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Typical atrioventricular bypass tracts
Types of bypass tracts
Bypass tracts (BTs) are remnants of the atrioventricular (AV) connections caused by incomplete embryological development of the AV annuli and failure of the fibrous separation between the atria and ventricles. There are several types of BTs, according to the structures they connect, including atrioventricular, atrionodal, atrio-Hisian, atriofascicular, fasciculoventricular, and nodofascicular BTs.
Atrioventricular bypass tracts
Atrioventricular BTs are strands of working myocardial cells connecting atrial and ventricular myocardium across the electrically insulating fibrofatty tissues of the AV junction, bypassing the atrioventricular node (AVN)-His-Purkinje system (HPS). In the older literature, these BTs were called Kent bundles, although incorrectly (Kent described AVN-like tissue in the right atrial [RA] free wall that did not connect to the ventricle). Thus, the use of the term bundle of Kent should be discouraged.
Atrionodal bypass tracts
Atrionodal BTs connect the atrium to the distal or compact AVN. They have been called James fibers and are of uncertain physiological significance.
Atrio-Hisian bypass tracts
Atrio-Hisian BTs connect the atrium to the His bundle (HB); these BTs are rare.
Atypical bypass tracts
The term “atypical BTs” is used here to describe variants of BTs that connect the atrium (atriofascicular BTs), AVN (nodofascicular and nodoventricular BTs), or HB (fasciculoventricular BTs) to distal Purkinje fibers or ventricular myocardium. Additionally, this term encompasses slowly conducting short atrioventricular BTs and long atrioventricular BTs. Although many of those variants of BTs are sometimes referred to as “Mahaim fibers,” it is more appropriate to describe them based on their anatomic connections.
Types of preexcitation syndromes
Several patterns of preexcitation can occur, depending on the anatomy of the bypass tract (BT) and the direction in which impulses are conducted. Conduction from the atria to the ventricles normally occurs via the AVN-HPS. Patients with preexcitation have an additional or alternative pathway, the BT, which directly connects the atria and ventricles and bypasses the AVN. The term syndrome is used when the anatomical variant is responsible for tachycardia.
Wolff-Parkinson-White syndrome
In the Wolff-Parkinson-White (WPW) syndrome, AV conduction occurs, partially or entirely, through an AV BT, which results in earlier activation (preexcitation) of the ventricles than if the impulse had traveled through the AVN.
Concealed bypass tracts
Concealed AV BTs refer to AV BTs that do not manifest anterograde conduction and therefore do not result in ventricular preexcitation. Because they do not result in alteration of the QRS complex in the ECG, they cannot be detected by inspection of the surface ECG; they are called concealed . However, the concealed BT can conduct in a retrograde fashion, thereby creating a reentrant circuit with impulses traveling from the atrium to the AVN, HPS, and ventricle, and then back to the atrium via the BT.
Lown-Ganong-Levine syndrome
In the Lown-Ganong-Levine (LGL) syndrome, preexcitation purportedly occurs via atrio-Hisian BTs or, alternatively, no BT is present and enhanced AVN conduction accounts for the ECG findings. The net effect is a short PR interval without delta wave or QRS prolongation. It is important to stress, however, that LGL is not a recognized syndrome with an anatomical basis but only an ECG description, and the use of the term should be discouraged.
Mahaim variant of preexcitation
The so-called “Mahaim” variant of preexcitation does not typically result in a delta wave, because these pathways, which usually terminate in the conducting system or in the ventricular myocardium close to the conducting system, conduct slowly, and the AVN-HPS has adequate time to activate most of the ventricular muscle mass.
It is worth noting that some of the older literature refers to eponymous pathways that were originally anatomically described, with subsequent attempts made to correlate these structures with physiologic findings. With more recent data from intracardiac recordings, many of these correlations have been shown to be incorrect and thus the use of the eponyms adds confusion to discussions about them. For instance, the pathways described by Kent (AV nodal-like tissue at the free-wall AV valve annulus) more resemble atriofascicular fibers than they do typical atrioventricular fibers, and atriofascicular fibers in turn possess the physiology initially (incorrectly) attributed to Mahaim fibers. Table 22.1 displays some of the terminology; it is evident why use of eponymous terms is discouraged.
TABLE 22.1
Historic Description of Different Bypass Tracts
| EPONYMOUS PATHWAY | ANATOMIC DESCRIPTION | PROPOSED PHYSIOLOGIC ROLE/SYNDROME | ACTUAL PHYSIOLOGIC ROLE |
|---|---|---|---|
| Kent bundle | Clusters of nodal cells at nonseptal AV junction | Anomalous AV connection/WPW syndrome | None (possibly atriofascicular pathways) |
| James fiber | Atrio-compact nodal connection | LGL syndrome | Enhanced AVN conduction |
| Brechenmacher fiber | Atrio-Hisian connection | LGL syndrome | Enhanced AVN conduction |
| Mahaim fiber | Connection from compact AVN to ventricle | Septal WPW pathways with decremental conduction | Probably none |
| Paladino fiber | Connection from proximal AVN to ventricle | Septal WPW pathways with decremental conduction | Probably none |
AV , atrioventricular; AVN , AV node; LGL , Lown-Ganong-Levine;. WPW , Wolff-Parkinson-White.
Pathophysiology
Wolff-Parkinson-White syndrome
WPW pattern refers to the constellation of ECG abnormalities related to the presence of a manifest AV BT (i.e., ventricular preexcitation: short PR interval, delta wave, wide QRS complex) in asymptomatic patients ( Fig. 22.1 ). WPW syndrome refers to the combination of ventricular preexcitation and either a documented tachyarrhythmia or symptoms of a tachyarrhythmia.
Because the AV BT typically conducts faster than the AVN, the onset of ventricular activation is earlier than if depolarization occurred only via the AVN, resulting in a shortened PR (P-delta) interval. Additionally, because the BT exhibits practically nondecremental conduction, the early ventricular activation (i.e., P-delta interval) remains almost constant at all atrial rates ( Fig. 22.2 ).
Preexcited intraventricular conduction in WPW propagates from the insertion point of the AV BT in the ventricular myocardium via direct muscle-to-muscle conduction. This process is inherently slower than ventricular depolarization resulting from rapid HPS conduction. Thus, although the initial excitation of the ventricles (via the BT) occurs earlier, it is followed by slower activation of the ventricular myocardium than occurs normally. The net effect is that the QRS complex consists of fusion between the early ventricular activation caused by preexcitation with the later ventricular activation resulting from impulse propagation through the AVN/HPS to the ventricles. The initial part of ventricular activation resulting in the upstroke of the QRS complex is slurred because of slow muscle-to-muscle conduction; this is termed a delta wave .
Depending on the relative contribution from ventricular activation by the normal AVN/HPS versus the manifest BT, a variable degree of preexcitation occurs. The more rapid the conduction along the BT in relation to the AVN, the greater the amount of myocardium depolarized via the BT, resulting in a more prominent or wider delta wave and increasing prolongation of the QRS complex duration.
Atrioventricular bypass tracts
The cardiac skeleton consists of four rings of dense connective tissue that surround the AV canals (mitral and tricuspid) and extend to the origins of the aorta and the pulmonary trunk, providing structure and support for the heart as well as electrical isolation between the atria and the ventricles. The aortic valve occupies the central position with the other valve rings attached to it. The right fibrous trigone is formed by the triangular formation between the aortic valve and the medial parts of the tricuspid and mitral valves, and it represents the largest thickening and strongest portion of the cardiac skeleton. Together with the membranous septum, the right fibrous trigone constitutes the central fibrous body ( Fig. 22.3 ).
The AV junctions are the areas of the heart where the atrial musculature connects to the annuli of the mitral and tricuspid valves. The AVN-HPS, which lies in the septal component of the AV junction, is the only normal electrical connection between the atria and the ventricles. The fibrous skeleton and AV valvular annuli (annulus fibrosus) act as insulators to prevent electrical impulses from conducting to the ventricles by any other route. The main function of the AVN is modulation of atrial impulse transmission to the ventricles; it receives, delays, and conveys atrial impulses to the ventricles, thereby coordinating atrial and ventricular contractions.
