The Ashman phenomenon refers to aberration occurring when a short cycle follows a long one (long-short cycle sequence) (see Fig. 10-1).^11,¹³ Aberrancy is caused by the physiological changes of the conduction system refractory periods associated with the R-R interval. Normally, the refractory period of the HPS lengthens as the heart rate slows and shortens as the heart rate increases, even when heart rate changes are abrupt. Thus, aberrant conduction can result when a short cycle follows a long R-R interval. In this scenario, the QRS complex that ends the long pause is conducted normally but creates a prolonged ERP of the bundle branches. If the next QRS complex occurs after a short coupling interval, it may be conducted aberrantly because one of the bundles is still refractory as a result of a lengthening of the refractory period (phase 3 block; Fig. 10-2).^6,⁷

FIGURE 10-2 Atrial tachycardia (AT) with variable atrioventricular (AV) conduction and intermittent aberrancy. Left, AT with 2:1 AV conduction and normal QRS morphology is observed. Once 1:1 AV conduction occurs (QRS 3), left bundle branch block (LBBB) aberration develops (caused by long-short cycle sequence, phase 3 block). LBBB aberration is also observed during QRS 4, likely secondary to concealed transseptal conduction causing perpetuation of aberration. After a pause, both bundle branches recover from refractoriness producing a normal QRS 5. Ashman phenomenon (long-short cycle sequence) explains phase 3 block during QRS 6, but this time it manifests as a right bundle branch block (RBBB) pattern. This is because activation during QRS 4 propagated down the right bundle branch (RB) and across the septum, thus activating the left bundle branch (LB) retrogradely after some delay (concealed transseptal conduction), so that the LB-LB interval (between QRS 4 and QRS 5) resulted in a shorter effective refractory period (ERP) of the LB following QRS 5. Conversely, the RB-RB interval (between QRS 4 and QRS 5) was longer and, consequently, the ERP of the RB was still prolonged following QRS 5, thereby setting the stage for a long-short cycle sequence of the RB but not the LB. Therefore, RBBB (rather than LBBB) aberration develops. RBBB aberration during QRS complexes 7 and 8 is secondary to either concealed transseptal conduction or rate-dependent aberration. However, because LBBB (rather than RBBB) was observed during QRS 4 (although QRS 4 occurred following a similar cycle length to that of QRS 8), concealed transseptal conduction is more likely to be the mechanism of aberration. After a pause, both bundle branches recover from refractoriness to produce a normal QRS 9. Long-short cycle sequence explains phase 3 block during QRS 10, but this time it manifests as LBBB. This is because activation during QRS 8 propagated down the LB and across the septum, thus activating the RB after some delay (concealed transseptal conduction). The result is that the RB-RB interval (following QRS 8) and ERP of the RB became shorter, whereas the LB-LB interval (following QRS 8) was longer. Consequently, the ERP of the LB was still prolonged following QRS 9, thereby setting the stage for a long-short cycle sequence and LBBB aberration. LBBB aberration during QRS 11 is caused by concealed transseptal conduction. The Ashman phenomenon (long-short cycle sequence) underlies RBBB aberration during QRS 14. HRA = high right atrium.

RBBB aberration is more common than LBBB in this setting because the RB has a longer ERP than the LB. The Ashman phenomenon can occur during second-degree AV block (see Fig. 9-9), but it is most common during atrial fibrillation (AF), whereby the irregularity of the ventricular response results in frequently occurring long-short cycle sequences.

The aberrancy can be present for one beat and have a morphology resembling a premature ventricular complex (PVC), or it can involve several sequential complexes, a finding suggesting ventricular tachycardia (VT). In the setting of aberrancy during AF, the long-short cycle sequence characteristic of the Ashman phenomenon may not be helpful in differentiating aberration from ventricular ectopy. Although a long cycle (pause) sets the stage for the Ashman phenomenon, it also tends to precipitate ventricular ectopy. Furthermore, concealed conduction occurs frequently during AF, and, therefore, it is never possible to know from the surface ECG exactly when a bundle branch is activated. Thus, if an aberrant beat does end a long-short cycle sequence during AF, it can be because of refractoriness of a bundle branch secondary to concealed conduction into it, rather than because of changes in the length of the ventricular cycle.⁸^–¹⁰ ¹²

Nevertheless, there are several features of ventricular ectopy that can help distinguish a PVC from an aberrantly conducted or Ashman beat during AF. PVCs are usually followed by a longer R-R cycle, indicating the occurrence of a compensatory pause, the result of retrograde conduction into the AVN and anterograde block of the impulse originating in the atrium. A ventricular origin is also likely when there is a fixed coupling cycle between the normal and aberrant QRS complexes. The presence of long and identical R-R cycles after the aberrated beats and the absence of a long-short cycle sequence associated with the wide or aberrant QRS complex also suggest ventricular ectopy. Additionally, the absence of aberrancy, despite the presence of R-R cycle length (CL) combinations that are longer and shorter than those associated with the wide QRS complex, suggests ventricular ectopy. QRS morphology inconsistent with LBBB or RBBB aberrancy argues against aberration (Fig. 10-3).^6,⁸^–¹⁰ ^,12

FIGURE 10-3 Premature ventricular complexes during atrial fibrillation. Several features suggest that the wide QRS complexes are caused by ventricular ectopy rather than aberration. QRS morphology is inconsistent with LBBB or RBBB aberrancy. Additionally, there is a fixed coupling cycle between the normal and aberrant QRS complex. The absence of a long-short cycle sequence associated with the wide QRS complex and the absence of aberrancy despite the presence of R-R cycle length combinations that are longer and shorter than those associated with the wide QRS complex also suggest ventricular ectopy.

Aberration caused by the Ashman phenomenon can persist for several cycles. The persistence of aberration can reflect a time-dependent adjustment of refractoriness of the bundle branch to the abrupt change in CL, or it can be the result of concealed transseptal activation (see later).

Aberration Caused by Heart Rate Acceleration

As the heart rate accelerates, the HPS refractory period shortens; normal conduction tends to be preserved because of this response. However, refractoriness of the HPS eventually reaches a critical value beyond which an increase in heart rate no longer abbreviates it; at this point, AV block may occur. Conversely, the refractory period lengthens as the heart rate slows. Acceleration-dependent BBB is a result of failure of the action potential of the bundle branches to shorten, or, paradoxically, the action potential lengthens in response to acceleration of the heart rate (Fig. 10-4).^3,¹¹ As noted, the ERP of the RB normally shortens at faster heart rates to a greater degree than that of the LB; this finding explains the more frequent RBBB aberration at longer CLs and LBBB aberration at shorter CLs.^4,^8,¹⁰

FIGURE 10-4 Supraventricular tachycardia (SVT) with tachycardia-dependent (phase 3) right bundle branch block. Delivery of a late ventricular extrastimulus (arrow) during the SVT preexcites the right bundle branch (and either peels back or shortens its refractoriness) and restores normal conduction.

