7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction : Philip J Podrid, MD, FACC : Peter J Zimetbaum, MD : Susan B Yeon, MD, JD, FACC All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jun 2023. This topic last updated: May 02, 2022. INTRODUCTION The term cardiac preexcitation was originally used to describe premature activation of the ventricle prior to activation via the normal atrioventricular (AV) node His-Purkinje system in patients with the Wolff-Parkinson-White syndrome (WPW). This term has been broadened to include all conditions in which antegrade ventricular activation or retrograde atrial activation occurs partially or totally via an anomalous (or accessory) pathway distinct from the normal cardiac conduction system. The classic form of cardiac preexcitation remains the WPW pattern, which is characterized by a short PR interval (less than 120 milliseconds) and a broad QRS complex due to a delta wave resulting from direct myocardial activation that is slower than activation via the normal conduction system and hence the delta wave. The anatomic substrate for WPW pattern is a band of myocytes (which has similar electrophysiologic properties as the His-Purkinje system), also known as the bundle of Kent, which bridges the fibrous AV junction (ie, a direct atrial-ventricular accessory pathway that bypasses the AV node and normal His-Purkinje system). The electrocardiographic (ECG) features are a result of premature and direct ventricular myocardial activation due to conduction over the accessory pathway that directly innervates the ventricular myocardium. (See "Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome" and "Wolff-Parkinson-White syndrome: Anatomy, epidemiology, clinical manifestations, and diagnosis", section on 'Electrocardiographic findings'.) https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 1/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate Several other pathways have been postulated to result in cardiac preexcitation. However, most lack the histopathologic correlation that has been demonstrated for the WPW syndrome. The Lown-Ganong-Levine (LGL) pattern and enhanced AV nodal conduction (EAVNC) share some common features and have often been considered to have a similar etiology. The mechanisms proposed to account for these conditions include more rapid conduction within the AV node ("slick AV node") or as a result of a bypass of the normal AV nodal tissue ( figure 1). The LGL syndrome and EAVNC will be discussed in detail here. Mahaim fiber tachycardia, another non- WPW form of preexcitation, is discussed separately. (See "Atriofascicular ("Mahaim") pathway tachycardia".) LOWN-GANONG-LEVINE PATTERN The Lown-Ganong-Levine (LGL) pattern is characterized by the presence of a short PR interval (120 milliseconds) and normal QRS complex on the surface ECG. This finding may represent a perinodal accessory pathway or enhanced AV nodal conduction ( waveform 1). This bypass tract or accessory pathway is known as a bundle of James. This pathway links the atrial myocardium with the bundle of His. Thus, there is a short PR interval but a normal QRS complex as ventricular activation is still via the normal His-Purkinje system. Patients with palpitations who had a short PR interval but normal QRS complex on the resting ECG (ie, LGL syndrome) were first described in 1938 and then further evaluated by Lown, Ganong, and Levine in 1952 [1,2]. The latter report consisted of a retrospective examination of 13,500 consecutive ECGs at a single tertiary care center. 200 subjects were identified with a short PR interval, most of whom had a normal QRS complex [2]. The incidence of paroxysmal supraventricular tachycardia was significantly higher in these patients when compared with a control group with a normal PR interval (11 versus 0.5 percent). Electrophysiologic properties The electrophysiologic mechanism for the short PR interval in the LGL pattern is abnormal (fast) AV conduction with normal His-Purkinje fiber conduction. There is a bypass of the AV node with a tissue pathway, known as a bundle of James, that enters the bundle of His. This accounts for a short PR interval with normal QRS complexes. The following abnormalities have been found in patients with LGL pattern [3-7]: Abbreviated AH intervals at rest. Shortened AV nodal refractory times. Abnormal responses to rapid atrial pacing (ie, AH intervals remain constant as the rate of atrial pacing increases until there is failure of conduction, ie, block). This is due to the fact https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 2/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate that unlike the AV node, the accessory pathway (similar to the His-Purkinje system) manifests all-or-none conduction (ie, the pathway either conducts or does not conduct, and there is no change in the velocity of conduction when heart rate increases). The first two represent abnormal AV nodal function while the third can also represent an accessory pathway ( figure 1). Enhanced atrioventricular nodal conduction Since the description of the LGL pattern, it has been recognized that some of these patients have enhanced AV nodal conduction (EAVNC) [8]. Although both LGL and EAVNC refer to a presumed abnormality in or bypass of normal AV nodal function, diagnosis of the LGL pattern is based upon clinical and ECG findings while diagnosis of EAVNC requires the following specific electrophysiologic criteria [8]: AH interval in sinus rhythm of less than or equal to 60 milliseconds (normal 80 to 120 milliseconds in most subjects) 1:1 conduction between atrium and His bundle maintained during right atrial pacing at cycle lengths below 300 milliseconds AH prolongation 100 milliseconds at the shortest 1:1 conduction when compared with the sinus rhythm value The definition of EAVNC was subsequently broadened by including patients who had increase in AH interval of not more than 100 milliseconds during pacing at a cycle length of 300 milliseconds compared with the value measured during sinus rhythm. In one study, the individual criteria for EAVNC were fulfilled in 20 to 50 percent of patients undergoing electrophysiologic study for the evaluation of cardiac arrhythmia or syncope; however, only 11 percent fulfilled all three criteria for EAVNC [9]. Caution with sympathomimetic agents Drugs that can increase the heart rate or cause atrial tachyarrhythmias (eg, sympathomimetic agents like dextroamphetamine) should be used with caution in patients with LGL or EAVNC. While there are no data about the risk of any sympathomimetic agent and developing arrhythmias with LGL, there would seem to be similar pathophysiologic considerations as are seen in patients with the Wolff-Parkinson-White (WPW) syndrome. Sympathomimetic agents would enhance conduction through the AV node (making an AVRT more likely), as well as change refractoriness of the atrial myocardium (making atrial arrhythmias more likely and hence conducted more rapidly to the ventricle as a result of the bypass tract). Electrophysiology of EAVNC By definition, patients with EAVNC have an abnormally short AH interval and an abnormal magnitude and pattern of response to decremental or rapid atrial pacing. (See "Invasive diagnostic cardiac electrophysiology studies".) https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 3/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate With a progressive increase in the rate of atrial pacing, the normal AV node demonstrates a progressive physiologic conduction delay (decremental conduction) as manifested by increases in the PR and AH intervals (AV nodal Wenckebach) until complete AV block occurs. The progressive prolongation of AV nodal conduction time or AH interval with rapid atrial pacing has a protective role, limiting the ventricular response to rapid atrial rates in atrial fibrillation or atrial flutter. In patients with EAVNC and LGL, different patterns of AH response to decremental atrial pacing (ie, increasing pacing rates) have been elicited ( figure 2) [3-5,10,11]: The response to pacing may be similar to that of normal subjects, but the initial AH interval and magnitude of prolongation is less (type 1 response). There may be an initial increase in the AH interval followed by a plateau phase and then a further increase in the AH interval at faster pacing rates (type 2 response). Approximately 10 percent of subjects have little or no increase in the AH interval with incremental atrial pacing (type 3 response); this is the most abnormal response and is what is typically seen with LGL as a result of stable conduction velocity through the accessory pathway or bundle of James. There may be an initial flat AH response until very short pacing cycle lengths when marked AH prolongation occurs (type 4 response). The patterns of nodal refractoriness also differ. Most subjects with LGL pattern and EAVNC exhibit shorter AV nodal effective and functional refractory periods than control subjects but similar atrial refractoriness [3,4,7,12]. The criteria for EAVNC are empirically derived, raising a question as to whether this represents a discrete electrophysiologic entity. In one study of electrophysiologic evaluation in 160 consecutive patients, 11 percent fulfilled all criteria for EAVNC; however, there was a unimodal and continuous frequency distribution for the three criteria for EAVNC and all of the criteria fell within the lower end of their calculated normal distribution curves [9]. No factor reliably distinguished the subgroup of patients with abnormally rapid AV nodal conduction. This suggests that the criteria used to define EAVNC represent one end of the normal spectrum of AV nodal function rather than identifying a distinct group of individuals with an abnormality of the AV node. RELATIONSHIP BETWEEN LGL PATTERN AND EAVNC https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 4/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate While there is significant overlap between these two conditions, they are diagnostically distinct. Since the patients in the original description of LGL were not studied electrophysiologically, the basis for the short PR interval and incidence of EAVNC in this group is unknown. Few studies have examined EAVNC in patients with LGL pattern, and most published data on the LGL pattern do not allow an incidence to be