AV BTs are aberrant muscle bundles that connect the atria to the ventricles outside of the normal AV conduction system. AV BTs are found most often in the parietal AV junctional areas, including the paraseptal areas. They breach the insulation provided by the fibrofatty tissues of the AV groove (sulcus tissue) and the hinge lines (fibrous annulus) of the valves. They are rarely found in the area of fibrous continuity between the aortic and mitral valves because in this area there is usually a wide gap between the atrial myocardium and ventricular myocardium to accommodate the aortic outflow tract ( Fig. 22.3 ). The remainder of the AV groove may be divided into segments consisting of the left free wall, right free wall, and posteroseptal and anteroseptal spaces ( Fig. 22.4 ). The distribution of BTs within these regions is not homogeneous—46% to 60% of BTs are found within the left free wall space; 25% are within the posteroseptal space; 13% to 21% of BTs are within the right free wall space; up to 7% are within the right superoparaseptal (formerly called anteroseptal) space; and less than 5% are located in the midseptum.
AV BTs are usually short and very thin muscular strands (typically 5–10 mm in length, with a maximal diameter of 0.1–7 mm) but can occasionally exist as broad bands of tissue. They can course through the AV groove at variable depth ranging from subepicardial to subendocardial locations. The AV BT can run in an oblique course rather than perpendicular to the transverse plane of the AV groove. As a result, the fibers can have an atrial insertion point that is transversely from less than one to several centimeters removed from the point of ventricular attachment. Some posteroseptal pathways insert into coronary sinus (CS) musculature rather than atrial myocardium and can be associated with the coronary venous system or diverticula from a CS branch vein.
Multiple AV BTs occur in 5% to 12% of patients. BTs are defined as multiple when they are separated by more than 1 to 3 cm at the AV junction. The most common combination of widely spaced multiple BTs is posteroseptal and right free-wall BTs. The incidence of multiple BTs is particularly high in patients with antidromic AV reentrant tachycardia (AVRT) (50%–75%), patients in whom atrial fibrillation (AF) resulted in ventricular fibrillation (VF), and patients with Ebstein anomaly.
Although the majority (approximately 60%) of AV BTs conduct both anterogradely and retrogradely (i.e., bidirectionally, 
), some AV BTs are capable of propagating impulses in only one direction. BTs that conduct only in the anterograde direction are uncommon (<5%), often cross the right AV groove, and frequently possess decremental conduction properties. On the other hand, BTs that conduct only in the retrograde direction occur more frequently, accounting for 17% to 37% of all BTs. When the BT is capable of anterograde conduction, ventricular preexcitation is usually evident during sinus rhythm, and the BT is referred to as manifest. BTs capable of retrograde-only conduction are referred to as concealed.
Because working myocardial cells make up the vast majority of AV BTs, conduction over those BTs is mediated by the rapid inward sodium current, similar to normal His-Purkinje tissue and working atrial and ventricular myocardium. Therefore, AV BTs have rather constant anterograde and retrograde conduction at all rates until the refractory period is reached, at which time conduction is completely blocked. Thus, conduction over AV BTs usually behaves in an all-or-none fashion (i.e., nondecremental conduction), although, with careful measurement, there is often a small amount of prolongation of conduction intervals (10–15 milliseconds) over the BT when the pacing rate is just below that at which block occurs. In contrast, the AVN, which depends on the slow inward calcium current for generation and propagation of its action potential, exhibits what has been called decremental conduction , whereby conduction time of the impulse propagating through the AVN prolongs as the atrial cycle length (CL) shortens. Thus, AV conduction is more rapid through the AV BT than through the AVN, a difference that is exaggerated at faster heart rates. This difference has potentially great clinical importance. A primary function of the AVN is to limit the number of impulses conducted from the atria to the ventricles, which is particularly important during fast atrial rates (e.g., AF or atrial flutter [AFL]) when only a fraction of impulses are conducted to the ventricles, whereas the remainder are blocked in the AVN. However, in the presence of nondecrementally conducting AV BTs with short refractory periods, these arrhythmias can lead to very fast ventricular rates that can degenerate into VF.
Atrioventricular reentry
AVRT is a macroreentrant tachycardia with an anatomically defined circuit that consists of two distinct pathways, the normal AV conduction system and an AV BT, linked by common proximal (atrial) and distal (ventricular) tissues. If sufficient differences in conduction time and refractoriness exist between the normal conduction system and the BT, a properly timed premature impulse of atrial or ventricular origin can initiate reentry. AVRTs are the most common (80%) tachycardias associated with the WPW syndrome. AVRT is classified as orthodromic and antidromic according to the direction of conduction in the AVN-HPS ( Fig. 22.5 ). Orthodromic indicates normal direction (anterograde) of conduction over AVN-HPS during the AVRT.
Orthodromic atrioventricular reentrant tachycardia
In orthodromic AVRT, the AVN-HPS serves as the anterograde limb of the reentrant circuit (i.e., the pathway that conducts the impulse from the atria to the ventricles), whereas an AV BT serves as the retrograde limb ( Figs. 22.5 and 22.6 ). Approximately 50% of BTs participating in orthodromic AVRT are manifest (able to conduct bidirectionally) and 50% are concealed (able to conduct retrogradely only). Therefore, a WPW pattern may or may not be present on the surface ECG during sinus rhythm. When preexcitation is present, the delta wave seen during sinus rhythm is lost during orthodromic AVRT, because anterograde conduction during the tachycardia occurs over the normal AV conduction system, and not via the BT (i.e., the ventricle is not preexcited). Orthodromic AVRT accounts for approximately 90% to 95% of AVRT episodes in patients with a manifest BT and 20% to 30% of all paroxysmal supraventricular tachycardias (SVTs).
Antidromic atrioventricular reentrant tachycardia
Antidromic AVRT is a preexcited AVRT whereby an AV BT serves as the anterograde limb and the AVN-HPS as the retrograde limb of the reentrant circuit ( Figs. 22.5 and 22.6 ). Consequently, the QRS complex during antidromic AVRT is fully preexcited (i.e., the ventricles are activated totally by the BT with no contribution from the normal conduction system). The BT involved in the antidromic AVRT circuit must be capable of anterograde conduction and, therefore, preexcitation is typically observed during sinus rhythm. Other, less frequent, forms of preexcited AVRT utilize one AV BT as the anterograde conduction and a second BT for retrograde conduction or a combination of one BT plus the AVN-HPS in either direction ( eFig. 22.1 ).
Clinically, antidromic AVRT is much less frequent than orthodromic AVRT, occurring in 3% to 8% of patients with WPW syndrome, and can be induced in the electrophysiology (EP) laboratory in less than 10%. The low prevalence of antidromic AVRT is related to the EP properties of the AVN, since good retrograde ventriculoatrial (VA) conduction is required to sustain tachycardia. Clinical presentation, sex, age, and orthodromic AVRT induction appear similar in patients with and without inducible antidromic AVRT. Patients with antidromic AVRT induction more frequently have BTs with more rapid conduction properties than other WPW patients.
Susceptibility to antidromic AVRT appears to be facilitated by a distance of at least 4 cm between the BT and the normal AV conduction system. Consequently, most antidromic AVRTs use a lateral (right or left) BT as the anterograde route for conduction. Because posteroseptal BTs are in close proximity to the AVN, those BTs are rarely part of antidromic AVRT if the other limb is the AVN and not a second free-wall BT. Up to 60% of patients with spontaneous antidromic AVRT have multiple BTs (manifest or concealed), which may or may not be utilized as the retrograde limb during the tachycardia.
Other arrhythmias associated with Wolff-Parkinson-White syndrome
Atrial tachycardia (AT), AFL, AF, and AVN reentrant tachycardia (AVNRT) can all coexist with a BT. In these preexcited tachycardias, the BT serves as a bystander route for ventricular or atrial activation and is not required for the initiation or maintenance of the arrhythmia.