This form of aberration is a marker of some type of cardiac abnormality when it appears at relatively slow heart rates (less than 70 beats/min), displays LBBB (Fig. 10-5), appears after several cycles of accelerated but regular rate, or appears with gradual rather than abrupt acceleration of the heart rate and at CL shortening by less than 5 milliseconds. Because of the small changes in the duration of the CL that may initiate aberration (critical cycle), recognition of acceleration-dependent aberration may require a long record to document the gradual, and at times minimal, shortening of the R-R interval.⁸^–¹⁰

FIGURE 10-5 Acceleration-dependent aberration. The lead II continuous rhythm strip demonstrates sinus rhythm with rate-related left bundle branch block (LBBB). Note that small changes in rate can result in acceleration-dependent aberration. LBBB develops at a sinus rate faster than 70 beats/min, and normal QRS complexes are observed at slower sinus rates. The LBBB and the slow rate at which aberration develops strongly suggest an abnormality in the left bundle branch (LB), rather than physiological aberration, often associated with underlying structural heart disease such as cardiomyopathy. Interestingly, the onset and offset of the LBBB demonstrate hysteresis in that the cycle length (CL) required to initiate the LBBB is shorter than the CL required to maintain it, probably because of retrograde concealed penetration of the LB.

With increasing heart rate or persistence of fast heart rate, acceleration-dependent aberration can occasionally disappear. The normalization of a previously aberrant QRS complex can be explained by a greater shortening of the ERP of the bundle branches than that of the AVN or by a time-dependent gradual shortening of the refractory period of the affected bundle branch (a phenomenon occasionally referred to as restitution).⁶

During slowing of the heart rate, intraventricular conduction often fails to normalize at the critical CL, and aberration persists at cycles longer than the critical cycle that initiated the aberration. Once acceleration-dependent BBB is established, the actual cycle for the blocked bundle does not begin until approximately halfway through the QRS complex because of concealed transseptal conduction (see later); thus, it is necessary for the heart rate to slow down more than would be expected to reestablish normal conduction.⁸^–¹⁰

Phase 4 Block

Phase 4 block occurs when conduction of an impulse is blocked in tissues well after their normal refractory periods have ended.^3,¹¹ Phase 4 block is governed by the same physiological principles as those for phase 3 block. Membrane responsiveness is determined by the relationship of the membrane potential at excitation with the maximum height of phase 0. The availability of the Na⁺ channels is reduced at less negative membrane potentials, and activation at a reduced membrane potential is likely to cause aberration or block.⁶

The cause of membrane depolarization (i.e., reduction of membrane potential) in the setting of phase 4 block, however, is different from that in phase 3 block. Enhanced phase 4 depolarization within the bundle branches can be caused by enhanced automaticity or partial depolarization of injured myocardial tissue, or both. In this setting, the maximum diastolic potential immediately follows repolarization, from which point the membrane potential is steadily reduced (by the pacemaker current). This reduction, in turn, results in inactivation of some Na⁺ channels. Thus, an action potential initiated early in the cycle (immediately after repolarization) would have a steeper and higher phase 0 and consequently better conduction than would an action potential initiated later in the cycle when the membrane potential at the time of the stimulus is reduced, with resulting reductions in the velocity and height of phase 0 and slower conduction.^8,^10,¹²

Phase 4 aberration is one explanation for the development of aberration at the end of a long cycle. As a result of a gradual spontaneous depolarization made possible by a prolonged cycle, the cell is activated from a less negative potential, and the result is impaired conduction. This type of aberration is sometimes referred to as bradycardia-dependent BBB (Fig. 10-6).⁶ Importantly, the membrane potential has to depolarize significantly before conduction becomes impaired. Mild depolarization can actually improve conduction because the voltage is closer to threshold.

FIGURE 10-6 Bradycardia-dependent left bundle branch block (LBBB). Normal intraventricular conduction is observed during sinus rhythm. Premature atrial complexes (hidden within the T waves) are not conducted to the ventricles, resulting in pauses. The sinus complex following the pause is conducted with LBBB pattern (arrows) presumably secondary to phase 4 block. See text for discussion.

Phase 4 aberration would be expected in the setting of bradycardia or enhanced normal automaticity. However, despite the fact that bradycardia is common and cells with phase 4 depolarization are abundant, phase 4 block is not commonly seen; most reported cases are associated with structural heart disease. One explanation for this phenomenon is that in normal fibers, conduction is well maintained at membrane potentials more negative than −70 to −75 mV. Significant conduction disturbances are first manifested when the membrane potential is less negative than −70 mV at the time of stimulation; local block appears at −65 to −60 mV. Because the threshold potential for normal His-Purkinje fibers is −70 mV, spontaneous firing occurs before the membrane can actually be reduced to the potential necessary for conduction impairment or block. Phase 4 block is therefore pathological when it does occur, and it requires one or more of the following: (1) the presence of slow diastolic depolarization, which needs to be enhanced; (2) a decrease in excitability (a shift in threshold potential toward zero) so that, in the presence of significant bradycardia, sufficient time elapses before the impulse arrives, thus enabling the bundle branch fibers to reach a potential at which conduction is impaired; and (3) a deterioration in membrane responsiveness so that significant conduction impairment develops at −75 mV instead of −65 mV; this occurrence would also negate the necessity for such a long cycle before conduction fails.^8,^10,¹²

Bradycardia-dependent or phase 4 block almost always manifests an LBBB pattern, likely because the left ventricular (LV) conduction system is more susceptible to ischemic damage and has a higher rate of spontaneous phase 4 depolarization than the right ventricle (RV).¹²

Both tachycardia-dependent and bradycardia-dependent BBB can be seen in the same patient with an intermediate range of CLs associated with normal conduction. The prognosis of rate-dependent BBB largely depends on the presence and severity of the underlying heart disease. Its clinical implications are not clear, and it usually occurs in diseased tissue and in the setting of myocardial infarction (MI), especially inferior wall MI.⁶

Aberration Caused by Concealed Transseptal Conduction

Concealed transseptal conduction is the underlying mechanism of aberration occurring in several situations, including perpetuation of aberrant conduction during tachyarrhythmias, unexpected persistence of acceleration-dependent aberration, and alternation of aberration during atrial bigeminal rhythm.⁸^–¹⁰ ^,12 ^,14 ^,15

Perpetuation of Aberrant Conduction during Tachyarrhythmias

During a supraventricular tachycardia (SVT) with normal ventricular activation, a PVC originating from the RV can retrogradely activate the RB early, whereas retrograde activation of the LB occurs later, following transseptal conduction of the PVC. Consequently, although the RB ERP expires in time for the next SVT impulse, the LB remains refractory because its actual cycle began later than the RB. Therefore, the next SVT impulse traveling down the His bundle (HB) encounters an excitable RB and a refractory LB; thus, it propagates to the RV over the RB (with an LBBB pattern, phase 3 aberration). Conduction subsequently propagates from the RV across the septum to the LV. By this time, the distal LB has recovered, allowing for retrograde penetration of the LB by the SVT impulse propagating transseptally, thereby rendering the LB refractory to each subsequent SVT impulse (Fig. 10-7). This process is repeated, and the LBBB pattern continues until another well-timed PVC preexcites the LB (and either peels back or shortens its refractoriness), so that the next impulse from above finds the LB fully recovered (see Fig. 10-4).^4,^14,¹⁵

FIGURE 10-7 Perpetuation of aberrant conduction during supraventricular tachycardia (SVT) secondary to concealed transseptal conduction. At left, SVT is associated with normal ventricular activation and narrow QRS complexes. Critically timed ventricular extrastimulus delivered from the right ventricle (arrow) precipitates left bundle branch block aberrancy during the SVT. See text for discussion.

