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 electrophysiology. 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 discernible 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 sinus 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 important 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

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. on on acquired AV block, the guidelines state

CİLT 7, SAYI 3, Ekim 2009 58 Türk Aritmi, Pacemaker ve Elektrofizyoloji Dergisi

“Permanent pacemaker implantation is reaso- mend advanced AV block as an additional ca- nable for asymptomatic type II second-degree tegory (outside of type I, type II, and 2:1 AV AV block with a narrow QRS as a Class IIa in- block) in which multiple consecutive P waves dication. When type II second-degree AV block are blocked but complete AV block is not pre- occurs with a wide QRS, including isolated sent. Asymptomatic 2:1 AV block with bund- right bundle-branch block, pacing becomes a le branch block does not automatically indica- Class I recommendation.” There is no eviden- te infranodal block because the site of block is ce that type II block with a narrow QRS has a in the AV node in 15–20% of the cases and thus better prognosis than type II block with a wide not necessarily a Class II indication for pacing. QRS. Thus, all type II blocks should receive a pacemaker regardless of symptoms. Is the Diagnosis of Type II AV Block The presence of type I second-degree AV Possible Based on the PR Interval block with a narrow QRS complex is almost in- After Block of 2 or More Consecutive P variably owing to a lesion in the A-V node, be- Waves? cause type I in the His bundle is rare. In the ca- When all PR intervals remain constant befo- se of type I block with a broad (0.12 sec) QRS re and after AV block of 2 or more consecutive complex, outside the setting of acute MI, the impulses in the setting of a stable sinus rhythm, block may be AV nodal in 30% to 40% of pa- the purist will insist on calling this pattern type tients, whereas in the His-Purkinje system in II AV block by citing the original description 60% to 70% of cases. Infranodal type I block by Mobitz. Some workers still respect the origi- carries the same prognostic significance as nal definition. According to the codified defini- type II second-degree AV block: Both indicate tions such an observation should not be labeled severe disease of the His-Purkinje system, with type II AV block (but advanced second-degree a relatively poor prognosis. Any believe that block). The 2008 ACC/AHA/HRS guidelines an asymptomatic patient with type I second- classify this pattern as advanced second-degree degree AV block and bundle branch block de- AV block (Figure 5C). Admittedly, the constant serves an electrophysiologic study to determine PR intervals before and after the block strongly the site of block. suggest infranodal block and the potential need for a pacemaker. When the first PR interval af- 2:1 AV Block ter the blocked P waves is not equal to previous P Although 2:1 AV block can occur in either waves the block can be either in the AV node or the AV node or the His-Purkinje system, this the His-Purkinje system. form of AV block cannot be classified into type I or type II, an important concept clearly emp- Problematic Definitions of Type II hasized in the 2008 ACC/AHA/HRS guidelines. Block: Historical Aspects 2:1 AV block is best considered advanced In 1906 John Hay from Liverpool, England second-degree AV block, as are higher degre- described a new form of second-degree AV block es of block such as 3:1, 4:1, and so on, accor- now considered type II second-degree AV block. ding to the definitions promulgated by the In 1924 Mobitz, using the electrocardiograph, WHO and ACC, although some experts recom- classified the well known Wenckebach form of

CİLT 7, SAYI 3, Ekim 2009 Continuing Problems with the Diagnosis of Mobitz Type II Second-Degree Atrioventricular Block 59

Type I describes electrocardiographically visible varying and generally decremental AV conduction with large or small increments in conduction time and type II describes what appears all-or-none conduction without visible changes in AV conduction time as it appeared to Mobitz. In 1955 Katz and Pick described type II block as a condition that develops “without FIGURE 5 warning, that is it occurs after a series of be- Diagramatic representation of various forms of second-degree ats with a constant PR prolongation.” It is note- AV block with the same format as in Fig 1. (A) Dropped beat followed by a 30ms shortening of the PR interval. This pattern worthy that the required number of constant PR should not be called type II AV block. It may be a type I AV block or unclassifiable if shortening of the PR interval is due to intervals before the blocked beat was missing an AV junctional escape beat. (B) Type II AV block according from this statement. The Katz and Pick descrip- to some of the old definitions. This is now labeled as a type I with very small increments in conduction. Some workers still tion of type II block created the still popular de- call this arrangement type II AV block. The diagnosis of type II block cannot be made if the PR interval after the blocked finition of type II block found in most textbo- beat is not equal to all the other PR intervals. (C) Advanced second-degree AV block (failure of conduction of 2 consecutive oks as an “electrocardiographic pattern charac- P waves without warning). All the PR intervals are constant including the first one after the block. This suggests infranodal terized by failure of a single impulse to conduct AV block. Some workers cling to the original Mobitz definition to the ventricles in the absence of antecedent and call this sequence type II block. (From Barold SS, Friedberg HD. Second-degree AV block. A matter of definition. lengthening of the PR interval (normal or pro- Am J Cardiol 1974;33:311–313, with permission.) longed)”. This definition of type II block in fact also describes type I block when the terminal second-degree AV block as type I and characteri- portion of a long atypical type I sequence exhi- zed type II second-degree AV block as “the occa- bits PR intervals with no discernible or measu- sional block of one or more P waves with no chan- rable change before the blocked impulse (Figu- ge in the PR interval before and after the noncon- re 7). Consequently by citing the Chicago defi- ducted P waves.” (Figure 6). Mobitz also classified nition of type II block, some workers have ma- 2:1, 3:1, etc. AV block as type II only when the PR de the diagnosis of type II block with only 2 or interval after the blocked impulse (or impulses) 3 constant PR intervals before block of single P remained unchanged. Mobitz believed that con- wave (and ignoring the first postblock PR inter- duction in type II block was an all-or-none phe- val). Further more the Chicago School allowed nomenon. The definitions of type I and II second- shortening of the postblock PR interval by 0.02 degree AV block remain descriptive to this day: sec for the diagnosis of type II block (Figure 5).