accurately estimated. In one study, the incidence of EAVNC (defined as the presence of all three of the above criteria) in a group of subjects with LGL was found to be 58 percent [3], but other reports found an incidence of less than 30 percent [4-6]. A short PR interval is not always found in patients with EAVNC since AV nodal conduction is only one component of the PR interval; in addition, conduction velocity does not necessarily correlate with AV refractoriness which is the limiting factor in the ability of the AV node to conduct 1:1. However, LGL and EAVNC share some common features and have often been considered to have a similar etiology. The mechanisms proposed to account for these conditions include more rapid conduction within or bypass of the normal AV nodal tissue. ANATOMIC-PHYSIOLOGIC CORRELATION Little progress has been made in correlating observed physiologic responses with anatomic abnormalities in these syndromes. LGL and EAVNC probably represent one end of the normal spectrum of AV nodal conduction properties. This hypothesis is supported by the unimodal and near normal distribution of the PR interval and AV nodal conduction properties [9,13]. Nevertheless, it is impossible to exclude a distinct AV nodal bypass tract or an abnormality in conduction characteristics as the cause in some cases. The proposed explanations fall into two main groups, anatomic and physiologic. Anatomic theories One anatomic explanation centers around the proposed existence of a bypass tract or accessory pathway arising from within the atria and inserting into the low portion of the AV node or proximal portion of the His bundle. Such a pathway would bypass the transitional portion of the AV node which is largely responsible for conduction delay and AV nodal decremental properties ( figure 1) [4,14-18]: James fibers The existence of tracts of atrial tissue running from the atria and inserting into the low AV node (James fibers) has been well established [14]. These tracts are, however, present in all hearts and probably represent a normal part of the complex anatomy of the AV node. Their functional significance has not been established, but these fibers have been felt to be the anatomic basis responsible for LGL and abnormal AV conduction. https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 5/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate Brechenmacher fibers Brechenmacher fibers (atrio-Hisian tracts) could theoretically be present in the patients with EAVNC who show no decremental conduction during decremental atrial pacing (type 3 response) [15]; however there has not been clinicopathologic support for such tracts as the cause of the LGL pattern or EAVNC. The reported frequency of these tracts is 0.03 percent which is considerably lower than the expected incidence if they were to account for a significant proportion of individuals with abnormal AV nodal conduction. Intranodal bypass tracts The existence of intranodal bypass tracts would permit rapid conduction through the AV node, avoiding the usual decremental pathways. The fact that most patients with EAVNC show some degree of decrementation with incremental atrial pacing makes complete bypass of the AV node unlikely but does not exclude partial bypass. The normal AV node demonstrates decremental conduction in which adjacent cells are unable to fully excite subsequent cells in the conduction pathway. This property is enhanced by a short coupling interval (ie, increased rate) between successive electrical stimuli. Decremental conduction within the AV node is clinically manifest as a progressive prolongation in AV conduction time with increased atrial rates, ultimately resulting in Wenckebach block. (See "Second-degree atrioventricular block: Mobitz type I (Wenckebach block)".) Decremental conduction is diminished in patients with EAVNC. This could be explained by abnormal tissue in the region of the AV node, such as functional myocardial fibers rather than AV nodal tissue. Alternatively, the usual arrangement of the cellular fibers and the relatively poor intercellular excitation coupling, which are responsible for decremental conduction, may be abnormal [18]. Decremental conduction is absent in patients with LGL as there is tissue which bypasses the AV node and the bypass tract does not demonstrate decremental conduction. Another possible cause for the enhanced AV nodal conduction is an underdeveloped or anatomically small node [4,8,19-21]. The conduction properties of the AV node change with aging; infants and children are generally able to sustain more rapid conduction than adults [22,23]. The basis of this may not be anatomic but related to autonomic tone. Accurate premortem measurement of AV nodal size is not possible, so correlations between AV nodal conduction properties and AV nodal size have not been performed in humans. However, the lack of influence of size on the conduction properties of the AV node in other species suggests that this factor alone is not sufficient to account for the variation seen among individuals [24]. Physiologic theories The physiologic theories suggest that there is either an abnormality of the intrinsic conduction