SUMMARY https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 11/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Patients with a preexcitation syndrome have an additional or alternative pathway, known as an accessory pathway, which directly connects the atria and ventricle and bypasses the atrioventricular (AV) node. During sinus rhythm, AV conduction through an accessory pathway (most commonly a direct AV connection) results in the earlier activation of the ventricles than if the impulse had traveled through the AV node, resulting in ventricular preexcitation. (See 'Introduction' above.) Some accessory pathways are associated with structural heart abnormalities. (See 'Anatomic considerations' above.) The classic accessory pathway is the AV bypass tract in patients with the Wolff-Parkinson- White (WPW) pattern. This pathway directly connects atrial and ventricular myocardium, bypassing the AV node/His-Purkinje system. (See 'Anatomic considerations' above.) Delta waves may not be apparent in sinus rhythm in patients with WPW who have a left- lateral bypass tract as the antegrade route for conduction. (See 'Minimal preexcitation in WPW' above.) Although AV accessory pathways usually conduct antegrade and retrograde, some AV bypass tracts are capable of propagating impulses in only one direction. (See 'Concealed accessory pathways' above.) The electrocardiographic (ECG) appearance of activation depends upon the extent of preexcitation and fusion; as a result, the same pathway may not always produce the identical ECG pattern. (See 'Relationship between accessory pathway site and the electrocardiographic pattern' above.) Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Wolff, L, Parkinson, et al. 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Importance of preexcited QRS morphology during induced atrial fibrillation to the diagnosis and localization of multiple accessory pathways. Circulation 1990; 81:578. 12. Lindsay BD, Crossen KJ, Cain ME. Concordance of distinguishing electrocardiographic features during sinus rhythm with the location of accessory pathways in the Wolff- Parkinson-White syndrome. Am J Cardiol 1987; 59:1093. 13. Milstein S, Sharma AD, Guiraudon GM, Klein GJ. An algorithm for the electrocardiographic localization of accessory pathways in the Wolff-Parkinson-White syndrome. Pacing Clin Electrophysiol 1987; 10:555. 14. Cain ME, Luke RA, Lindsay BD. Diagnosis and localization of accessory pathways. Pacing Clin Electrophysiol 1992; 15:801. 15. Otomo K, Gonzalez MD, Beckman KJ, et al. Reversing the direction of paced ventricular and atrial wavefronts reveals an oblique course in accessory AV pathways and improves localization for catheter ablation. Circulation 2001; 104:550. 16. MacRae CA, Ghaisas N, Kass S, et al. Familial Hypertrophic cardiomyopathy with Wolff- Parkinson-White syndrome maps to a locus on chromosome 7q3. J Clin Invest 1995; 96:1216. 17. Dai CC, Guo BJ, Li WX, et al. Dyssynchronous ventricular contraction in Wolff-Parkinson- White syndrome: a risk factor for the development of dilated cardiomyopathy. Eur J Pediatr 2013; 172:1491. https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 13/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 18. Chiu SN, Chang CW, Lu CW, Wu MH. Restored cardiac function after successful resynchronization by right anterior and anteroseptal accessory pathway ablation in Wolff- Parkinson-White syndrome associated dilated cardiomyopathy. Int J Cardiol 2013; 163:e19. 19. Chauvin M, Shah DC, Ha ssaguerre M, et al. The anatomic basis of connections between the coronary sinus musculature and the left atrium in humans. Circulation 2000; 101:647. 20. Sun Y, Arruda M, Otomo K, et al. Coronary sinus-ventricular accessory connections producing posteroseptal and left posterior accessory pathways: incidence and electrophysiological identification. Circulation 2002; 106:1362. 21. Prystowsky EN, Page RL. Electrophysiology and autonomic influences of the human atrioven tricular node. In: Electrophysiology of the Sinoatrial and Atrioventricular Nodes, Mazgalev T, Dreifus L, Michelson EL (Eds). 22. Sternick EB, Timmermans C, Sosa E, et al. The electrocardiogram during sinus rhythm and tachycardia in patients with Mahaim fibers: the importance of an "rS" pattern in lead III. J Am Coll Cardiol 2004; 44:1626. 23. Gaita F, Haissaguerre M, Giustetto C, et al. Catheter ablation of permanent junctional reciprocating tachycardia with radiofrequency current. J Am Coll Cardiol 1995; 25:648. 24. de Chillou C, Rodriguez LM, Schl pfer J, et al. Clinical characteristics and electrophysiologic properties of atrioventricular accessory pathways: importance of the accessory pathway location. J Am Coll Cardiol 1992; 20:666. 