Figure 2). In is associated with bundle branch block in at le- such a situation the pattern is either type I or ast 70% of the cases and should be a Class I pa- unclassifiable. The shorter PR interval after the cemaker indication regardless of QRS duration. blocked P wave may either be caused by imp- The 2008 ACC/AHA/HRS guidelines sho- roved conduction (type I block) or AV disso- uld remove confusing recommendations for ciation from an escape AV junctional beat be- Type II second-degree AV block. In the secti- aring no relationship to the preceding P wave.
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