Atrioventricular nodal reentrant tachycardia and atrial tachycardia
Both AVNRT and AT can use the bystander BT to transmit impulses to the ventricle ( Fig. 22.5 ). When AVNRT occurs in the WPW syndrome, the arrhythmia can be difficult to distinguish from AVRT without EP testing.
Atrial fibrillation
The overall incidence of AF in patients with WPW syndrome varies from 12% to 39%. AF is typically paroxysmal. Persistent AF is rare in these patients. AF is most common in patients with anterogradely conducting BTs. Patients with antidromic AVRT, multiple BTs, and BTs that have a short anterograde ERP are more prone to develop AF. In individuals with WPW, AF is often preceded by AVRT that degenerates into AF ( Fig. 22.7 ).
The frequency with which intermittent AF occurs in patients with the WPW syndrome is striking because of the low prevalence of coexisting structural heart disease or other predisposing factors for AF. This observation suggests that the AV BT itself can be related to the genesis of AF, supported by the fact that when the BT is ablated, AF may not recur.
The mechanisms by which AVRT precipitates AF are not well understood. The rapid atrial rate can cause disruption in atrial activation and reactivation, creating an EP substrate conducive to AF. The observation that most patients with BT and AF who undergo BT ablation are cured of both AVRT and AF is compatible with this hypothesis. Another possibility is that the complex geometry of networks of BTs predisposes to AF by fractionation of the activation wavefronts. Localized reentry has been recorded in some patients, using direct recordings of the activation of the BTs. Hemodynamic changes, atrial stretch caused by atrial contraction against closed AV valves during ventricular systole, can also play a role. Ablation of the BT can cure AF in more than 90% of patients; however, vulnerability to AF persists in up to 56%, and the response to atrial extrastimulation (AES) is also unaltered by ablation.
Nonetheless, recent studies have challenged the role of BT ablation in modulating the risk of AF particularly in adult patients, showing persistently higher rates of AF in WPW patients (hazard ratio, 4.77) compared with the general population despite catheter ablation of the BT. Trends of increased risk of AF in ablated WPW patients suggest that mechanisms other than those directly related to the presence of a BT, such as underlying atrial myopathy, may play a role in AF genesis.
Atrial flutter
About 4% of WPW patients present with AFL. AFL is the most common (60%) regular preexcited tachycardia in patients with WPW syndrome. AFL is caused by a macroreentrant circuit within the RA and therefore exists independently of the BT, and AFL does not have the same causal association with AV BTs as AF. In some patients with WPW syndrome who develop AFL, AVRT often is the initiating event. This relationship can be mediated by contraction-excitation feedback into the atria during the AVRT.
AFL, like AF, can conduct anterogradely via a BT causing a preexcited tachycardia. Depending on the various refractory periods of the normal and pathological AV conduction pathways, AFL potentially can conduct 1:1 to the ventricles during a preexcited tachycardia, making the arrhythmia difficult to distinguish from VT (see Fig. 13.8 ).
Ventricular fibrillation and sudden cardiac death
The mechanism of sudden cardiac death (SCD) in patients with WPW is likely the occurrence of AF or AFL with a very rapid ventricular rate which provokes VF. Although the frequency with which AF having rapid AV conduction via a BT degenerates into VF is unknown, the incidence of SCD in patients with WPW syndrome is rather low, ranging from 0% to 0.39% annually in several large case series. The trigger for AF in this population of patients is generally an episode of AVRT. In fact, most patients who have been resuscitated from VF secondary to preexcitation have previous history of AVRT, AF, or both. Nonetheless, SCD can be the first manifestation of WPW syndrome.
Several factors can help identify the patient with WPW who is at increased risk for VF, including symptomatic AVRT, septal location of the BT, presence of multiple BTs, and male gender. Additionally, the risk of SCD associated with WPW appears highest in the first 2 decades of life. Nonetheless, it is clear that the most important factor for the occurrence of VF in these patients is the ability of the BT to conduct rapidly to the ventricles. This is best measured by determining the shortest and average preexcited R-R intervals during AF or, alternatively, by measuring the anterograde effective refractory period (ERP) of the BT. If the BT has a very short anterograde ERP (<250 milliseconds), a rapid ventricular response can occur with degeneration of the rhythm to VF. A short preexcited R-R interval during AF (≤220 milliseconds) appears to be a sensitive clinical marker for identifying patients at risk for SCD in children, although its positive predictive value in adults is only 19% to 38%.
Drug therapy can be an additional determinant of the risk of VF in patients with preexcitation. Several pharmacological agents can potentially enhance BT conduction and increase the ventricular rate during AF and, hence, increase the risk of VF (as discussed later).
Ventricular tachycardia
Coexisting VT is uncommon in patients with WPW syndrome, since structural heart disease is very infrequent in this young patient population. Naturally, older patients are subject to coronary artery disease and other pathologies that can cause VT.
Mechanical complications of preexcitation
In patients with manifest ventricular preexcitation, the ventricular myocardium adjacent to the BT contracts earlier than the other ventricular segments, which can cause ventricular dyssynchrony and regional wall motion abnormality. For left free-wall AV BTs, only a distant segment of the lateral left ventricle (LV) is affected. In contrast, ventricular preexcitation mediated by right-sided or septal BTs can potentially cause abnormal electrical and mechanical excitation of the LV (analogous to left bundle branch block [LBBB]), leading to LV remodeling, dilated cardiomyopathy (referred to as “BT-induced cardiomyopathy”) and heart failure, irrespective of the presence of tachyarrhythmia. Several reports have described non–tachycardia-mediated cardiomyopathy in pediatric patients from dyssynchrony owing to ventricular preexcitation. Recovery of cardiac function has been observed within weeks to months after BT ablation and disappearance of ventricular preexcitation. ,
Epidemiology and natural history
Wolff-Parkinson-White pattern
The prevalence of WPW pattern on the surface ECG is 0.1% to 0.3% in the general population. The prevalence is increased to 0.55% among first-degree relatives of affected patients, suggesting a familial component. The annual incidence of newly diagnosed cases of preexcitation in the general population is estimated at 0.004%. The incidence in men is twice that in women and is highest in the first year of life, with a secondary peak in young adulthood.
The WPW pattern on the surface ECG can be intermittent and can even permanently disappear with loss of anterograde but preserved retrograde conduction. Loss of preexcitation has been observed in up to 31% of adults and in 0% to 33% of children and adolescents over a 5-year time period. Intermittent and persistent loss of preexcitation may indicate that the BT has a relatively longer baseline ERP, which makes it more susceptible to age-related degenerative changes and variations in autonomic tone.
Wolff-Parkinson-White syndrome
The prevalence of the WPW syndrome is substantially lower than that of the WPW ECG pattern. At present, it is estimated that approximately 65% of adolescents and 40% of adults over 30 years with a WPW pattern on a resting ECG are asymptomatic.
The occurrence of arrhythmias is related to the age at the time preexcitation is discovered and can vary with location and EP properties of the BT. AVRT manifests early in life, with an average of more than 10 years separating the time of clinical presentation of AVRT versus that of AVNRT. Up to 70% of children (8–12 years of age) who are asymptomatic at the time of diagnosis of WPW ECG pattern remain asymptomatic over median follow-up of 57 months. About 30% eventually develop an arrhythmic event, which can be potentially life-threatening in approximately 10%. In contrast, only a minority (10%) of adults who are asymptomatic at the time of diagnosis of ventricular preexcitation develop cardiac arrhythmias over median follow-up of 67 months, which can be potentially life-threatening in approximately 5%. The vast majority of patients in whom preexcitation is first uncovered after the age of 40 remain asymptomatic. In a large meta-analysis, the annual risk of developing SVT was 0.25% of WPW patients.
A male predominance among WPW patients has been observed. Furthermore, men tend to have a higher incidence of antidromic AVRT, more prevalent left-sided BTs, and shorter anterograde BT ERP. Additionally, men usually present with AVRT at an older age than women. In contrast, women were found to have a higher prevalence of multiple BTs, orthodromic AVRT, and right-sided BTs. Notably, Asians appear to have right free-wall BTs substantially more frequently than other races.