Unexpected Persistence of Acceleration-Dependent Aberration

Acceleration-dependent BBB develops at a critical rate faster than the rate at which it disappears (Fig. 10-8). This paradox is most commonly ascribed to concealed conduction from the contralateral conducting bundle branch across the septum with delayed activation of the blocked bundle. Such concealed transseptal activation results in a bundle branch–to–bundle branch (RB-RB or LB-LB) interval shorter than the manifest R-R cycle. The reason is that the actual cycle for the blocked bundle does not begin until approximately halfway through the QRS complex, because it takes 60 to 100 milliseconds for the impulse to propagate down the RB and transseptally reach the blocked LB. Consequently, for normal conduction to resume, the cycle (R-R interval) during deceleration must be longer than the critical cycle during acceleration by at least 60 to 100 milliseconds.⁸^–¹⁰ ^,12 ^,14 ^,15

FIGURE 10-8 Unexpected persistence of acceleration-dependent aberration. During incremental-rate atrial pacing from the coronary sinus (CS), acceleration-dependent bundle branch block develops at a cycle length (CL) of up to 370 milliseconds. Aberrancy continues despite progressively increasing the pacing CL and disappears only when pacing at a CL of more than 600 milliseconds. See text for discussion.

However, unexpected delay of normalization of conduction cannot always be explained by concealed conduction. Conduction sometimes normalizes with slowing of the heart rate, only to recur at cycles that are still longer than the critical cycle. Such a sequence excludes transseptal concealment as the mechanism of recurrence of the aberration. Similarly, when the discrepancy between the critical cycle and the cycle at which normalization finally occurs is longer than the expected transseptal activation time (approximately 60 milliseconds in the normal heart and 100 milliseconds in the diseased states), transseptal concealment alone cannot explain the delay (see Fig. 10-8). Fatigue and overdrive suppression have been suggested as possible mechanisms of the delayed normalization of conduction.⁸^–¹⁰

Alternation of Aberration during Atrial Bigeminal Rhythm

A bigeminal rhythm can be caused by atrial bigeminy, 3:2 AV block, or atrial flutter with alternating 2:1 and 4:1 AV conduction. The alternation can be between a normal QRS complex and BBB or between RBBB and LBBB.

When alternation occurs between a normal QRS complex and RBBB during atrial bigeminy, the ERP of both RB and LB starts simultaneously following the normally conducted PAC, and the ERP of both branches is relatively short because of the preceding short cycle. After the pause, the sinus beat conducts normally, and the ERP of both bundle branches starts simultaneously but is relatively long because of the preceding long cycle. However, because the RB ERP is relatively longer than that of the LB, the next PAC encroaches on the RB refractoriness and conducts with an RBBB pattern (phase 3 block). Subsequently, that PAC is conducted down the LB and across the septum. The PAC activates the RB retrogradely after some delay (concealed transseptal conduction), so that the RB-RB interval (during the following pause) and the RB ERP become shorter. As a result, by the time the next PAC reaches the RB, the RB is fully recovered because of its abbreviated ERP (reflecting the shorter preceding RB-RB interval, which is shorter than the manifest R-R interval during the preceding pause), and normal conduction occurs (see Fig. 10-2).^14,¹⁵

The same phenomenon (concealed transseptal conduction) explains alternating RBBB and LBBB during bigeminal rhythms. In the presence of RBBB, transseptal concealed conduction from the LB to the RB shortens the RB-RB interval relative to the now longer LB-LB interval. As a result, the ERP of the LB is longer and conduction in the LB fails. In the presence of a refractory LB, conduction propagates through the RB. The delayed transseptal activation of the LB shortens the LB-LB interval. The ERP of the RB is now relatively longer, because RB conduction is blocked.^9,^10,¹²

Chronic Bundle Branch Block

Anatomical Considerations

Normal ventricular activation requires the synchronized participation of the distal components of the specialized conduction system (i.e., the main bundle branches and their ramifications). An IVCD is the result of abnormal activation of the ventricles caused by conduction delay or block in one or more parts of the specialized conduction system. Abnormalities of local myocardial activation can further alter the specific pattern of venticular activation.^1,⁶

Traditionally, three major fascicles are considered to be operative in normal persons: the RB, the left anterior fascicle (LAF), and the left posterior fascicle (LPF). An estimated 65% of individuals have a third fascicle of the LB, the left median fascicle (LMF). The HB divides at the junction of the fibrous and muscular boundaries of the intraventricular septum into the RB and LB. The RB is an anatomically compact unit that travels as the extension of the HB after the origin of the LB. The LB and its divisions are, unlike the RB, diffuse structures that fan out just beyond their origin. The LAF represents the superior (anterior) division of the LB, the LPF represents the inferior (posterior) division of the LB, and the LMF represents the septal (median) division of the LB.⁶

The bundle branches and fascicles consist of bundles of Purkinje cells covered by a dense sheath of connective tissue. The terminal Purkinje fibers connect the ends of the bundle branches to the ventricular myocardium. Purkinje fibers form interweaving networks on the endocardial surface of both ventricles and penetrate only the inner third of the endocardium, and they tend to be less concentrated at the base of the ventricle and at the papillary muscle tips. Purkinje cells are specialized to conduct rapidly, at 1 to 3 m/sec, because phase 0 of the action potential is dependent on the rapid inward Na⁺ current (I_Na). This characteristic results in almost simultaneous depolarization of the terminal HPS and propagation of the cardiac impulse to the entire RV and LV endocardium.⁶

Characteristics of the Right Bundle

The RB courses down the right side of interventricular septum near the endocardium in its upper third, deeper in the muscular portion of the septum in the middle third, and then again near the endocardium in its lower third. The RB is a long, thin, discrete structure. It does not divide throughout most of its course, and it begins to ramify as it approaches the base of the right anterior papillary muscle, with fascicles going to the septal and free walls of the RV.

The RB is vulnerable to stretch and trauma for two thirds of its course when it travels subendocardially. Chronic RBBB pattern can result from three levels of conduction delay in the RV: proximal, distal, or terminal.³ Proximal RBBB is the most common site of conduction delay. Distal RBBB occurs at the level of the moderator band, and it is an unusual site of conduction delay unless there has been transection of the moderator band during surgery. Terminal RBBB involves the distal conduction system of the RB or, more likely, the ventricular muscle itself, and it can be produced by ventriculotomy or transatrial resection of parietal bands in repair of tetralogy of Fallot. In addition, RBBB can be induced by events in the HB, because certain fibers of the HB are organized longitudinally and predestined to activate only one fascicle or bundle branch. Disease in these areas can result in activation that is asynchronous from that in the rest of the infranodal conducting system, possibly with resulting bundle branch or fascicular block.

The development of RBBB after a normal pattern can suggest the presence of structural heart disease, although less often than the development of LBBB; however, RBBB in younger individuals generally is not indicative of serious underlying heart disease. Causes of RBBB include increased RV pressure, RV hypertrophy or dilatation, ischemic heart disease, myocarditis, cor pulmonale, acute and chronic pulmonary embolism, hypertension, cardiomyopathies, congenital heart disease, and mechanical damage, as well as Lev and Lenègre diseases.