characteristics of the AV node or disturbed autonomic tone https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 6/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate [3,4,8,20,21,25]. Supporting a physiologic basis is the observation that most patients with EAVNC do show some degree of AH prolongation with decremental atrial pacing, a feature not usually seen with AV bypass tracts. In addition, many of these patients respond to a pharmacologic challenge in a manner similar to those with normal AV nodes [21,26-29]. Another theory is EAVNC represents conduction primarily over the fast pathway in patients with dual AV nodal physiology, thereby masking slow pathway conduction. However, the incidence of AV nodal reentry in patients with LGL or EAVNC is not appreciably higher than in subjects with normal AV nodal conduction [3,10,30,31]. Furthermore, in patients with EAVNC and dual pathways, both the fast and the slow pathways demonstrate enhanced conduction when compared with control subjects with dual AV nodal pathways but no evidence of EAVNC [7]. This finding suggests that a generalized abnormality of AV nodal conduction exists in patients with EAVNC. There is limited information concerning the role of the autonomic nervous system in patients with EAVNC. Although autonomic blockade may partially reverse the conduction abnormalities in these patients, rapid conduction is usually still present, suggesting that increased sympathetic and/or reduced vagal tone is not the sole factor responsible for this condition [20,27]. ARRHYTHMIAS ASSOCIATED WITH LGL SYNDROME AND EAVNC It is uncertain if an abnormality in AV nodal conduction is associated with arrhythmia. Although the LGL syndrome implies recurrent palpitations, it has not been conclusively established whether this is due to an increased incidence of tachyarrhythmias or symptoms resulting from a more rapid heart rate during sinus tachycardia. The incidence of EAVNC in the general population is difficult to establish since the diagnosis requires formal electrophysiologic testing. Thus, the reports of arrhythmias associated with EAVNC are based upon a very select group of individuals who are undergoing electrophysiologic testing. Etiology of arrhythmia EAVNC has been shown to coexist with dual AV nodal pathways and both overt and concealed accessory AV pathways [3,7,8,10,26,28,32,33]. Although the presence of overt preexcitation (ie, Wolff-Parkinson-White [WPW] pattern) precludes a diagnosis of LGL, this has been found in association with dual AV nodal pathways and concealed accessory AV connections. LGL The most common mechanism for arrhythmia in patients with the LGL syndrome, accounting for more than 50 percent of cases, is an orthodromic AV reentry tachycardia https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 7/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate (AVRT) using an accessory AV connection (ie, fast conduction to the ventricles via the accessory pathway [bundle of James] and retrograde activation of the atria via the AV node) [3,10,30]. (See "Atrioventricular reentrant tachycardia (AVRT) associated with an accessory pathway".) EAVNC In patients with EAVNC, the most common mechanism is AV nodal reentry tachycardia (AVNRT) if there are dual AV nodal pathways present. Although the relative incidence of these arrhythmias varies in different studies, it does not seem different from that reported in patients without LGL or EAVNC [34,35]. (See "Atrioventricular nodal reentrant tachycardia".) The presence of rapid conduction over the AV node is unlikely to increase the frequency of tachycardia in patients with AVNRT or AVRT since both result from slow conduction or complete block within the AV node. Thus, enhanced conduction in EAVNC should not favor initiation of these arrhythmias. Atrial fibrillation, atrial flutter, and ventricular tachycardia have also been described in patients with the LGL syndrome and EAVNC ( figure 3) [2,3,27,36,37]. A rapid ventricular response is possible with atrial fibrillation or atrial flutter. The most likely mechanism by which EAVNC may predispose to ventricular arrhythmias is the degeneration of rapid ventricular response to atrial arrhythmia into ventricular tachycardia and fibrillation. This occurrence has been reported in patients with and without underlying cardiac disease but is uncommon [3,36,38]. It is also a complication of atrial fibrillation in the WPW syndrome. (See "Wolff-Parkinson-White syndrome: Anatomy, epidemiology, clinical manifestations, and diagnosis".) Influence of EAVNC on the tachycardia rate The contribution of EAVNC to the ventricular rate during a tachycardia is dependent upon the type of tachycardia. Some studies have suggested that EAVNC is associated with a more rapid heart rate during AVRT and atrial fibrillation or flutter [3,8,27,33]. AVRT The rate of an AVRT is limited by the conduction through the weakest limb of the circuit, which in orthodromic AVRT is conduction over the AV node. In this situation, the presence of EAVNC may result in more rapid heart rates as AV conduction is enhanced [10,14]. In LGL, the antegrade conduction to the ventricles during orthodromic