25. Bogun F, Kalusche D, Li YG, et al. Septal Q waves in surface electrocardiographic lead V6 exclude minimal ventricular preexcitation. Am J Cardiol 1999; 84:101. 26. Yoshida T, Ikeda H, Hiraki T, et al. Detection of concealed left sided accessory atrioventricular pathway by P wave signal averaged electrocardiogram. J Am Coll Cardiol 1999; 33:55. 27. Josephson ME. Preexcitation Syndromes. In: Clinical Cardiac Electrophysiology, Lea & Febige r, Philadelphia 1993. p.311. 28. German LD, Gallagher JJ. Functional properties of accessory atrioventricular pathways in Wolff-Parkinson-White syndrome. Clinical implications. Am J Med 1984; 76:1079. 29. Nalos, PC, Deng, et al. Intermittent preexcitation: Clinical recognition and management. Pract Cardiol 1985; 11:49. 30. Chen SA, Tai CT, Chiang CE, et al. Electrophysiologic characteristics, electropharmacologic responses and radiofrequency ablation in patients with decremental accessory pathway. J Am Coll Cardiol 1996; 28:732. https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 14/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 31. Murdock CJ, Leitch JW, Teo WS, et al. Characteristics of accessory pathways exhibiting decremental conduction. Am J Cardiol 1991; 67:506. 32. Tchou P, Lehmann MH, Jazayeri M, Akhtar M. Atriofascicular connection or a nodoventricular Mahaim fiber? Electrophysiologic elucidation of the pathway and associated reentrant circuit. Circulation 1988; 77:837. 33. Gillette PC, Garson A Jr, Cooley DA, McNamara DG. Prolonged and decremental antegrade conduction properties in right anterior accessory connections: Wide QRS antidromic tachycardia of left bundle branch block pattern without Wolff-Parkinson-White configuration in sinus rhythm. Am Heart J 1982; 103:66. 34. Ross DL, Uther JB. Diagnosis of concealed accessory pathways in supraventricular tachycardia. Pacing Clin Electrophysiol 1984; 7:1069. 35. Gillette PC. Concealed anomalous cardiac conduction pathways: a frequent cause of supraventricular tachycardia. Am J Cardiol 1977; 40:848. 36. Meiltz A, Weber R, Halimi F, et al. Permanent form of junctional reciprocating tachycardia in adults: peculiar features and results of radiofrequency catheter ablation. Europace 2006; 8:21. 37. Fitzpatrick AP, Gonzales RP, Lesh MD, et al. New algorithm for the localization of accessory atrioventricular connections using a baseline electrocardiogram. J Am Coll Cardiol 1994; 23:107. 38. Gallagher JJ, Sealy WC, Kasell J, Wallace AG. Multiple accessory pathways in patients with the pre-excitation syndrome. Circulation 1976; 54:571. 39. Arruda, M, Wang, et al. ECG algorithm for predicting sites of successful radiofrequency ablation of accessory pathways (abstract). Pacing Clin Electrophysiol 1993; 16(Pt 2):865. 40. Epstein AE, Kirklin JK, Holman WL, et al. Intermediate septal accessory pathways: electrocardiographic characteristics, electrophysiologic observations and their surgical implications. J Am Coll Cardiol 1991; 17:1570. 41. Arruda MS, McClelland JH, Wang X, et al. Development and validation of an ECG algorithm for identifying accessory pathway ablation site in Wolff-Parkinson-White syndrome. J Cardiovasc Electrophysiol 1998; 9:2. 42. Moss JD, Gerstenfeld EP, Deo R, et al. ECG criteria for accurate localization of left anterolateral and posterolateral accessory pathways. Pacing Clin Electrophysiol 2012; 35:1444. 43. Sternick EB, Sosa EA, Timmermans C, et al. Automaticity in Mahaim fibers. J Cardiovasc Electrophysiol 2004; 15:738. https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 15/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 44. Sternick EB, Gerken LM. The 12-lead ECG in patients with Mahaim fibers. Ann Noninvasive Electrocardiol 2006; 11:63. Topic 953 Version 28.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 16/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate GRAPHICS Terminology and anatomic connections of preexcitation pathways Old Proposed or commonly used Anatomic connections terminology terminology Kent bundle* Accessory AV connection or AV bypass Atrium to ventricle tract James fiber Atrionodal bypass tract Atrium to low AV node Atriofascicular bypass tract Atrium to bundle of His Mahaim fiber Atriofascicular bypass tract Atrium to bundle branch Nodofascicular bypass tract AV node to bundle branch Nodoventricular bypass tract AV node to ventricular tissue Fasciculoventricular bypass tract Bundle branch to ventricular tissue AV: atrioventricular. These bypass tracts result in delta waves and the Wolff-Parkinson-White syndrome. These bypass tracts result in the Lown-Ganong-Levine syndrome and enhanced AV nodal conduction. Graphic 53343 Version 6.