Characteristics of the Left Bundle and Its Fascicles

The predivisional portion of the LB penetrates the membranous portion of the interventricular septum under the aortic ring and then divides into three discrete branches: the LAF, the LPF, and the LMF.³ The LAF crosses the anterobasal LV region toward the anterior papillary muscle and terminates in the Purkinje system of the anterolateral wall of the LV. The LPF appears as an extension of the main LB and is large in its initial course. It then fans out extensively toward the posterior papillary muscle and terminates in the Purkinje system of the posteroinferior wall of the LV. The LMF runs to the interventricular septum; it arises in most cases from the LPF, less frequently from the LAF, or from both, and in a few cases it has an independent origin from the central part of the main LB at the site of its bifurcation.¹⁶

The clinical presentation of conduction disturbances, in order of decreasing incidence, is LAF block, RBBB, LBBB, and LPF block. This rank depends not only on the intrinsic, anatomical, and genetically determined differences among branches and fascicles but also on the manner in which the intraventricular conduction system is exposed to the various pathological processes of the surrounding cardiac structures. LBBB is usually caused by ischemic or mechanical factors, and the site of block is typically in the predivisional segment at the junction of the LB and HB. LBBB is encountered usually in patients with structural heart disease involving dilation, hypertrophy, or fibrosis of the LV such as ischemic heart disease, valvular heart disease, and various cardiomyopathies. LBBB can also be caused by Lev and Lenègre diseases. The location of conduction delay in LBBB can be proximal (particularly in diffuse myocardial disease), distal, or a combination of both.

The LAF can be injured by diseases that involve primarily the LV basal septum, the anterior half of the ventricular septum, and the anterolateral LV wall. In fact, isolated LAF block is the most common type of IVCD seen in acute anterior MI. Other disorders and situations that can also cause LAF block include hypertension, cardiomyopathies, aortic valve disease, Lev and Lenègre diseases, spontaneous and surgical closure of a ventricular septal defect, and other surgical procedures.¹⁶^–¹⁸

In contrast, the LPF is the least vulnerable segment of the whole system because it is short and wide and is located in the inflow tract of the LV, which is a less turbulent region than the outflow tract. Additionally, the LPF has a dual blood supply, from the anterior and posterior descending coronary arteries, and it is not related to structures that are so potentially dangerous.¹⁶ LPF block is a rare finding, and rather nonspecific for cardiac disease. LPF block is almost always associated with RBBB.

Site of Block

Interest in the site of block stems from the fact that bifascicular block (especially RBBB and LAF block) is the most common ECG pattern preceding the development of AV block. Determining the site of block therefore can help predict the risk of AV block.

Despite ECG anatomical correlates, the site of block producing BBB patterns is not certain in all cases. Data suggest that fibers to the RV and LV are already predestined within the HB and that lesions in the HB can produce characteristic BBB patterns. Longitudinal dissociation with asynchronous conduction in the HB can give rise to abnormal patterns of ventricular activation; hence, the conduction problem may not necessarily lie in the individual bundle branch. Moreover, it is not uncommon for intra-Hisian disease to be accompanied by BBB (especially LBBB).³ Pacing distal to the site of block can normalize the QRS. Furthermore, the problem may not be actual block, because conduction delay within the bundle in the range of 10 milliseconds can give rise to an ECG pattern of BBB.

Clinical Relevance

The prognosis of BBB is related largely to the type and severity of the underlying heart disease and to the possible presence of other conduction disturbances. Isolated BBB in the absence of apparent structural heart disease and not associated with block in the other fascicles is usually benign.

Bifascicular block (especially RBBB and LAF block) is the most common ECG pattern preceding complete heart block in adults. Other forms of IVCD precede the bulk of the remaining cases of complete intra-Hisian and infra-Hisian AV block. The incidence of progression to complete AV block is approximately 2% in asymptomatic patients with an IVCD and approximately 6% in patients with an IVCD and neurological symptoms (e.g., syncope).^19,²⁰

Patients with BBB have an unusually high incidence of cardiac disease and sudden cardiac death. The highest incidence of sudden cardiac death is among patients with LBBB and cardiac disease.¹⁹^–²² However, most sudden cardiac deaths are caused by VT or ventricular fibrillation, do not seem to be related to AV block, and are not prevented by pacemakers (although pacing can potentially relieve symptoms such as syncope). Complete electrophysiological (EP) testing and ventricular stimulation are therefore necessary in patients with syncope and BBB because VT can be found in up to 30% to 50%. Although the poor prognosis associated with BBB is related to myocardial dysfunction, heart failure, and ventricular fibrillation rather than heart block, symptoms such as syncope are often related to heart block.²³^–²⁵

RBBB is a common finding in the general population. In patients without evidence of structural heart disease, RBBB has no prognostic significance. However, new-onset RBBB does predict a higher rate of coronary artery disease, heart failure, and cardiovascular mortality. When cardiac disease is present, the coexistence of RBBB suggests advanced disease. RBBB is an independent predictor of all-cause mortality in patients with known or suspected coronary heart disease, and in the setting of an acute MI, RBBB is associated with a significant increase in mortality.²⁶

The prevalence of LBBB in the general population has been reported to vary considerably according to population size and sampling criteria; it ranges from 0.1% to 0.8%. The presence of isolated LBBB has no adverse prognostic significance. In patients with LBBB associated with ischemic heart disease, hypertension, or cardiomyopathy, the prognosis depends on the severity of the underlying heart disease. Nevertheless, among patients with acute MI, cardiomyopathy, and heart failure, the presence of LBBB is associated with a worse prognosis. Because the presence of LBBB can represent the clinical onset of LV structural disease such as dilated cardiomyopathy or an infective, hypertensive, or valvular heart disease, the finding of LBBB on resting ECG requires further evaluation with echocardiogram and Holter monitoring for assessment of LV function, as well as identifying both advanced degrees of AV block and heart disease-related tachyarrhythmias. Evaluation for ischemic heart disease can be necessary based on the presence of risk factors and typical symptoms.²⁶

The prevalence of LAF block in the general population ranges from 0.9% to 6.2%. Isolated LAF block does not itself imply a risk factor for cardiac mortality or morbidity, and in a healthy population it should be regarded as an incidental ECG finding.¹⁶ The prognosis of LAF block is primarily related to the underlying heart disease. LAF block in the setting of acute MI is probably associated with increased mortality.