AVRT is via the accessory pathway, which conducts rapidly while retrograde conduction back to the atria is via the AV node, which conducts slower. (See "Atrioventricular reentrant tachycardia (AVRT) associated with an accessory pathway".) https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 8/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate AVNRT In patients with the typical form of AVNRT, the slow pathway within the AV node is the weak limb. Since the rate of the tachycardia is not critically dependent upon the conduction characteristics of the fast AV nodal pathway, the limb presumed to be abnormal in EAVNC, the presence of EAVNC does not alter the rate of a typical AVNRT [10,29]. Atrial fibrillation or atrial flutter The ventricular response during atrial fibrillation or flutter is significantly faster in patients with EAVNC or LGL than in control subjects [3,27,36,38]. Cases of 1:1 AV conduction in excess of 300 beats/min have been documented in patients with EAVNC or LGL [27]. Similar to the situation with the Wolff-Parkinson-White syndrome, the propensity for rapid ventricular responses to atrial fibrillation and atrial flutter makes the aggressive treatment of these arrhythmias necessary, particularly in patients who have underlying ischemic heart disease or cardiomyopathy who not are likely to tolerate rapid ventricular rates. ANTIARRHYTHMIC DRUGS The effects of various classes of antiarrhythmic drugs in patients with EAVNC compared with controls have not been extensively studied. This is complicated by the fact that the LGL syndrome and EAVNC probably have multiple causes. Digitalis Digitalis and other cardiac glycosides exert their major effect in supraventricular tachycardia by enhancement of vagal tone at the level of the AV node. It has been postulated that patients with EAVNC have reduced parasympathetic tone and an attenuated AV nodal response to acetylcholine [21]. In one series of eight patients with EAVNC and medically refractory AVRT, digitalis produced little or no slowing of antegrade AV nodal conduction or prolongation of AV nodal refractoriness [33]. Digitalis does not have any effect on the accessory pathway in LGL, although it may be of benefit because of slowing conduction via the AV node. The change in retrograde AV nodal conduction and refractoriness may prevent AVRT in these patients. Beta blockers The effects of beta blockers in patients with EAVNC has varied widely. In some studies, beta blockers are equally effective in slowing AV nodal conduction and the tachycardia rate in patients with and without EAVNC [11,26]. However, other reports have found no effect in EAVNC [33,39]. Beta blockers have no direct effect on the accessory pathway in LGL but like digitalis may slow conduction through the AV node and may be of benefit, similar to what can be seen with digitalis. https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 9/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate Calcium channel blockers The effects of calcium blockers, in particular verapamil, are variable in LGL and EAVNC. In one study, verapamil had little effect on AV nodal conduction characteristics or refractory periods in the subjects with a short PR interval in contrast to significant depressant effects seen in a control group [40]. In contrast, another report found that verapamil substantially affected both AV conduction time and refractoriness of the AV node in patients with EAVNC or LGL [41]. Similar to digitalis and beta blockers, there is no direct effect on the accessory pathway properties. The ability of oral verapamil to prevent tachycardia in patients with a short PR interval has not been evaluated in a controlled trial. In one series, verapamil was ineffective or only marginally effective in six of nine patients with recurrent supraventricular tachycardia and EAVNC [30]. (See "Calcium channel blockers in the treatment of cardiac arrhythmias".) Class I and III antiarrhythmic drugs There is even less information about the effectiveness of class I and III antiarrhythmic drugs in patients with the LGL syndrome or EAVNC. It has been proposed that AV nodal conduction in these patients is less calcium channel-dependent and less influenced by parasympathetic tone [11]. Thus, drugs with predominant sodium channel- blocking activity may have more of an effect on AV conduction than in normal individuals. As the accessory pathway in LGL (bundle of James) is similar to His-Purkinje tissue, these antiarrhythmic drugs may slow conduction or prolong refractoriness in this pathway and hence may prevent AVRT. Agents such as sotalol or amiodarone which have multiple effects may be particularly effective, but this remains to be confirmed. RADIOFREQUENCY CATHETER ABLATION In recent years, catheter ablation has become the preferred therapy for various supraventricular tachycardias and for tachyarrhythmias in the Wolff-Parkinson-White (WPW) syndrome. The majority of patients with a short PR interval or EAVNC who have reentrant tachycardias are suitable for ablation directed at an accessory pathway (if documented) or one limb of the AV node (if dual AV nodal pathways are present). In patients who have