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The electro cardiographic patterns of type I and type II second-degree atrioventricular (AV) block during sinus rhythm describe the behaviour of the PR intervals (in sinus rhythm) in sequences (with at least two consecutive conducted PR intervals) where a single P wave fails to conduct to the ventricles.1 Unfortunately, the diagnosis of Mobitz type II second-degree AV block continues to be an important clinical problem because the standard definitions of second-degree AV block are often misinterpreted.1 The literature is replete with cases of type I second-degree AV block labelled as type II block, and similar errors have crept into the sports literature with claims that narrow QRS type II AV block can also occur in otherwise healthy young athletes2,–,7 and less commonly after exercise in the form of vasovagal syncope (table 1).8 These reports of type II block in athletes provided either no ECG or misdiagnosed ones. Furthermore, when stated, the definitions of type II block were inappropriate. The occurrence of type II block in young athletes is counterintuitive because it would imply serious disease of the His–Purkinje system and an absolute indication for a permanent pacemaker regardless of symptoms.1 We were unable to find a single case of precisely documented type II block in young athletes. Yet, many review articles continue to list type II block as one of the manifestations of the athlete's heart.9,–,12 Accurate diagnosis of type II block in athletes is critical and will affect treatment recommendations. Based on these considerations, there is a need for reviewing the pitfalls surrounding the diagnosis of type II block with emphasis on how errors can be avoided by strict adherence to definitions.
Type I second-degree atrioventricular block
Type I second-degree atrioventricular (AV) block should be defined as the occurrence of a single non-conducted P wave associated with inconstant PR intervals before and after the blocked impulse, provided there are at least two consecutive conducted P waves (ie, 3:2 AV block) to determine the behaviour of the PR interval. The PR interval after the blocked impulse always shortens, provided the P wave is conducted to the ventricle in the absence of obvious escape beats. The term ‘inconstant’ PR or AV intervals is important, as a large proportion of type I sequences are atypical and do not conform to the traditional mathematical behaviour of the PR intervals. For example, the second PR interval (after a blocked impulse) often does not exhibit the greatest increment.1 The PR interval may actually shorten in the middle of a type I sequence. The term ‘progressive’ is misleading because PR intervals may stabilise and show no discernible change in the middle or at the end of a type I sequence. The presence of type I second-degree AV block with a narrow QRS complex is almost invariably localised in the AV node. One should remember that what appears to be narrow QRS type II block may be a type I variant. Failure to suspect type I block in the presence of miniscule increments of the PR interval is an important pitfall in the diagnosis of type I block.
Type II second-degree AV block
According to the definitions codified by the WHO and the American College of Cardiology (ACC) (both in 1978),13 14 type II second-degree AV block should be defined as the occurrence of a single non-conducted P wave associated with constant PR intervals before and after the blocked impulse, provided the sinus rate or the P–P interval is constant, and there are at least two consecutive conducted P waves (ie, 3:2 AV block) to determine the behaviour of the PR interval, and the P wave after the block must be conducted.1 13 14 The pause encompassing the blocked P wave should equal two (P–P) cycles1 15,–,18 (figure 1). The stability of the sinus rate is an important criterion because a vagal surge can cause simultaneous sinus slowing and AV nodal block, generally a benign condition, that can superficially resemble type II second-degree AV block.17 19,–,21 Type II block according to the strict definition always occurs in the His–Purkinje system.22,–,28
Potential errors in the diagnosis of second-degree type II AV block
Atypical type I sequence mistaken for type II block
Many type I sequences are atypical and do not conform to the traditional mathematical structure of the Wenckebach phenomenon with progressive prolongation of the PR intervals.1 29,–,32 A popular definition of type II block found in most textbooks describes it as an ‘electrocardiographic pattern characterised by failure of a single impulse to conduct to the ventricles in the absence of antecedent lengthening of the PR interval (normal or prolonged).’ This definition of type II block in fact also describes type I block when the terminal portion of a long atypical type I sequence contains PR intervals with no discernible or measurable change before the blocked impulse (figure 2).29 Thus, according to this incorrect definition, the diagnosis of type II block can also be entertained with only two or three constant PR intervals before block of a single P wave (and ignoring the first postblock PR interval).
Vagally mediated AV block
Vagally mediated AV block shows obvious irregular PP intervals and is bradycardia-associated (not bradycardia-dependent). The stability of the sinus rate is an important criterion of type II block because a vagal surge (generally a benign condition) that occurs invariably at the level of the AV node usually causes simultaneous sinus bradycardia (sinus node) and depression of AV conduction (AV node), a situation that can superficially resemble type II second-degree AV block.19,–,21 Sinus slowing with AV block essentially rules out type II block.