As noted, LPF block is a rare finding and rather nonspecific for cardiac disease. LPF block is almost always associated with RBBB, in that LPF block and RBBB share cause, pathogenesis, and prognosis. The combination of LPF block and RBBB in acute MI is associated with a high mortality rate (80% to 87%) during the first weeks after the coronary event. Similarly, the risk of progression toward complete AV block (a form of trifascicular block) is also considerable (42%), and approximately 75% of these patients die of pump failure.¹⁶

Electrocardiographic Features

Bundle Branch Block

The ECG criteria for different types of fascicular blocks and BBB are listed in Table 10-1. The ECG pattern of BBB can represent complete block or conduction delay (relative to the other fascicles) that produces asynchronous ventricular activation without necessarily implying complete failure of conduction in the diseased fascicle. Therefore, an ECG pattern of complete BBB can have varying degrees or alternate with contralateral complete BBB pattern, phenomena explained by delay, rather than complete block, as the underlying pathophysiologic feature of the ECG pattern.^3,^20,²⁷

TABLE 10-1 ECG Criteria for Fascicular and Bundle Branch Block

Complete Right Bundle Branch Block

• QRS duration ≥120 msec in adults

• Broad, notched secondary R waves (rsr′, rsR′, or rSR′ patterns) in right precordial leads (V₁ and V₂); R′ or r′ deflection usually wider than the initial R wave; in a minority of patients, a wide and often notched R wave pattern may be seen in lead V₁ and/or V₂

• Wide, deep S waves (qRS pattern) of greater duration than R wave or greater than 40 msec in leads I and V₆

• Normal R wave peak time in leads V₅ and V₆ but >50 msec in lead V₁

• Of the foregoing criteria, the first three should be present to make the diagnosis; when a pure dominant R wave with or without a notch is present in V₁, criterion 4 should be satisfied

Complete Left Bundle Branch Block

• QRS duration ≥120 msec in adults

• Broad notched or slurred R wave in leads I, aVL, V₅, and V₆ and an occasional RS pattern in V₅ and V₆ attributed to displaced transition of QRS complex

• Absent q waves in leads I, V₅, and V₆, but in the lead aVL, a narrow q wave may be present in the absence of myocardial disease

• R wave peak time >60 msec in leads V₅ and V₆ but normal in leads V₁, V₂, and V₃, when small initial r waves can be discerned in the above leads

• ST and T waves usually opposite in direction to QRS

• Positive T wave in leads with upright QRS may be normal (positive concordance)

• Depressed ST segment and/or negative T wave in leads with negative QRS (negative concordance) are abnormal

• Appearance of LBBB may change the mean QRS axis in the frontal plane to the right, to the left, or superiorly, in some cases in a rate-dependent manner

Left Anterior Fascicular Block

• Frontal plane axis between −45 and −90 degrees

• qR pattern in lead aVL

• R wave peak time in lead aVL ≥45 msec

• QRS duration <120 msec

Left Posterior Fascicular Block

• Frontal plane axis between 90 and 180 degrees in adults

• rS pattern in leads I and aVL

• qR pattern in leads III and aVF

• QRS duration <120 msec

BBB leads to prolongation of the QRS duration and sometimes to alterations in the QRS vector. The degree of prolongation of the QRS duration depends on the severity of the impairment. With complete BBB, the QRS is 120 milliseconds or longer in duration; with incomplete BBB, the QRS duration is 100 to 120 milliseconds. The QRS vector in BBB is generally oriented in the direction of the myocardial region in which depolarization is delayed.¹

BBB is characteristically associated with secondary repolarization (ST-T) abnormalities. The T wave is typically opposite in polarity to the last deflection of the QRS, a discordance that is caused by the altered sequence of repolarization that occurs secondary to altered depolarization.

Right Bundle Branch Block

Development of RBBB alters the activation sequence of the RV but not the LV. Because the LB is not affected, the initial septal activation (the initial 30 milliseconds of the QRS complex), which depends on the LB, remains normal, occurring from left to right, and results in septal q waves in leads I, aVL, and V₆ and r waves in leads V₁, V₂, and aVR.²⁶ Thus, the Q wave of a prior MI remains unchanged. Septal activation is followed by LV activation (within the subsequent 40 to 60 milliseconds), occurring over the LB in a leftward and posterior vector and resulting in R waves in the leftward leads (I, aVL, and V₆), as well as s (or S) waves in the anterior precordial leads (V₁ and V₂). This appearance is usually similar to that in normal subjects.

The asynchronous depolarization caused by RBBB is primarily manifested in the later portion of the QRS, at 80 milliseconds and beyond. During this time, RV activation spreads slowly by conduction through working muscle fibers rather than the specialized Purkinje system, and it occurs predominantly after activation of the LV has completed. The forces generated by the late, unopposed RV free wall activation result in a terminal rightward and anterior positivity, manifesting as a second positive deflection that can be small (r′) or large (R′) in the anterior precordial leads (V₁ and V₂) and S waves in the leftward leads (I, aVL, and V₆; Fig. 10-9).^3,²⁶ The QRS axis is unaffected by RBBB; left or right axis deviation can indicate concurrent LAF or LPF block, respectively (see Fig. 10-9).

FIGURE 10-9 Surface ECG of incomplete and complete right bundle branch block (RBBB) and bifascicular block. LAF = left anterior fascicle.

RBBB also results in an abnormality of ventricular repolarization of the RV myocardium. Thus, there are often secondary ST segment and T wave changes present in the right precordial leads. The ST segment change is usually small and, when present, is discordant (i.e., has an axis in the opposite direction) to the terminal mean QRS spatial vector. The T wave also tends to be discordant to the terminal conduction disturbance, resulting in inverted T waves in the right precordial leads (where there is a terminal R′ wave) and upright T waves in the left precordial leads (where there is a terminal S wave).

The time interval necessary for full depolarization of the ventricular free wall (from the endocardium to the epicardium) beneath any given ECG electrode corresponds to the interval from the beginning of the QRS complex to the time of initial downstroke of the R wave after it has peaked (or to the time of initial upstroke of the S wave after it has reached its nadir). This interval is termed R wave peak time (in preference to the term intrinsicoid deflection). In the right precordial leads, the upper limit of normal for R wave peak time is 35 milliseconds, whereas in the left precordial leads, it is 45 milliseconds. In RBBB, the R wave peak time is delayed in the right precordial leads (more than 50 milliseconds).¹

Fascicular Block

Hemiblocks in the LB system affect the LAF, LPF, or LMF. Fascicular block generally does not substantially prolong QRS duration, but alters only the sequence of LV activation. The primary ECG change is a shift in the frontal plane QRS axis because the conduction disturbance primarily involves the early phases of activation. The QRS duration is usually less than 100 milliseconds (unless complicated by BBB or hypertrophy), although some investigators allow a QRS duration of up to 120 milliseconds, or 20 milliseconds higher than the previous baseline.¹⁶^–¹⁸

Left Anterior Fascicular Block

The LAF normally initiates activation in the upper part of the septum, the anterolateral LV free wall, and the left anterior papillary muscle. Delayed activation of these regions secondary to damage to the LAF causes unopposed activation wavefronts by the LPF and LMF early during the QRS complex and unopposed anterosuperior forces late during ventricular activation. All these changes occur with a QRS that widens no more than 20 milliseconds in pure and uncomplicated LAF block.¹⁶

As a consequence, the initial (first 20 milliseconds) QRS vector is normal in time, but it has an abnormal direction. Rather than proceeding superiorly and to the left, the QRS vector depicts an inferior and rightward shift in the frontal plane (frontal plane axis more than +120 degrees) that produces a small, sharp r wave in the inferior leads (II, III, and aVF) and a small q wave in leads I, aVL, V₅, and V₆. Additionally, the inferior and rightward shift in the initial QRS forces can occasionally result in a small, sharp q wave in leads V₂ and V₃ (mimicking old anteroseptal infarction patterns) when the electrodes are placed at the normal level and, in almost all cases, when they are placed in a higher position.¹⁶

During the midportion of the QRS, the main forces of LV activation are oriented superiorly and to the left (frontal plane axis more than –45 or –60 degrees), with a wide-open counterclockwise-rotated loop in the frontal plane caused by the delayed activation of the high lateral wall, which is normally activated by the LAF. This results in deep S waves in leads II, III, and aVF (S wave deeper in lead III than in lead II), and R waves in aVR and aVL. The net effect is an rS pattern in leads II, III, and aVF and a qR or R pattern in leads I, aVL, V₅, and V₆. Additionally, as a result of the superiorly directed forces, deeper S waves are recorded in leads V₅ and V₆; these waves tend to disappear when the electrodes are placed above the normal level and are deeper when the electrodes are placed below the normal level.^1,¹⁶^–¹⁸

The ECG pattern of LAF block can simulate LV hypertrophy in limb leads I and aVL. Conversely, it can conceal signs of LV hypertrophy in the left precordial leads, and it can also hide signs of inferior ischemia.