rapid ventricular responses to atrial fibrillation or atrial flutter, an effective treatment is the creation of complete heart block using radiofrequency ablation followed by implantation of a permanent pacemaker [42]. SUMMARY AND RECOMMENDATIONS Definitions The Lown-Ganong-Levine (LGL) pattern is characterized by the presence of a short PR interval and normal QRS complex on the surface ECG, a finding which may https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 10/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate represent a perinodal accessory pathway or enhanced atrioventricular nodal conduction (EAVNC). Although both LGL and EAVNC refer to a presumed abnormality in or bypass of normal AV nodal function, diagnosis of the LGL pattern versus EAVNC is based upon clinical and ECG findings as well as specific electrophysiologic criteria measured during invasive electrophysiology studies. (See 'Lown-Ganong-Levine pattern' above.) Pathophysiology Despite evidence to partially support numerous theories, the precise mechanisms for the LGL pattern and EAVNC, as well as the anatomic and physiologic correlation, remain undetermined. (See 'Anatomic-physiologic correlation' above.) Arrhythmias (See 'Arrhythmias associated with LGL syndrome and EAVNC' above.) LGL The most common mechanism for arrhythmia in patients with the LGL syndrome, accounting for more than 50 percent of cases, is an orthodromic AV reentry tachycardia (AVRT) using an accessory AV connection (the bundle of James). Paroxysmal supraventricular tachycardia (particularly AVRT) occurs much more frequently in patients with LGL syndrome when compared with persons with a normal PR interval. (See 'Lown-Ganong-Levine pattern' above.) EAVNC In EAVNC, the mechanism for arrhythmia is most likely AV nodal reentry tachycardia (AVNRT) due to dual AV nodal pathways. Atrial fibrillation, atrial flutter, and ventricular tachycardia have also been described. Management The optimal treatment of arrhythmias in the LGL syndrome and EAVNC is unclear, but radiofrequency catheter ablation appears to be a suitable and effective option for such patients. (See 'Antiarrhythmic drugs' above and 'Radiofrequency catheter ablation' above.) Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Clerc, A, Robert-Levy, et al. A propos du raccourcissement permanent de l'espace P-R del'electrocardiogramme sans deformation du complexe ventriculaire. Arch Mal Coeur 1938; 31:569. 2. LOWN B, GANONG WF, LEVINE SA. 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Role of electrophysiologic testing in the diagnosis and treatment of patients with known and suspected bradycardias and tachycardias. Prog Cardiovasc Dis 1981; 24:25. 35. Josephson ME. Paroxysmal supraventricular tachycardia: an electrophysiologic approach. Am J Cardiol 1978; 41:1123. 36. Myerburg RJ, Sung RJ, Castellanos A. Ventricular tachycardia and ventricular fibrillation in patients with short P-R intervals and narrow QRS complexes. Pacing Clin Electrophysiol 1979; 2:568. 37. Blanc, JJ, Fontaliran, et al. Electrophysiologic and histopathologic correlation (abstract). Pacing Clin Electrophysiol 1981; 4:A. 38. Castellanos A, Vagueiro MC, Befeler B, Myerburg RJ. Syndrome of short P-R, narrow QRS and repetitive supraventricular tachyarrhythmias: the possible occurrence of the R-on-T phenomenon and the limits of this syndrome. Eur J Cardiol 1975; 2:337. 39. Prystowsky EN, Greer S, Packer DL, et al. Beta-blocker therapy for the Wolff-Parkinson-White syndrome. 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Topic 1000 Version 17.0 https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 14/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate GRAPHICS Types of accessory pathways Diagram of 4 types of abnormal conduction pathways: classic accessory atrioventricular pathway (red), atriofascicular pathway (green), fasciculoventricular pathway (light blue), and nodoventricular pathway (dark blue). The atrioventricular node and His-Purkinje system are represented in yellow. Atrioventricular, atriofascicular, and nodoventricular pathways are capable of supporting atrioventricular reciprocating tachycardias. In contrast, an isolated fasciculoventricular pathway can cause subtle preexcitation but does not support clinical tachycardia. Graphic 138850 Version 1.0 https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 15/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate Lown-Ganong-Levine syndrome The 12 lead ECG of a patient with the Lown-Ganong-Levine syndrome shows a short PR interval of 0.10 second and a normal QRS complex. Graphic 74221 Version 2.0 Sinus rhythm The normal P wave in sinus rhythm is slightly notched since activation of the right atrium precedes that of the left atrium. The P wave is upright in a positive direction in leads I and II. A P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals are identical from beat to beat. Graphic 69872 Version 2.0 https://www.uptodate.com/contents/lown-ganong-levine-syndrome-and-enhanced-atrioventricular-nodal-conduction/print 16/19 7/6/23, 11:14 AM Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction - UpToDate Patterns of AV nodal conduction in EAVNC