Sudden death
The incidence of SCD among patients with asymptomatic preexcitation is difficult to ascertain. A large meta-analysis (including 1869 patients with asymptomatic ventricular preexcitation from 20 studies with 11,722 patient-years of follow-up) found a total of 10 cases of SCD, with SCD rates between 0 and 4.5 events per 1000 person-years of follow-up, and an overall risk of SCD in adults and children of 2.5 per 1000 person-years (or 3%–4% over a lifetime). Of note, all SCD events, except one, were reported in Italian studies, which questions the degree to which the enrolled population in those studies was representative of patients seen in clinical practice.
In general, children seem to have a numerically higher event rate than adults. The majority of victims were between the ages of 10 and 40 years. Patients with WPW most susceptible to SCD are symptomatic; however, SCD can be the first event in patients with asymptomatic preexcitation. Several characteristics have been reported among patients who experienced potentially life-threatening events, including younger age (<30 years), male gender, history of AF, prior syncope, associated congenital or other heart disease, and familial WPW. Low-risk WPW patients also showed a characteristic EP profile (older age, lower tachyarrhythmia inducibility, longer anterograde refractory ERP of BTs, and low likelihood of baseline retrograde BT conduction or multiple BTs).
Associated cardiac abnormalities
Most patients with AV BTs do not have coexisting structural cardiac abnormalities, except for those that are age-related. The association of ventricular preexcitation with structural congenital heart defects is well recognized. Up to 20% of children with WPW also have congenital heart disease. Associated congenital abnormalities, when present, are more likely to be right-sided than left-sided. Ebstein anomaly is the congenital lesion most strongly associated with the WPW syndrome. As many as 10% of such patients have one or more BTs; most of these are located in the right free-wall or the right posteroseptal space. Ventricular preexcitation has also been described in patients with transposition of great arteries, pulmonary atresia, patent ductus arteriosus, tetralogy of Fallot, total anomalous pulmonary venous return, and ventricular septal defects.
Familial Wolff-Parkinson-White syndrome
Typically, WPW syndrome occurs sporadically; however, in a minority of cases it is inherited. A familial form of WPW has infrequently been reported and is usually inherited as an autosomal dominant trait. Among patients with the WPW syndrome, 3.4% have first-degree relatives with a preexcitation syndrome. The genetic cause of a rare form of familial WPW syndrome has been described. The clinical phenotype is characterized by the presence of preexcitation on the ECG, frequent SVTs (including AF), progressive conduction system disease, and LV hypertrophy (distinct from sarcomeric hypertrophic cardiomyopathy). Patients typically present in late adolescence or the third decade with syncope or palpitations. Premature SCD occurred in 10% of patients. Paradoxically, by the fourth decade of life, progression to advanced SND or AV block (with the loss of preexcitation) requiring pacemaker implantation was common. Approximately 80% of the patients older than 50 years had chronic AF. Causative mutations in the PRKAG2 gene were identified in these families. The PRKAG2 gene encodes the gamma-2 regulatory subunit of the adenosine monophosphate (AMP)–activated protein kinase, which is a key regulator of metabolic pathways, including glucose metabolism. The penetrance of the disease for WPW syndrome was complete, but the expression was variable. The described phenotype of this syndrome is similar to the autosomal recessive glycogen storage disease, Pompe disease. Given the function of the AMP-activated protein kinase and this similarity, the PRKAG2 syndrome is likely a cardiac-specific glycogenosis syndrome. This syndrome thus belongs to the group of genetic metabolic cardiomyopathies, rather than to the congenital primary arrhythmia syndromes. The annulus fibrosus, which normally insulates the ventricles from the atria, is thinned and disrupted by glycogen-filled myocytes, and these anomalous microscopic AV connections, rather than morphologically distinct bypass tracts, appear to provide the anatomic substrate for ventricular preexcitation. Of note, certain mutations in PRKAG2 have been associated with nodoventricular BTs.
Another genetic form of WPW syndrome is associated with mutations in the bone morphogenetic protein-2 ( BMP2 ) gene, which belongs to the class of transforming growth factor, TGF-β-superfamily of proteins, and is involved in the development of the annulus fibrosus. This syndrome is characterized by variable cognitive deficits and dysmorphic features in addition to ventricular preexcitation.
Concealed bypass tracts
The true prevalence of concealed BTs is unknown because, unlike the situation with the WPW ECG pattern, these BTs are concealed on the surface ECG and are only expressed during AVRT; only symptomatic patients undergo EP testing. As noted, orthodromic AVRT accounts for approximately 95% of AVRTs and 35% of all paroxysmal SVTs, and 50% of the BTs that participate in orthodromic AVRT are concealed. SVTs using a concealed BT have no gender predilection and tend to occur more frequently in younger patients than in those with AVNRT; however, significant overlap exists.
Clinical presentation
The majority of patients with preexcitation are asymptomatic and are discovered incidentally on an ECG obtained for unrelated reasons. The two most common types of arrhythmias in the WPW syndrome are AVRT and AF. Patients with AVRT experience symptoms characteristic of paroxysmal SVT with abrupt onset and termination, including rapid and regular palpitations, chest pain, dyspnea, presyncope, and rarely, syncope. Episodes can last from seconds to several hours. Symptoms are usually mild and short-lived and terminate spontaneously or with vagal maneuvers. However, occasionally patients present with disabling symptoms, especially in the presence of structural heart disease.
AVRT, which in general is well tolerated by the patient when additional heart disease is absent, can deteriorate into AF; the latter can be a life-threatening arrhythmia if the BT has a short anterograde refractory period, resulting in very fast ventricular rates, with possible degeneration into VF and SCD. The incidence of SCD in patients with the WPW syndrome has been estimated to range from 0.15% to 0.39% over a 3- to 10-year follow-up. It is unusual for cardiac arrest to be the first symptomatic manifestation of WPW syndrome. Conversely, in about 50% of cardiac arrest cases in WPW patients, it is the first manifestation of WPW syndrome.
Rarely, significant ventricular preexcitation during NSR can result in ventricular dysfunction secondary to dyssynchronous ventricular contraction. Improvement of LVEF has been described after ablation of septal BTs in pediatric patients.
Initial evaluation
History, physical examination, and 12-lead ECG constitute an appropriate initial evaluation. In patients with brief, self-terminating episodes of palpitations, an event recorder is the most effective way to obtain ECG documentation of arrhythmias. Also, echocardiographic examination is recommended to exclude structural heart disease.
Several other noninvasive tests have been proposed as useful for evaluating symptomatic patients and risk-stratifying patients for SCD risk. However, the sensitivity and specificity of noninvasive testing have been shown to be limited. Invasive EP testing may be considered in patients with arrhythmias as well as in asymptomatic patients with WPW ECG pattern when noninvasive testing does not lead to the conclusion that the anterograde ERP of the BT is relatively long. However, a strategy to perform an EP study for all asymptomatic patients with the WPW ECG pattern for the purpose of risk stratification is still controversial and not widely accepted.
Methods for evaluation of bypass tract refractory period
Demonstration of intermittent preexcitation
Intermittent preexcitation has historically been thought to confer a lower risk of SCD than persistent preexcitation. Observation of intermittent loss of the preexcitation pattern on ambulatory monitoring or serial ECGs is generally correlated with a long BT anterograde ERP. The longer refractory period of the BT decreases the frequency of manifest preexcitation during sinus rhythm and is expected to lower the risk of mediating rapid preexcited ventricular activation during AF. Nevertheless, EP studies in symptomatic patients with intermittent preexcitation found that 10% to 24% can have BTs capable of conducting at rapid rates during AF (with anterograde ERP or shortest preexcited R-R interval of less than 250 milliseconds), perhaps related to autonomic influences. However, similar findings have not yet been demonstrated in asymptomatic patients with intermittent preexcitation.