Left Posterior Fascicular Block

The early unopposed activation of the anterolateral wall of the LV by the normally conducting LAF and LMF causes the initial forces to be oriented superiorly and to the left, producing initial small r waves in leads I, aVL, V₁, and V₆ and small q waves in leads II, III, and aVF. However, the main and terminal forces of the QRS are directed posteriorly, inferiorly, and to the right with a wide-open clockwise-rotated loop, because of the late unopposed activation of the areas normally activated by the LPF (the inferoposterior LV free wall). This is responsible for the characteristic rightward frontal plane axis of +120 to +180 degrees. As a result, there is a qR morphology in leads II, III, and aVF and an rS morphology in leads I and aVL. In fact, the ECG pattern of LPF block is the exact mirror image of LAF block in the standard and unipolar leads. LPF block is almost always associated with RBBB. Isolated LPF block is extremely rare, and a firm diagnosis requires that other causes of right axis deviation have been excluded.^1,¹⁶

Left Median Fascicular Block

The ECG pattern seen with LMF block is probably determined by the differences in the sites of insertion of the LMF, LAF, and LPF. Functional block in the LMF can lead to the apparent loss of anterior forces and can result in the transient development of q waves in leads V₁ and V₂, which normally have a positive initial deflection caused by septal depolarization. These changes are similar to those that occur in septal MI. On the other hand, prominent R waves are seen in the right precordial leads when LMF block leads to a gain of anterior forces. These changes are similar to those caused by true posterior MI. The prominence of the R waves may be increased when LMF block occurs in association with RBBB.

Other Types

Nonspecific Intraventricular Conduction Defects

A nonspecific IVCD is the result of diffuse slowing of impulse conduction involving the entire HPS that causes a generalized and uniform delay in activation of the ventricular myocardium. A nonspecific IVCD is diagnosed in the presence of a QRS duration longer than 120 milliseconds and a QRS morphology that does not resemble either LBBB or RBBB and may even resemble the normal QRS complex. Nonspecific IVCDs can be classified as LV or RV IVCD, depending on the site of delayed R wave peak time and the direction of the terminal forces.

Bifascicular Blocks

Bifascicular block refers to different combinations of fascicular block and BBB. Examples of bifascicular block include RBBB with LAF block (most common; see Fig. 10-9), RBBB with LPF block, and LAF block plus LPF block (which manifests as LBBB).

Trifascicular Blocks

Trifascicular block involves conduction delay in the RB and either the main LB or both the LAF and LPF. The resulting ECG pattern depends on the relative degree of conduction delay in the affected fascicles. Ventricular activation starts at the insertion site of the fastest conducting fascicle, with subsequent spread of activation from that site to the remainder of the ventricles. ECG documentation of trifascicular block during 1:1 AV conduction is rare. ECG manifestations of trifascicular block include the following: (1) complete AV block with a slow ventricular escape rhythm with a wide, bizarre QRS; (2) alternating RBBB and LBBB; and (3) fixed RBBB with alternating LAF and LPF block.

The combination of bifascicular block (RBBB plus LAF block, RBBB plus LPF block, or LBBB) with first-degree AV block on the surface ECG (Fig. 10-11) cannot be considered as trifascicular block because the site of AV block can be in the AVN or HPS, and such a pattern can reflect slow conduction in the AVN with concomitant bifascicular block. In such circumstances, the PR interval on the ECG does not appear to be helpful in selecting those individuals with a prolonged His bundle–ventricular (HV) interval because a normal PR interval can easily conceal a significantly prolonged HV interval and a prolonged PR interval can be caused by a prolonged atrial–His bundle (AH) interval. However, two criteria can be of value: (1) a short PR interval (less than 160 milliseconds) makes a markedly prolonged HV interval (i.e., more than 100 milliseconds) unlikely, and (2) a markedly prolonged PR interval (more than 300 milliseconds) almost always indicates that at least some of the abnormality, if not all, is caused by AVN conduction.

FIGURE 10-11 Surface ECG of sinus rhythm with first-degree atrioventricular (AV) block and bifascicular block (right bundle branch block and left anterior fascicular block). The PR interval is markedly prolonged (480 msec), which suggests that at least some of the AV conduction delay, if not all, occurs in the atrioventricular node and is not exclusively in the His-Purkinje system. A diagnosis of trifascicular block cannot be made solely based on this ECG.

Alternating Bundle Branch Block

Alternating RBBB and LBBB is manifested by QRS complexes with LBBB morphology coexisting with complexes with RBBB morphology (Fig. 10-12). Often, every other complex is an RBBB or LBBB. Spontaneous alternating BBB, especially when associated with a change in the PR interval, represents the most common ominous sign for progression to AV block (Fig. 10-13). Beat-to-beat alternation is the most concerning, whereas a change in BBB noted on different days is less ominous.

FIGURE 10-12 Alternating bundle branch block: surface ECG precordial leads during atrial fibrillation. QRS complexes with left bundle branch block configuration are observed mostly at long cycle lengths, and complexes with right bundle branch block configuration are observed mostly at shorter cycle lengths.

FIGURE 10-13 Alternating bundle branch block. Continuous Holter three-lead recordings demonstrating sinus rhythm with first-degree atrioventricular (AV) block and right bundle branch block (RBBB) alternating with left bundle branch block (LBBB). Arrows denote sinus P waves, which are commonly masked by the preceding T waves. Note that sinus beats conducted with RBBB are associated with shorter PR intervals compared with those conducted with LBBB. This suggests that conduction delay in the right bundle branch (RB) is more severe than that in the left bundle branch (LB); hence, the PR interval is longer when conduction occurs only over the RB (i.e., with LBBB pattern), but shorter when conduction occurs only over the LB (i.e., with RBBB pattern). The first sinus P waves to conduct with LBBB (blue arrows) are associated with the longest PR intervals, with subsequent P waves conducting with somewhat shorter PR intervals, resulting in gross irregularity of the QRS complexes despite relatively regular sinus P waves. These findings are suggestive of severe His-Purkinje system disease and high risk for complete AV block.