The large variety of ECG patterns seen with vagal AV block depends on the interplay of several factors influencing the net vagal effect in: (1) the moment in the cycle when the vagal effect occurs, (2) the intensity (and speed) of the vagal surge, (3) the atrial rate, (4) the sensitivity of the AV node and (5) the background sympathetic activity.
Vagally induced AV block may be followed by either a shorter or longer PR interval and rarely by an unchanged PR interval. An unchanged PR interval after the blocked P wave may be due to a non-conducted P wave occurring fortuitously with an escape AV junctional beat so that the P–QRS relationship or PR interval is equal to that seen before the blocked P waves. Alternatively, a residual vagal effect on the AV node may prevent the expected PR shortening. These mechanisms simulate type II second-degree AV nodal block. The clue to the diagnosis of AV nodal block is the presence of sinus slowing, no matter how slight, in the setting of a narrow QRS complex.
Vagally mediated AV block occurs commonly during sleep when parasympathetic activity predominates. In this respect, a recent statement that ‘episodes of Mobitz type II second-degree block may be present in some high-level athletes at night (24 h Holter) but if present in baseline ECGs during daytime, structural disease or subjacent conditions should be ruled out’ implies erroneously that type II block at night is benign.33
Type I and type II in the same individual
The presence of both narrow-QRS type I and II second-degree AV block patterns in a Holter recording essentially excludes the diagnosis of type II second-degree AV block in this situation.1 The occurrence of both wide QRS type I and II block in the same recoding is quite rare.34 Thus, this combination with a narrow QRS is so rare that one can confidently label the tracing as showing only type I block in virtually all the cases. On this basis, we believe that the reported cases of both type I and II in athletes represent only type I blocks.
Shortage of PR interval: no truly conducted first postblock P wave
The literature of type II block is replete with errors because the diagnostic importance of an unchanged PR interval after a single blocked impulse is often ignored. A constant PR after the blocked beat is a sine qua non of type II block. The diagnosis of type II cannot be made if there is no P wave after a blocked impulse or the P wave is not conducted with the same PR interval as all the other conducted P waves. In such a situation, the pattern is either type I or unclassifiable. The shorter PR interval after the blocked P wave may be due to either improved conduction (type I block) or AV dissociation due to an escape AV junctional beat that bears no relationship to the preceding P wave.1 32
2:1 AV block
Although 2:1 AV block can occur in either the AV node or the His–Purkinje system, this form of AV block cannot be classified into type I or type II second-degree AV block. 2:1 AV block is best considered as advanced second-degree AV block, as are higher degrees of block such as 3:1, 4:1, etc, according to the definitions promulgated by the WHO and ACC.35
Other characteristics of type II block versus vagally induced block
True narrow QRS type II block is relatively rare and occurs without sinus slowing. It is typically associated with AV conduction ratios >2 (3:1, 4:1 which are rare in vagal block) and without associated type I structures.17 Sustained advanced second-degree AV block is far more common in association with true type II block than type I block or its variant. Although intense vagal tone can be associated with block of multiple consecutive P waves, vagally mediated AV block rarely involves more than block of two consecutive P waves.17 The diagnosis of type II can only be made with confidence if the same pattern occurs repeatedly without sinus slowing and in the absence of any sequence suggesting type I second-degree AV block. Obviously the PR intervals before and after the blocked impulse must remain constant.
Impact of exercise
Exercise induces an increase in sympathetic tone with improvement of AV nodal conduction. Consequently, type I block commonly improves or disappears with exercise. In contrast, enhanced sympathetic activity exerts no effect below the AV node so that type II block commonly deteriorates on exercise. The purported cases of type II block in athletes appeared to behave like type I blocks on exercise, and no aggravation of the block occurred, providing further proof against the diagnosis of type II block.2,–,7
We believe that the recommendation of the 26th Bethesda Conference that ‘Mobitz type II block should be treated with a permanent pacemaker before any athletic activity’ is appropriate, provided decisions are based on the correct definition of type II block.36 The 2002 ACC/American Heart Association/North American Society of Pacing and Electrophysiology guidelines for pacemaker implantation also recommend permanent pacing for type II second-degree AV block regardless of symptoms.37 38 Therefore, we cannot agree with the recommendations of the European Society of Cardiology that low moderate dynamic exercise can be performed in the setting of type II block in the absence of symptoms, cardiac disease, ventricular arrhythmias on exercise and if the resting rate >40 bpm.39
Much of the prevailing confusion surrounding type II second-degree AV block in young athletes would disappear with the proper application of strict and uniform definitions. The diagnosis of second-degree AV block is basically an exercise in clinical logic that centres on understanding the definitions of type I and type II block.
Competing interests None.