Intermittent preexcitation can be observed on ambulatory monitoring in up to 67% of patients. It is important, however, to distinguish intermittent preexcitation from inapparent preexcitation (see later) and from a bigeminal ventricular rhythm with a long coupling interval. Although intermittent preexcitation is a predictor of poor anterograde conduction through the BT, it has rarely been observed in some patients with cardiac arrest. Furthermore, the presence of intermittent preexcitation does not predict retrograde conduction properties of the BT nor preclude the development of AVRT.
Loss of preexcitation during exercise
Demonstration of a sudden loss of preexcitation (indicated by abrupt loss of the delta wave associated with prolongation of the PR interval and normalization of the QRS) during exercise is consistent with anterograde conduction block in the BT and is consistent with a long BT ERP (>300 milliseconds). Importantly, rapid AVN conduction during exercise can potentially mask persistent preexcitation. Therefore, only abrupt and complete loss of preexcitation during exercise should be sought as a surrogate of a long anterograde ERP of the BT.
Loss of preexcitation during exercise is a good predictor that the patient is not at risk for VF even during sympathetic stimulation. However, the frequency of block in the BT during exercise is low (approximately 10%–20%), and thus, sensitivity of this test is poor. On the other hand, persistence of preexcitation during exercise stress has a sensitivity of 96% but a specificity of only 17% in predicting either a shortest preexcited R-R ≤250 milliseconds during AF or a BT ERP of ≤250 milliseconds (positive predictive value of 40% and negative predictive value of 88%).
Bypass tract conduction block in response to antiarrhythmic agents
When the administration of ajmaline (1 mg/kg intravenous [IV] over 3 minutes) or procainamide (10 mg/kg IV over 5 minutes) results in complete block of the BT during NSR, a long anterograde ERP (>270 milliseconds) of the BT is likely. The shorter the BT ERP, the less likely it would be blocked by these drugs. Also, the amount of ajmaline required to block conduction over the BT correlates with the duration of the anterograde ERP of the BT. However, the incidence of BT block in response to these drugs is low and, although the occurrence of block predicts a long ERP of the BT, failure to produce block does not necessarily suggest a short ERP. Moreover, pharmacological testing is carried out at rest and therefore does not indicate what effect the drug will have on the BT ERP during sympathetic stimulation, such as exercise, emotion, anxiety, and recreational drug use. Importantly, the specificity of loss of preexcitation after administration of sodium channel blockers is poor compared to the shortest preexcited R-R intervals during inducible AF. Given these limitations, pharmacologic challenge is no longer routinely utilized.
Evaluation of ventricular response during atrial fibrillation
During spontaneous or induced AF, the propensity for rapid AV conduction can be judged by the interval between consecutively preexcited QRS complexes. A mean preexcited R-R interval >250 milliseconds and a shortest preexcited R-R >220 milliseconds predict low risk for SCD, with a negative predictive value of more than 95%; however, the positive predictive value is low (20%).
Response of preexcitation to transesophageal atrial stimulation
There is good correlation between the value of the anterograde ERP of the BT obtained during single-test programmed atrial stimulation and atrial pacing at increasing rates and the ventricular rate during AF. Programmed electrical stimulation of the atrium can be performed by the transesophageal route and the value of the anterograde ERP of the BT can be determined.
Electrophysiological testing
Programmed atrial stimulation is used to evaluate the anterograde ERP of the BT. Because BT refractoriness shortens with decreasing pacing CL (PCL), the ERP should be determined at multiple PCLs (preferably ≤400 milliseconds). Additionally, atrial stimulation should be performed close to the BT atrial insertion site to obviate the effect of intraatrial conduction delay. Incremental-rate atrial pacing is performed to determine the maximal rate at which 1:1 conduction over the BT occurs. Induction of AF should be performed to determine the average and the shortest R-R interval during preexcited AF. Atrial and ventricular stimulation is also performed to evaluate inducibility of AVRT as well as the number and location of BTs.
A shortest preexcited R-R interval less than 220 to 250 milliseconds during AF has been shown to be the best discriminator of those at risk of VF, with a high sensitivity (88%–100%) and a high negative predictive value for identifying children and young adults with WPW syndrome at risk for VF. However, the positive predictive value is low (19%–38%), largely due to the very low incidence of SCD in these patients. Also, a shortest preexcited R-R interval less than 250 milliseconds during AF has been noted in 20% to 26% of asymptomatic adults with a WPW pattern, and in up to 67% when isoproterenol is administered. Thus, although isoproterenol raises the sensitivity of invasive EP testing, it markedly reduces the specificity. It should be noted that recent studies found that BT properties characterized during invasive EP testing under anesthesia do not correlate well with how the BT may function during an episode of clinical AF.
On the other hand, BT ERP less than 240 milliseconds appears to significantly correlate with only AVRT inducibility but, as an isolated variable, it is less predictive of life-threatening events and exhibits significant overlap between WPW patients with VF and those without VF. The presence of multiple BTs and the ability to induce sustained AVRT, especially when AVRT spontaneously degenerates into AF, have been proposed to predict risk of malignant ventricular arrhythmias. The lack of retrograde conduction over the BT appears to predict at lower risk for SCD. However, the predictive value of these criteria remains limited and significant overlap exists.
Principles of management
Acute management
Symptomatic patients with concealed bypass tracts
Patients with orthodromic AVRT utilizing a concealed BT are treated in a similar fashion to those with paroxysmal SVT. Vagal maneuvers (including Valsalva and carotid sinus massage) are the first-line intervention for acute conversion of the tachycardia, though the overall success rate is limited (approximately 28%). If SVT persists, adenosine is recommended, and it offers a success rate of 90% to 95%. For refractory tachycardia, IV diltiazem, verapamil, or beta-blockers can terminate orthodromic AVRT in the majority of patients. Synchronized cardioversion is recommended for hemodynamically unstable patients and for those refractory or intolerant to drug therapy ( Fig. 22.8 ).
Symptomatic patients with manifest bypass tracts
In patients with manifest preexcitation during sinus rhythm presenting with AVRT (orthodromic or antidromic), vagal maneuvers are the first-line intervention for tachycardia termination ( Fig. 22.8 ). For persistent SVT, adenosine is recommended. Importantly, adenosine should be used with caution because it can induce AF with a rapid ventricular rate in the presence of an anterogradely conducting BT. This is unusual and should not be viewed as a contraindication to adenosine use, but one should be prepared for emergency cardioversion before administering adenosine to SVT patients.
For refractory AVRT, IV diltiazem, verapamil, or beta-blockers can be considered to block conduction in the AVN, which represents either the retrograde or anterograde limb in the AVRT circuit. AVN blocking drugs, however, are ineffective in patients with preexcited AVRT that utilizes two separate BTs for anterograde and retrograde conduction. Drug treatment directed at the BT (ibutilide, procainamide, flecainide) may also be considered. When drug therapy fails or hemodynamic instability is present, electrical cardioversion should be considered. It is important to note that IV diltiazem and verapamil can result in hemodynamic collapse if the AVRT does not terminate or AF is induced with rapid conduction over the BT degenerating to VF, and one must be prepared for immediate electrical cardioversion if that occurs (see below).
Preexcited atrial fibrillation
In patients presenting with AF or AFL with ventricular preexcitation and rapid ventricular response, prompt direct-current cardioversion is recommended, especially when hemodynamic compromise is present. IV procainamide or ibutilide to restore sinus rhythm or slow the ventricular rate may be considered in hemodynamically stable patients. Both drugs also slow BT conduction.
Importantly, drugs that preferentially slow AVN conduction without prolonging BT refractoriness (such as verapamil, diltiazem, beta-blockers, adenosine, oral or IV digoxin, and IV amiodarone) can accelerate the ventricular rate and potentially precipitate hemodynamic collapse and VF in high-risk patients. Several mechanisms are probably involved; hypotension produced by some of those medications is followed by a sympathetic discharge that enhances BT conduction. Furthermore, slowing or blocking conduction through the AVN prevents competitive concealed retrograde conduction into the BT by normally conducted beats and, as a result, can potentially enhance conduction over the BT. Additionally, the rapid and irregular ventricular rate, hypotension, and sympathetic discharge probably result in fractionation of the ventricular wavefront and VF. Also, digoxin increases the ventricular rate by shortening BT refractoriness. Lidocaine, for reasons that are unclear, has also been associated with degeneration of AF into VF. It is occasionally used in patients with WPW who have a wide QRS complex tachycardia that might be misinterpreted as VT. Unlike the IV route of administration, chronic oral amiodarone therapy can slow or block BT conduction.