This phenomenon implies instability of the HPS and a disease process involving the HB or bundle branches. In most patients with diffuse HPS disease, delay or block in one of the bundle branches consistently predominates, and alternating BBB is uncommon. The HV interval in alternating BBB is almost universally prolonged and typically varies with the change in BBB. This group has the highest incidence of HV interval exceeding 100 milliseconds.³

Alternating BBB is infrequent but is associated with the most predictable progression to complete AV block (70% of patients develop high-grade AV block within weeks of diagnosis). As a rule, AV conduction delay or block can be assumed to be caused by BBB only in the presence of an alternating or intermittent RBBB and LBBB with a changing PR interval. Not infrequently, the bilateral BBB is caused by acceleration-dependent aberration.

Intermittent Bundle Branch Block

Intermittent BBB, either right or left, is diagnosed on the surface ECG when there are occasional QRS complexes with RBBB or LBBB morphology, interspersed with QRS complexes that have a normal morphology. Most often, the intermittent BBB is rate-related; thus, the R-R intervals of the QRS complexes manifesting the BBB are shorter when compared with the intervals of the normal QRS complexes. In other cases, there is no rate-related change in the QRS intervals, but the occurrence of the BBB is a random or sporadic event.

Electrophysiological Testing

Baseline Intervals

His Bundle–Ventricular Interval

The use of multipolar catheters to record distal, middle, and proximal HB potentials can help localize the site of conduction delay or block within the HB. The value of prolonged HV interval in predicting the risk of AV block is controversial. Studies have shown that an HV interval longer than 70 milliseconds predicts a higher risk of AV block, especially in symptomatic patients. The risk of AV block, however, even in the high-risk group, approaches at most only 6% per year. An HV interval exceeding 100 milliseconds identifies a group of patients at a very high risk of AV block (25% over 22 months).³

In the presence of RBBB with or without additional fascicular block, the HV interval should be normal as long as conduction is unimpaired in the remaining fascicle (Fig. 10-14). However, 50% of patients with RBBB plus LAF block and 75% of those with LBBB have a prolonged HV interval; thus, a prolonged HV interval itself is nonspecific as a predictor of AV block.

FIGURE 10-14 Catheter-induced His bundle–ventricular (HV) interval prolongation. Right bundle branch block with a normal HV is present at baseline (at left). Prolongation of the HV interval is observed after introducing a mapping catheter into the left ventricle that traumatized a portion of the His-Purkinje system. The QRS is also slightly different from baseline. His_prox = proximal His bundle; RVA = right ventricular apex.

In the presence of LBBB, and in the absence of a change in the HB-RB and HB-RV intervals, the HV interval may be slightly prolonged because the earliest site of depolarization on the left side of the septum via the LB precedes activation of the RB by 5 to 15 milliseconds. Therefore, an HV interval of up to 60 milliseconds in the presence of LBBB should be considered normal and does not by itself indicate associated RB or HB disease (Fig. 10-15).³

FIGURE 10-15 Catheter-induced His bundle–ventricular (HV) interval prolongation. Left bundle branch block (LBBB) and HV interval at the upper limit of normal are observed at baseline (at left). Manipulation of the His bundle recording catheter results in prolongation of the HV interval. This is not an artifact of the location of His recording, because the PR interval is also prolonged. There is no change in the QRS complex (LBBB pattern), suggesting trauma to the right bundle (RB) or His fibers destined to the RB. His_dist = distal His bundle; His_prox = proximal His bundle; HRA = high right atrium.

Catheter manipulation in the LV or RV can inadvertently produce prolongation of the HV interval and varying degrees of AV block or BBB, or both, usually temporarily (see Figs. 10-14 and 10-15). Complete heart block can occur during right-sided heart catheterization in a patient with preexisting LBBB or during LV catheterization (LV angiography or ablation procedures) in a patient with preexisting RBBB. Among patients with chronic LBBB, the presence of an r wave of more than 1 mm in lead V₁ appears to identify a subgroup at lower risk for complete AV block in response to catheter trauma to the RB. This ECG sign likely suggests intact conduction over the septal fibers of the LBB.²⁹

Localization of the Site of Block in Right Bundle Branch Block

As noted, a chronic RBBB pattern can result from three levels of conduction delay in the RV: proximal RBBB, distal RBBB at the level of the moderator band, and terminal RBBB involving the distal conduction system of the RB or, more likely, the muscle itself.

With proximal RBBB, loss of RB potential occurs where it is typically recorded. Activation at these RV septal sites is via transseptal spread following LV activation. The transseptal activation begins at the apex and then sequentially activates the midanterior wall and base of the RV. The midseptum and apical septum are activated at least 30 milliseconds after the onset of the QRS.

In the setting of distal RBBB, activation of the HB and proximal RB is normal. RB potentials persist at the base of the moderator band and are absent at the midanterior wall (where the moderator band normally inserts). The apical and midseptum are normally activated, but activation of the free wall at the level of the moderator band is delayed, as is the subsequent activation of the RV outflow tract (RVOT) and the remaining RV.

With terminal RBBB, activation along the HB and RB remains normal up the Purkinje-myocardium junction. Activation of the midanterior wall also remains normal, and only the RVOT shows delayed activation.

Measurement of activation times at different areas of the RV has been used to assess conduction properties of the RB indirectly. In the presence of RBBB, measuring the HV interval, HB–proximal RB interval, ventricular-RV (V-RV) apex interval (from onset of the surface QRS to RV apical local activation), and V-RVOT interval can help localize the site of block (Fig. 10-16). In the presence of RBBB, a normal HV interval and a V-RV apex interval of less than 30 milliseconds suggest terminal RBBB. On the other hand, a normal HV interval and a V-RV apex interval of less than 30 milliseconds, with delayed activation in the anterior wall (prolonged V-RVOT interval), suggest distal RBBB. With proximal RBBB, the V-RV apex interval is longer than 30 milliseconds.

FIGURE 10-16 Localization of the site of block in right bundle branch block (RBBB). Two complexes from different patients are shown, with proximal (left panel) and distal (right panel) RBBB, distinguished by the interval from QRS onset to right ventricle apical (RVA) recording. His_dist = distal His bundle; His_mid = middle His bundle; His_prox = proximal His bundle; RVA = right ventricular apex; RVOT = right ventricular outflow tract.

Localization of the Site of Block in Left Bundle Branch Block

Measurement of activation times at different areas of the LV to assess conduction properties of the LB, LAF, and LPF indirectly has many limitations. It is not as feasible as that used for evaluation of the RB, mainly because the LB does not divide into two discrete fascicles, but it rapidly fans out over the entire LV. Therefore, for practical reasons, clinical evaluation has primarily focused on the surface ECG pattern and the HV interval.

In chronic LBBB, RV activation occurs relatively earlier in the QRS. A discrete LV breakthrough site is absent, in contrast to normal findings, in which two or three breakthrough sites may be seen. Moreover, transseptal conduction is slow. In LBBB with normal axis, the latest LV site activated is the AV sulcus, as it is with normal conduction.

A common observation in patients with LBBB is a delay in transseptal activation. The pattern in which the LV is activated initially (i.e., the site of breakthrough), as well as the remainder of the LV endocardium and transmural activation, depends critically on the nature of the underlying heart disease; the bizarreness of the QRS width and morphology is more a reflection of the underlying LV disease than of the primary conduction disturbance.³ Patients with normal hearts and those with cardiomyopathy appear to have an intact distal conducting system and, hence, early engagement and rapid spread throughout the rest of the intramural myocardium. In patients with large infarcts, the bulk of their distal specialized conducting system has been destroyed; consequently, endocardial activation occurs via muscle-to-muscle conduction and thus is much slower.