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 17/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 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/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 18/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Schematic showing anatomic locations of accessory pathways as they cross the AV junction The atrioventricular (AV) valve plane of the heart is viewed from the cardiac apex. The tricuspid valve is on the left in this figure and the mitral valve on the right; the coronary sinus, which provides the venous drainage of the heart, passes posterior to the mitral valve and empties into the right atrium above the tricuspid valve. All AV bypass tracts cross the AV valve plane; their locations are named with respect to this orientation (eg, an anteroseptal bypass tract crosses the AV valve plane from atrium to ventricle in the region marked "anteroseptal" in the figure, at approximately 10 to 11 o'clock with respect to the mitral valve ring). Reproduced with permission from: Cosio FG, Anderson RH, Becker A, et al. Living anatomy of the atrioventricular junctions. A guide to electrophysiological Statement from the Cardiac Nomenclature Study Group, Working Group of Arrythmias, European Society of Cardiology, and the Task Force on Cardiac Nomenclature from NASPE. North American Society of Pacing and Electrophysiology. Eur Heart J 1999; 20:1068. Copyright 1999 European Society of Cardiology. Graphic 77941 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 19/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Schematic showing anatomic locations of accessory pathways in the left ventricular (LV) free wall Longitudinal cross section of the free wall of the left ventricle at the level of the atrioventricular (AV) groove. The accessory pathways are present in the AV groove fat pad at variable depths in relation to the mitral annulus and the epicardium overlying the AV groove. Graphic 76316 Version 2.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 20/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate AV conduction with a concealed accessory pathway Schematic representation of AV conduction. The normal pacemaker is in the sinoatrial (SA) node at the junction of the superior vena cava and the right atrium. The SA node activates the right and left atria (shown in green). In the absence of an accessory pathway (AP) or, as in this case, if the AP is concealed, ventricular activation results from the impulse traversing the AV node, the specialized infranodal conducting system (His bundle, bundle branches, and fascicular branches, shown in red), thereby activating the ventricular myocardium (shown in yellow). The ECG shows a normal PR interval and a narrow QRS complex. The inset on the right shows the timing of SA node (SAN), right (RA) and left atrial (LA), His bundle (H), and the beginning of normal ventricular activation (V ). All of ventricular activation (shown in yellow) is due to normal AV nodal and infranodal conduction. N AV: atrioventricular; ECG: electrocardiogram. Graphic 73740 Version 5.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 21/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate AV conduction through an overt accessory pathway Compared with normal conduction in the preceding diagram, the accessory pathway (AP) is now overt. As a result, ventricular activation results from both early activation (pre-excitation) of the free wall of the left ventricle (shown in blue) and from normal activation (shown in yellow). The degree of unopposed pre-excitation depends upon the time required to conduct through the right and left atria, the AP, and the ventricular myocardium as compared with conduction through the normal pathways. The inset on the right shows the ECG timing of these events. The net effect is a QRS complex that is a fusion of ventricular pre-excitation (blue) and normal excitation (yellow). Early activation throughout the AP (V ) P occurs at about the same time as His bundle depolarization (H). This leads to a shorter PR interval, a small delta wave (arrow), and some prolongation of the QRS duration. AV: atrioventricular. Graphic 53773 Version 6.