Chronic management
Symptomatic patients with concealed bypass tracts
Catheter ablation is considered first-line therapy (class I) for patients with paroxysmal SVT involving a concealed BT (i.e., orthodromic AVRT). However, because concealed BTs are not associated with an increased risk of SCD in these patients, catheter ablation should be presented as one of a number of potential therapeutic approaches, including pharmacological therapy and clinical follow-up alone. When pharmacological therapy is selected for patients with concealed BTs, it is reasonable to consider a trial of beta-blocker therapy, diltiazem, or verapamil ( Fig. 22.9 ). These agents are effective for preventing recurrent tachycardia in approximately 50% of patients. Antiarrhythmic agents may be considered for patients with refractory tachycardia; however, the risk and benefits of these drugs should be carefully considered.
Symptomatic patients with manifest bypass tracts
Catheter ablation is considered the treatment of choice for patients with the WPW syndrome—that is, patients with manifest preexcitation along with documented arrhythmias (antidromic or orthodromic AVRT or preexcited AF) or symptoms consistent with cardiac arrhythmias. Catheter ablation is curative in more than 95% of patients with a relatively low complication rate (approximately 3%), and it also obviates the unwanted side effects of pharmacological therapy.
For patients with WPW syndrome who are not candidates for, or prefer not to undergo, catheter ablation, antiarrhythmic drugs to block BT conduction are considered ( Fig. 22.9 ). Class IC agents such as flecainide and propafenone (in patients without structural or ischemic heart disease), and class III drugs, such as sotalol and dofetilide, may be considered. Oral amiodarone is a last resort therapy, given the associated risks of long-term therapy. In general, antiarrhythmic drug therapy can offer symptomatic improvement in up to 90% of patients, although complete disappearance of symptom is observed in only 30%.
Chronic oral beta-blockers, verapamil, and diltiazem may be used for the treatment of patients with WPW syndrome, particularly if their BT has been demonstrated to be incapable of rapid anterograde conduction. However, these agents must be used with caution and after a discussion with the patient the potential risk of rapid conduction over the BT if AF develops. Digoxin, on the other hand, should be avoided since it can shorten the refractory period of the BT and, hence, is potentially harmful in patients with manifest BTs.
Asymptomatic patients with manifest bypass tracts
Asymptomatic young WPW patients have about 30% risk of becoming symptomatic, as well as a very small but definite risk of life-threatening arrhythmias and SCD. Therefore, in the recent Pediatric and Congenital Electrophysiology Society (PACES) and the Heart Rhythm Society (HRS) expert consensus document on treatment of asymptomatic young WPW subjects, risk stratification is recommended to identify a potential subgroup of patients with BTs with “high-risk” properties that may confer an increased risk for lethal cardiac arrhythmias and in whom the risk-to-benefit ratio favors prophylactic ablation ( Fig. 22.10 ).
Initial risk stratification utilizes noninvasive testing (e.g., Holter monitoring, exercise stress testing) to ascertain true loss of preexcitation at physiological heart rates. Complete and abrupt loss of preexcitation during exercise testing or intermittent loss of preexcitation during ECG or ambulatory monitoring indicate long anterograde ERP of the BT and help identify patients at low risk of rapid conduction over the BT and SCD ( Table 22.2 ).
TABLE 22.2
Finding Suggestive of Lower Risk of Life-Threatening Events in Patients With Ventricular Preexcitation
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BT , bypass tract.
Inability to clearly demonstrate absolute loss of ventricular preexcitation on noninvasive testing warrants consideration for EP testing. However, the benefits and risk of invasive risk stratification should be based on individual considerations such as age, gender, occupation, and athletic involvement and should be thoroughly discussed with the patient or, in the case of a child, with the parents. Given the low risk associated with invasive EP testing (0.1%–1%), it is reasonable to consider this approach for risk stratification in asymptomatic WPW patients.
Several EP findings help identify high-risk patients who may benefit from catheter ablation, including: (1) shortest preexcited R-R interval less than 250 milliseconds during induced AF; (2) the presence of multiple BTs; (3) spontaneous degeneration of induced AVRT into preexcited AF; and (4) BT anterograde ERP less than 240 milliseconds. The critical obligatory condition for VF is the presence of a short anterograde functional refractory period of the BT, which is best reflected in the shortest R-R interval between preexcited beats in AF. Although the ability to induce sustained AVRT has been considered by some as a potential risk factor, this has not been supported by recent studies.
The role of isoproterenol challenge during EP testing has not yet been clearly defined. Isoproterenol administration can significantly shorten the shortest preexcited R-R interval and, as a result, increase the proportion of the asymptomatic patients in the “high-risk” category. Thus, some investigators suggested the use of a more stringent threshold for the shortest preexcited R-R interval (≤220 milliseconds) instead of the threshold adopted by the 2012 PACES/HRS Guidelines (≤250 milliseconds) for definition of “high-risk” BTs when EP testing is performed under the influence of isoproterenol.
In low-risk patients, as determined by noninvasive or invasive testing, it is appropriate to pursue a strategy of follow-up with ECGs and reevaluation at selected intervals with a high degree of suspicion for new arrhythmia symptoms. This strategy should incorporate patient education about the potential risks associated with preexcitation and about the symptoms of arrhythmias, which should prompt them to seek medical attention. It is also advisable to give the patient a copy of his or her ECG and a short note about the fact that the WPW pattern is present to help prevent the misdiagnosis of myocardial infarction (MI) and to explain the basis of cardiac arrhythmias in case they develop later. Importantly, WPW patients most susceptible to SCD are symptomatic. Thus, the evolution of the clinical status from an asymptomatic state to symptoms (e.g., syncope or palpitations) likely portends a higher risk for SCD. Once WPW patients become symptomatic, catheter ablation needs to be considered, regardless of the prior risk assessment.
In patients with high-risk BT characteristics, prophylactic catheter ablation is reasonable. A combination of inducible AVRT and short R-R interval during preexcited AF provides the best indication for ablation. Catheter ablation of the BT, regardless of BT characteristics, is also reasonable in asymptomatic patients with high-risk occupations (e.g., school bus drivers, police, and pilots) and is probably reasonable in: (1) patients involved in moderate- to high-intensity competitive sports; (2) patients with structural heart disease; (3) patients with ventricular dysfunction secondary to dyssynchronous contractions; and (4) patients with BT locations associated with lower risk of procedural complications (such as AV block or coronary artery injury) which can counterbalance the potential benefit of ablation. However, because knowledge about the success and complication rates plays a major role in decision making, the physician must consider his or her own success and complication rates for ablation of the specific location of the BT identified, and that information should be made available to the patient.
It is important to recognize that the majority of adult patients with asymptomatic preexcitation have a benign course with few clinically significant arrhythmic events occurring over time, and the risk of SCD is small and is seen mainly in children and is very rare to be the initial clinical manifestation. Hence, observation, without further evaluation or treatment, remains a reasonable option in these patients, even when noninvasive low-risk markers are not evident. The key is a clear understanding by the patient of the relative merits of each strategy. The well-informed patient needs to choose between a very small risk of potentially life-threatening arrhythmia over a long period of time and a one-time small procedural risk associated with EP testing and catheter ablation. Certain patients such as athletes and those in higher-risk occupations will generally choose ablation. Others, especially patients older than 30 years, may prefer the small risk of a conservative strategy.
It should be noted that both invasive and noninvasive EP markers, despite their high sensitivity and negative predictive value, lack specificity for identifying patients at risk of life-threatening ventricular arrhythmias, largely due to the very low incidence of SCD. The great majority of individuals with WPW, even with shortest preexcited R-R interval of less than 250 milliseconds during AF, will not experience SCD; thus, the positive predictive value to predict SCD remains very low. Therefore, the management of asymptomatic patients with a WPW pattern remains controversial.