Diagnostic Maneuvers

Atrial Extra stimulation

Atrial premature stimulation helps determine the ERP of HPS. Normally, the HPS ERP is 450 milliseconds or less and it decreases with decreasing pacing drive CL. Because the AVN functional refractory period usually exceeds the HPS ERP, it can be difficult to evaluate the HPS ERP. Administration of atropine can help decrease AVN refractoriness and allow impulses to reach the HPS earlier so that HPS ERP can be determined. A grossly prolonged HPS ERP or a paradoxical increase in the HPS ERP in response to shortening of the pacing drive CL indicates an abnormal HPS and predicts a higher risk for progression to AV block.

Atrial Pacing

During incremental-rate atrial pacing, normal shortening of the HPS refractoriness at decreased pacing CLs facilitates 1:1 AV conduction. The development of second- or third-degree AV block within the HPS (in the absence of changing AH intervals) at pacing CLs longer than 400 milliseconds is abnormal and suggests a high risk (50%) for progression to high-grade AV block (Fig. 10-17).

FIGURE 10-17 His-Purkinje system (HPS) disease. The PR interval is borderline (210 milliseconds), and the His bundle–ventricular (HV) interval is prolonged (75 milliseconds) during normal sinus rhythm (at right). Atrial pacing at a cycle length of 330 milliseconds results in stressing the HPS and further prolonging the HV interval (108 milliseconds). Right bundle branch block also develops during atrial pacing, a finding suggesting slower conduction over the right bundle than the left bundle. His_dist = distal His bundle; His_prox = proximal His bundle; HRA = high right atrium; PCL = pacing cycle length.

His Bundle Pacing

Normalization of the QRS during HB pacing is suggestive of HB disease proximal to the pacing site (Fig. 10-18).

FIGURE 10-18 Left bundle branch block (LBBB) secondary to intra-Hisian disease. Left, ECG leads and intracardiac recordings in a patient with complete LBBB. Right, Pacing from the His bundle (HB) pacing normalizes the QRS complex, a finding suggesting that the LBBB is caused by HB disease proximal to the pacing site. His_dist = distal His bundle; His_prox = proximal His bundle; NSR = normal sinus rhythm; RVA = right ventricular apex.

Ventricular Pacing

Assessment of retrograde VH (or VA) conduction is not useful as an indicator of anterograde HPS reserve. Anterograde RBBB is usually proximal, whereas during RV stimulation, block usually occurs at the gate, which is at the distal HPS–myocardial junction. Bidirectional RBBB is characterized by His activation following local ventricular activation in the HB recording, caused by propagation from the RV pacing site across the interventricular septum to the LB and then to the HB, rather than the more direct retrograde route up the RB (Fig. 10-19).

FIGURE 10-19 Bidirectional right bundle branch block (RBBB). The sinus complex at left has proximal RBBB, indicated by a very prolonged ventricular–right ventricular apical (V-RVA) time (more than 100 milliseconds). RV pacing with the next two complexes show His activation following local ventricular activation in the His bundle (HB) recording, caused by propagation from the RV pacing site across the interventricular septum to the left bundle and then to the HB, rather than the more direct route retrogradely up the right bundle. This is consistent with both anterograde and retrograde proximal RBBB. His_dist = distal His bundle; His_prox = proximal His bundle; HRA = high right atrium; RVA = right ventricular apex.

Procainamide Challenge

The administration of drugs known to impair the HPS (e.g., procainamide) can unmask extraordinary sensitivity to the usual therapeutic doses of the drug, which can itself indicate poor HPS reserve. In normal persons and in most patients with a moderately increased HV interval (55 to 80 milliseconds), procainamide typically produces a 10% to 20% increase in the HV interval. Abnormal HV interval responses to procainamide representing evidence of a higher risk for infra-Hisian AV block include any of the following: (1) doubling of the HV interval, (2) prolongation of the HV interval to more than 100 milliseconds, or (3) second- or third-degree infra-Hisian block.

Role of Electrophysiological Testing

The guidelines for pacing in chronic BBB are listed in Table 10-2. EP testing is used to obtain information that could predict which patients are at risk for syncope, AV block, or sudden cardiac death (Table 10-3).^3,¹⁸ Complete EP testing with programmed atrial and ventricular stimulation is necessary in patients with syncope and BBB because VT can be induced in 30% to 50% of such patients. Pacemaker therapy clearly can help prevent syncope in those patients in whom that event most likely was caused by transient bradyarrhythmia, but it has not been shown to prevent sudden cardiac death or reduce cardiac mortality.^3,^18,³⁰

TABLE 10-2 Guidelines for Permanent Pacing in Chronic Bifascicular and Trifascicular Block

Class I

• Advanced second-degree AV block or intermittent third-degree AV block

• Type 2 second-degree AV block

• Alternating bundle branch block

Class IIa

• Syncope not demonstrated to result from AV block, when other likely causes have been excluded, especially ventricular tachycardia

• Incidental finding at EP study of HV interval ≥100 msec

• Incidental finding at EP study of pacing-induced block below the bundle of His that is not physiological

Class IIb

• Neuromuscular diseases, with or without symptoms

Class III

• Fascicular block without AV block or symptoms

• Fascicular block with first-degree AV block without symptoms

AV = atrioventricular; EP = electrophysiological; HV = His bundle–ventricular.

TABLE 10-3 Role of Electrophysiological Testing in Patients with Bundle Branch Block

EP Indicators of HPS Disease

• HV interval >55 msec

• Infra-Hisian block at atrial pacing CL ≥400 msec

• HPS ERP ≥450 msec

• HPS ERPs inversely related to pacing CLs

• Abnormal response to procainamide: prolongation of the HV interval by 100% or to >100 msec, or second- or third-degree infra-Hisian AV block

EP Indicators of High Risk for AV Block in Patients with IVCD

• HV interval >100 msec

• Infra-Hisian AV block or HV interval prolongation at atrial pacing CL >400 msec

• HPS-ERPs inversely related to pacing CLs

• Infra-Hisian AV block or doubling of the HV interval following procainamide in patients with neurological symptoms compatible with bradyarrhythmia

Recommendations for Pacing in Patients with IVCD

• LBBB, RBBB, IVCD, RBBB + LAF block, or RBBB + LPF block associated with the following: HV interval >100 msec or HV interval = 60-99 msec in the presence of unexplained syncope or presyncope

• Infra-Hisian block at atrial pacing CL ≥400 msec (regardless of HV interval or presence of symptoms)

• Alternating BBB (regardless of HV interval or presence of symptoms)

AV = atrioventricular; BBB = bundle branch block; CL = cycle length; EP = electrophysiological; ERP = effective refractory period; HPS = His-Purkinje system; HV = His bundle–ventricular; IVCD = intraventricular conduction defect; LAF = left anterior fascicle; LBBB = left bundle branch block; LPF = left posterior fascicle; RBBB = right bundle branch block.

References

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