FIGURE 6

Diagrammatic representation of second-degree type II AV block from Mobitz’s original 1924 article.

CİLT 7, SAYI 3, Ekim 2009 60 Türk Aritmi, Pacemaker ve Elektrofizyoloji Dergisi

in the shorter cycle) because a vagal effect can be ruled on the basis of rate increase and provided there are no associated episodes of type I block. One should not diagnose type II block with a narrow QRS if the P-P interval lengthens before the block, even if the PR intervals are constant before and after the block. This situation is almost always a type I variant with AV nodal block.

All Type II Blocks are Really Type I FIGURE 7 Blocks Narrow QRS type I block registered in a 3-lead Holter recording. There is sinus arrhythmia. The last 3 PR intervals It has been shown that type II blocks are in before the blocked beat (arrow) are constant. This pattern should not be classified as type II block when conduction of effect type I blocks with increments in AV con- the post-block P wave is not seen. Actually the P wave after the duction so miniscule that they cannot be recor- block was conducted with a shorter PR interval consistent with type I block. ded or measured with standard equipment. Alt- hough type II block is an artifact of the standard electrocardiograph because it cannot re- Definitions of type II block with statements cord ultra-short changes in AV conduction, the such as “all conducted PR intervals are constant resultant all-or-none pattern remains immen- or all the conducted P waves have constant PR sely useful to localize the site of block. intervals ” also contain a loophole. They could be interpreted to mean that an absent or non- Does Narrow QRS Type II AV Block conducted P wave after the blocked impulse Really Exist in Inferior Myocardial carries no diagnostic value and can be ignored Infarction? by the very fact that it is nonconducted. The documentation of occasional intra- and infra-Hisian blocks in inferior myocardial in- Influence of Heart Rate on Diagnosis farction (MI) suggests that true type II second- A modest increase or decrease in sinus ra- degree AV block is theoretically possible in this te permits the diagnosis of type I block. Classi- situation. Despite claims by many authors that cally, the sinus rate must be constant when PR narrow QRS type II block can occur in acute intervals are measured to determine the presen- inferior MI, there are no published electrocar- ce of type II block. However block in the His- diograms showing true Mobitz type II second- Purkinje system can be tachycardia-dependent degree AV block during an inferior MI, even in so that the diagnosis of type II can be made in patients exhibiting various infranodal conducti- the setting of an increasing sinus rate. on disturbances The diagnosis of type II block has not been carefully studied during sinus arrhythmia. Vagally-Mediated AV Block Type II block can be diagnosed when there is Vagally-mediated AV block occurs in the AV an increase in the sinus rate (and block occurs node (at the level of the AH interval) and is the-