The ability of noninvasive risk stratification to identify low-risk patients is low (<20%). Hence, according to the 2012 PACES/HRS Guidelines, invasive EP evaluation would be recommended for the majority of young asymptomatic WPW patients, and the majority of those would undergo catheter ablation. Although complications of a diagnostic EP study generally are minor and not life-threatening, the risks associated with an ablation procedure likely are at least similar to the risk of SCD in asymptomatic WPW patients. In three large series, procedure-related complications occurred in 1.8% to 8.2% of cases, and death as a consequence of ablation occurred in 0.07% to 0.19%. If routine EP testing were to be performed in the majority of asymptomatic WPW patients, many patients would proceed immediately to catheter ablation and, in others, there would be a strong temptation to ablate when catheters are in place (regardless of predicted SCD risk), especially given the fact that the criteria for ablation usually will not be black or white. This greatly increases the risk to the patient, which can potentially nullify the benefit of elimination of SCD risk achieved by BT ablation.
Finally, both the physician and the patient need to have a shared understanding regarding the value of invasive EP study for risk stratification, rather than a therapeutic tool. These issues have to be resolved before proceeding with invasive measures, and the risks and benefits of proceeding with ablation of BT found not to have high-risk characteristics should be discussed thoroughly with patients in advance of the EP procedure.
Wolff-Parkinson-White patients and sports participation
WPW syndrome accounts for approximately 1% of deaths in athletes. While many of the cases of SCD with WPW have an association with exercise, training does not alter the EP properties in WPW.
Catheter ablation is the treatment of choice for symptomatic WPW patients (whether or not engaged in athletics). For asymptomatic WPW patients engaged in moderate- to high-level competitive sports, noninvasive and, if needed, invasive risk stratification is advisable. For those with high-risk BT characteristics, ablation of the BT is recommended before clearance for competitive sports because of risk for life-threatening arrhythmias. In low-risk patients, as determined by noninvasive or invasive testing, BT ablation may still be appropriate, but competitive sports may be allowed without ablation, especially when BT ablation confers an unacceptable potential risk (e.g., AV block in the setting of midseptal or superoparaseptal BTs).
Electrocardiographic features
Electrocardiography of preexcitation
Anterogradely conducting AV BTs produce the classic WPW ECG pattern characterized by a fusion between conduction via the BT and the normal AVN-HPS: (1) short PR (P-delta) interval (<120 milliseconds); (2) slurred upstroke of the QRS (delta wave); and (3) wide QRS (>120 milliseconds) ( Figs. 22.1 and 22.2 ).
The degree of preexcitation depends on several factors, including conduction time over the AVN-HPS, conduction time from the sinus node to the atrial insertion site of the BT (which depends on the distance, conduction, and refractoriness of the intervening atrial tissue), and conduction time through the BT (which depends on the length, thickness, and conduction properties of the BT).
Pharmacological and/or physiological maneuvers that alter AVN conduction (e.g., carotid sinus massage, Valsalva maneuvers, adenosine, beta-blockers) can be used to alter the degree of preexcitation, thereby confirming the diagnosis of the presence of an anterogradely conducting AV BT.
The ECG pattern displayed by some patients with the WPW syndrome can simulate the pattern found in other cardiac conditions and can alter the pattern seen in the presence of other cardiac disease. A negative delta wave (presenting as a Q wave) can mimic an MI pattern. Conversely, a positive delta wave can mask the presence of a previous MI. Intermittent WPW can also be mistaken for frequent PVCs ( Fig. 22.11 ). If the WPW pattern persists for several beats, the rhythm can be misdiagnosed as an accelerated idioventricular rhythm or, if sufficiently rapid, VT. The WPW pattern is occasionally seen on alternate beats and may suggest ventricular bigeminy. An alternating WPW and normal pattern can occasionally suggest electrical alternans. On the other hand, late-coupled PVCs ( eFig. 22.2 ) and ventricular pacing ( eFig. 22.3 ) with inapparent pacing artifacts can occasionally mimic ventricular preexcitation.
Inapparent preexcitation
With inapparent preexcitation, ventricular preexcitation is absent on the surface ECG despite the presence of an anterogradely conducting AV BT, because conduction over the AVN-HPS reaches the ventricle faster than that over the BT. In this setting, the PR interval is shorter than what the P-delta interval would be if preexcitation were present. Therefore, the transition from manifest to inapparent preexcitation is characterized by normalization of the QRS in conjunction with shortening of the PR interval, reflecting the now better AVN-HPS conduction.
Inapparent preexcitation is usually caused by: (1) enhanced AVN conduction so that it is faster than conduction over the BT; (2) prolonged intraatrial conduction from the site of atrial stimulation to the atrial insertion site of the BT (most often left lateral), favoring anterograde conduction and depolarization of the ventricle over the AVN-HPS; or (3) prolonged conduction over the BT so that it is slower than AVN-HPS conduction.
Intermittent preexcitation
Intermittent preexcitation is defined as the presence and absence of preexcitation on serial ECGs or ambulatory cardiac monitoring ( Fig. 22.12 ). True intermittent preexcitation is characterized by an abrupt loss of the delta wave (independently of how fast or slow AVN conduction is), with prolongation (normalization) of the PR interval (reflecting the loss of the faster BT conduction, and the subsequent conduction over the slower AVN), and normalization of the QRS in the absence of any significant change in heart rate.
The mechanism of intermittent preexcitation is poorly understood but is likely related to BT refractory period as well as cellular connectivity within the BT. Potential mechanisms include (1) phase 3 (i.e., tachycardia-dependent) or phase 4 (i.e., bradycardia-dependent) block in the BT (see Chapter 11 ); (2) anterograde or retrograde concealed conduction produced by PVCs, PACs, or atrial arrhythmias; (3) BTs with long ERP and the gap phenomenon in response to PACs; and (4) BTs with long ERP and supernormal conduction.
Preexcitation alternans is a form of intermittent preexcitation in which a QRS complex manifesting a delta wave alternates with a normal QRS complex ( Fig. 22.11 ). Concertina preexcitation is another form of intermittent preexcitation in which the PR intervals and QRS complex durations show a cyclic pattern; that is, preexcitation becomes progressively more prominent over a number of QRS complex cycles followed by a gradual diminution in the degree of preexcitation over several QRS cycles, despite a fairly constant heart rate.
Differentiation between intermittent preexcitation and inapparent preexcitation on an ECG showing QRS complexes with and without preexcitation can be achieved by comparing the P-delta interval during preexcitation and the PR interval when preexcitation is absent. Loss of preexcitation associated with a PR interval longer than the P-delta interval is consistent with intermittent preexcitation ( Fig. 22.12 ), whereas loss of preexcitation associated with a PR interval shorter than the P-delta interval is consistent with inapparent preexcitation. Furthermore, maneuvers that slow AVN conduction (e.g., carotid sinus massage, AVN blockers) would unmask inapparent preexcitation but would not affect intermittent preexcitation.
Supraventricular tachyarrhythmias associated with Wolff-Parkinson-White syndrome
Orthodromic atrioventricular reentrant tachycardia
The ECG during orthodromic AVRT shows retrograde P waves inscribed within the ST-T wave segment with an RP interval that is usually less than half of the tachycardia R-R interval (i.e., RP interval < PR interval) ( eFig. 22.4 ). The RP interval remains constant, regardless of the tachycardia CL (TCL), because it reflects the nondecremental retrograde conduction over the BT. QRS morphology during orthodromic AVRT is generally normal and not preexcited, even when preexcitation is present during normal sinus rhythm (NSR) ( Fig. 22.6 ). Functional bundle branch block (BBB) can be observed frequently during orthodromic AVRT at fast rates ( eFig. 22.4 ). The presence of BBB during SVT in a young person (<40 years) should raise the suspicion of orthodromic AVRT incorporating a BT ipsilateral to the blocked bundle, because the longer conduction time through the involved ventricle engendered by the BBB facilitates orthodromic reentry by enabling all portions of the circuit enough time to recover excitability from the prior cycle. This is particularly true with LBBB, which is very uncommon in younger patients.
Orthodromic AVRT tends to be a rapid tachycardia, with rates ranging from 150 to more than 250 beats/min, generally faster in younger patients. A beat-to-beat oscillation in QRS amplitude (QRS alternans) is present in up to 38% of cases and is most commonly seen when the rate is very rapid ( Fig. 22.13 ). The mechanism for QRS alternans is not clear but may partly result from oscillations in the relative refractory period of the distal portions of the HPS.
Ischemic-appearing ST segment depression also can occur during orthodromic AVRT, even in young individuals who are unlikely to have coronary artery disease. An association has been observed between repolarization changes (ST segment depression or T-wave inversion) and the underlying mechanism of the tachycardia, because such changes are more common in orthodromic AVRT than AVNRT (57% versus 25%). Several factors can contribute to ST segment depression in these arrhythmias, including changes in autonomic tone, intraventricular conduction disturbances, a longer VA interval, and a retrograde P wave of longer duration that overlaps into the ST segment. The location of the ST segment changes can vary with the location of the BT; ST segment depression in leads V 3 to V 6 is almost invariably seen with a left lateral BT, whereas ST segment depression and a negative T wave in the inferior leads is associated with a posteroseptal or posterior BT. A negative or notched T wave in leads V 2 or V 3 with a positive retrograde P wave in at least two inferior leads suggests an anteroseptal BT. However, ST segment depression occurring during orthodromic AVRT in an older patient mandates consideration of possible coexisting ischemic heart disease.
Antidromic atrioventricular reentrant tachycardia
Antidromic AVRT is characterized by a wide (fully preexcited) QRS complex, usually regular R-R intervals, and ventricular rates of up to 250 beats/min ( Fig. 22.6 ). The width of the preexcited QRS complex and the amplitude of the ST-T wave segment usually obscure the retrograde P wave on the surface ECG. When the P waves can be identified, they are inscribed within the ST-T wave segment with an RP interval that may be more than half of the tachycardia R-R interval because retrograde conduction occurs slowly via the AVN-HPS. The PR (P-delta) interval remains constant, regardless of the TCL, because it represents nondecremental anterograde conduction over the BT.
Atrioventricular nodal reentrant tachycardia and atrial tachycardia
Both AVNRT and AT can be associated with partially or fully preexcited QRS complex secondary to anterograde conduction to the ventricles over the bystander BT. When AVNRT occurs in the WPW syndrome, the arrhythmia can be difficult to distinguish from orthodromic AVRT without EP testing.
Atrial fibrillation
There are several characteristic findings on the ECG in patients with AF conducting over a BT, so-called preexcited AF. The rhythm is irregularly irregular and can be associated with very rapid ventricular response caused by the nondecremental anterograde AV conduction over the BT ( Fig. 22.14 ). However, a sustained rapid ventricular rate of more than 180 to 200 beats/min will often create R-R intervals that appear to be regular when the ECG is recorded at 25 mm/sec. Although the QRS complexes are conducted aberrantly, resembling those during preexcited NSR, their duration can be variable and they can become normalized. This is not related to the R-R interval (i.e., it is not a rate-related phenomenon) but rather is related to the variable relationship between conduction over the BT and AVN-HPS. Preexcited and normal QRS complexes often appear “clumped.” This can result from concealed retrograde conduction into the BT or the AVN ( Fig. 22.15 ).
The QRS complex during preexcitation is a fusion between the impulse that preexcites the ventricles caused by rapid conduction through a BT and the impulse that takes the usual route through the AVN. The number of impulses that can be transmitted through the BT and the amount of preexcitation depend on the refractoriness of both the BT and AVN. The shorter the anterograde ERP of the BT, the more rapid is anterograde impulse conduction over the BT and, because of more preexcitation, the wider the QRS complexes. Patients who have a BT with a very short ERP and rapid ventricular rates represent the group at greatest risk for development of VF.
Anterograde block in the BT during AF abolishes retrograde concealment into the AVN, which in turn allows the AVN to recover its excitability and conduct anterogradely. In turn, these conducted impulses through the AVN can result in retrograde concealment into the BT, causing anterograde block of the BT and, thereby, slowing the ventricular rate.
Atrial flutter
AFL, like AF, can conduct anterogradely via a BT resulting in a preexcited tachycardia. Depending on the various refractory periods of the normal and pathological AV conduction pathways, AFL potentially can conduct 1:1 to the ventricles during a preexcited tachycardia, making the arrhythmia difficult to distinguish from VT (see Fig. 13.8 ).
Electrocardiographic localization of the bypass tract
Careful analysis of the preexcitation pattern during NSR can potentially allow an accurate approximation of the location of the BT. This provides the electrophysiologist with important information that can guide patient counseling regarding the risks and benefits of ablation, in particular, providing some guidance about the proximity of the BT to the normal conduction system and the subsequent risk of AV block associated with an ablation attempt, as well as the need for left heart catheterization and atrial septal puncture and their potential complications. Additionally, it can allow planning the ablation procedure, such as the use of cryoablation for septal BTs or the need for special equipment for atrial septal puncture for left-sided BTs.
Localization using the delta wave
Delta wave morphology reflects the ventricular insertion site of the BT and, hence, is helpful in approximating BT location, especially when maximal preexcitation is present. However, during NSR, only partial ventricular preexcitation is usually observed, which limits the accuracy of BT localization based on the surface ECG. Additionally, the degree of preexcitation can vary in an individual, depending on heart rate, autonomic tone, and AVN function, as well as the location and EP characteristics of the BT. Therefore, it is important first to assess the degree of preexcitation visible throughout the entire ECG and then use only delta wave polarity for localization (the first 20–60 milliseconds of the QRS in most cases, unless fully preexcited) rather than the overall QRS polarity, which may vary from each other.
Several algorithms have been developed to predict the anatomic location of manifest BTs based on delta wave polarity on the surface ECG ( Figs. 22.16 and 22.17 ; eFigs. 22.5–22.7 ). , Although these algorithms facilitate prediction of the location of a BT, they are inherently limited by biological variability in anatomy (e.g., rotation of the heart within the thorax), variable degree of preexcitation and QRS fusion, the presence of more than one manifest BT, intrinsic ECG abnormalities (such as prior MI and ventricular hypertrophy), patient body habitus, and technical variability in ECG acquisition and electrode positioning. The accuracy of these algorithms in more recent studies did not reach the previously reported accuracy by their designers. Therefore, these algorithms should be regarded as an orientation rather than a precise localization tool. No single published algorithm offers extremely high sensitivity and specificity for all BT locations. The algorithms appear most accurate in predicting left free-wall BTs locations and least accurate in predicting midseptal or right anteroseptal BT locations. Hence, it may be more realistic to initially identify a general area in which the BT is located and then to apply more subtle criteria from one or more of the algorithms to attempt a more precise localization. This can be achieved by applying some basic rules while maintaining a mental three-dimensional (3-D) representation of the mitral and tricuspid annuli and their anatomical relationships to adjacent structures as they lie within the chest (i.e., “attitudinally correct orientation”) while analyzing the ECG and predicting morphology of the preexcited QRS.
Of note, the mere presence and the degree of preexcitation can sometimes help in predicting the location of the BT. For example, posteroseptal and right-sided BTs tend to be associated with a prominent degree of preexcitation because of the proximity to the sinus node, whereas left lateral BTs are often associated with subtle preexcitation. Nevertheless, when preexcitation is prominent, the accuracy of surface ECG localization of manifest BTs tends to be higher for the diagnosis of left free-wall BTs than for BTs in other locations.
On the surface ECG, analysis of delta wave polarity and amplitude progression in precordial leads, and frontal plane horizontal and vertical axis, are valuable for predicting the site of origin of manifest BTs ( Fig. 22.18 ).
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