CİLT 7, SAYI 3, Ekim 2009 Continuing Problems with the Diagnosis of Mobitz Type II Second-Degree Atrioventricular Block 61 refore associated with a narrow QRS complex. sinus slowing no matter how slight, especially It is generally benign and must be carefully dif- if the QRS complex is narrow. The diagnosis of ferentiated from true type II block which indi- type II can only be made with confidence if the cates the presence of an irreversibly damaged same pattern type II occurs repeatedly without His-Purkinje system that always requires a per- sinus slowing and in the absence of any sequen- manent pacemaker. In contrast, vagally medi- ces, suggesting type I second-degree AV block. ated second-degree AV block rarely requires a permanent pacemaker. Conclusion Vagally mediated AV block is characteristi- The correct diagnosis of second-degree AV cally paroxysmal and often associated with si- block is basically an exercise in logic based on nus slowing. As a rule, AV nodal block shows ob- the clinical application of relatively simple de- vious irregular PP intervals and is bradycardia- finitions and knowledge of the various diagnos- associated (not bradycardia-dependent), that is, tic pitfalls (Table 1). Many of problems related both AV block and sinus slowing result from to the diagnosis of type II block are best un- vagal effects. An acute increase in vagal to- derstood by reviewing the historical aspects of ne may occasionally produce AV block witho- second-degree AV block. Unfortunately many ut preceding prolongation of the AH interval misconceptions about type II block continue to (constant PR), giving the appearance of a type complicate the evaluation of second-degree AV II AV block mechanism when there is no PR block. prolongation before the blocked beat. In this situation, AH prolongation may occur during the TABLE 1 initial few beats when AV conduction resumes. Electrocardiographic pitfalls Vagally induced AV block may be followed eit- · Nonconducted atrial premature beats masquerading as AV Block her by a shorter or longer PR interval or rarely · What appears to be narrow QRS type II block may by an unchanged PR interval (the latter mimic- be a type I variant. Failure to suspect type I block in the presence of miniscule increments of the PR king type II second-degree AV block). An unc- interval hanged PR interval after the blocked P wave · Atypical type I sequence mistaken for type II block. This occurs most commonly when an atypical type I may be caused by a nonconducted P wave oc- sequence displays a string of constant PR intervals in its terminal part before the block. curring fortuitously with an escape AV juncti- · Failure to realize that type I and type II almost never onal beat so that the P–QRS relationship or PR occur in the same ECG or Holter recording. What appears to be type II is a type I sequence with tiny interval is equal to that seen before the blocked increments of AV conduction. P waves. Alternatively, a residual vagal effect · Making the diagnosis of type II block without seeing a truly conducted first postblock P wave (“shortage” on the AV node may prevent the expected PR of PR intervals) · Concealed His or ventricular extrasystoles causing shortening. These mechanisms simulate type II pseudo-AV block. (Look for associated unexpected second-degree AV nodal block. The clue to the sudden PR prolongation, combination of what appears to be type I and type II and isolated diagnosis of AV nodal block is the presence of retrograde P waves from retrograde conduction of the concealed extrasystole)

CİLT 7, SAYI 3, Ekim 2009 62 Türk Aritmi, Pacemaker ve Elektrofizyoloji Dergisi

8. El-Sherif N, Aranda J, Befeler B, Lazzara R. Atypical REFERENCES Wenckebach periodicity simulating Mobitz type II AV block. Brit Heart J 1978;40:1376-1383. 1. WHO/ISC Task Force. Definition of terms related to 9. Barold SS, Hayes DL. Second-degree atrioventricular cardiac rhythm. Am Heart J 1978;95:796–806. block: a reappraisal. Mayo Clin Proc 2001;76:44-57. 2. Surawicz B, Uhley H, Borun R, et al. The quest for opti- 10. Epstein AE, DiMarco JP, Ellenbogen KA, et al. mal electrocardiography. Task Force I: standardization of American College of Cardiology/American Heart terminology and interpretation. Am J Cardiol 1978;41:130- Association Task Force on Practice Guidelines (Writing 45. Committee to Revise the ACC/AHA/NASPE 2002 3. Hay J. Bradycardia and cardiac arrhythmia produced by Guideline Update for Implantation of Cardiac Pacemakers depression of certain of the functions of the heart. Lancet and Antiarrhythmia Devices); American Association for 1906:1:139-143. Thoracic Surgery; Society of Thoracic Surgeons. ACC/ AHA/HRS 2008 Guidelines for Device-Based Therapy of 4. Möbitz W. Uber die unvollstandige storung der erregung- Cardiac Rhythm Abnormalities: a report of the American suberleitung zwischen vorhof und kammer des menschli- College of Cardiology/American Heart Association Task chen herzems. Z Ges Exp Med 1924;41:180-237. Force on Practice Guidelines (Writing Committee to 5. Katz LN, Pick A. Clinical Electrocardiography. Part I. Revise the ACC/AHA/NASPE 2002 Guideline Update for The Arrhythmias. Philadelphia, Lea and Febiger 1956:540- Implantation of Cardiac Pacemakers and Antiarrhythmia 658. Devices) developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic 6. Lange HW, Ameisen O, Mack R, Moses JW, Kligfield Surgeons. J Am Coll Cardiol. 2008;51(21):e1-62. P. Prevalence and clinical correlates of non-Wenckebach narrow QRS complex second degree atrioventricular block detected by ambulatory ECG. Am Heart J 1988;115-120. 7. Barold SS, Friedberg HD. Second degree atrioventricular block. A matter of definition. Am J Cardiol 1974; 33:311- 315.

CİLT 7, SAYI 3, Ekim 2009