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 22/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Conduction through an accessory pathway with AV nodal delay Compared with conduction through an AP with normal AV node conduction, delayed conduction through the AV node allows more of the ventricular myocardium to be activated by pre-excitation (shown in blue). The inset on the right shows the ECG timing of these events. The atrial to His interval is increased due to the AV nodal delay (RA to H); His activation is so delayed that it follows activation caused by the AP (V ). The PR interval is short due to the pre- excitation, the delta wave (arrow) is more pronounced due to the greater and unopposed early forces (blue), and the QRS duration is P prolonged due to the later than normal ventricular activation caused by the AV nodal delay (yellow). AV: atrioventricular; ECG: electrocardiogram. Graphic 62454 Version 6.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 23/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) showing concertina effect in Wolff-Parkinson-White syndrome The 12-lead ECG and rhythm strip during sinus rhythm show a short PR interval and QRS complex which is aberrated and has a delta wave. After the fifth complex on the rhythm strip, there is a sinus pause after which the QRS compex is still aberrated but the PR interval is somewhat longer. Subsequently, the PR interval becomes progressively shorter. This variability in the PR interval during sinus rhythm is known as the "concertina effect." Graphic 53508 Version 6.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 24/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Single-lead electrocardiogram (ECG) strip showing concertina effect in Wolff-Parkinson-White syndrome Single channel rhythm strip from a patient with the Wolff-Parkinson- White syndrome shows progressive shortening of the PR interval with the development of a widened QRS complex due to a delta wave. The P wave appears to merge with the QRS complex (arrows). The last two QRS complexes again show a normal PR interval and a narrow QRS complex. Graphic 65293 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 25/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Algorithm (Milstein) to localize accessory pathway in preexcitation syndrome Mapping algorithm for localization of accessory pathway in the preexcitation syndrome using the morphology of the delta wave on the electrocardiogram. LBBB: left bundle branch block; RAS: right anteroseptal; LL: left lateral; PS: posteroseptal; RL: right lateral. + QRS 90 msec in L1 and rS in V1 and V2. Reproduced with permission from Milstein S, Sharma AD, Guiraudon GM, et al. Pacing Clin Electrophysiol 1987; 10:555. Copyright 1987 Futura Publishing Company. Graphic 70648 Version 5.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 26/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Algorithm (Arruda) to localize accessory pathway in preexcitation syndrome https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 27/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 28/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Mapping algorithm for localization of accessory pathway in the preexcitation syndrome using the morphology of the delta wave on the electrocardiogram. R/S: R-S wave ratio; PPV: positive predictive value; LL: left lateral; LAL: left anterolateral; LP: left posterior; LPL: left posterolateral; CS/MCV: coronary sinus/middle cardiac vein; AS: right anteroseptal; MS: midseptal; LPS: left posteroseptal; RPS: right posteroseptal; RA: right anterior; RAP: right anterior paraseptal; RAL: right anterolateral; RL: right lateral; RP: right posterior; RPL: right posterolateral. Data from Arruda M, Wang X, McClelland J, et al, Pacing Clin Electrophysiol 1993; 16:865. Graphic 57692 Version 5.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 29/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a right anterior septal accessory AV pathway Electrocardiogram from a patient with Wolff-Parkinson-White syndrome during sinus tachycardia shows a maximally preexcited QRS complex which has a left bundle branch block pattern and is markedly positive in the inferior leads. The delta wave is isoelectric in leads aVL and V6. These features localize the accessory pathway to the right anterior septum. Graphic 50415 Version 4.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 30/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a left lateral accessory AV pathway Electrocardiogram in sinus rhythm from a patient with the Wolff-Parkinson- White syndrome shows a maximally preexcited QRS. The PR is short, the QRS has a right bundle branch block morphology, there is a prominent RS complex in V1, the QRS is negative, and the delta wave is isoelectric in leads I and aVL, localizing the accessory pathway to the left lateral annulus. Graphic 74370 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 31/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a left lateral accessory AV pathway Electrocardiogram in sinus rhythm from a patient with Wolff-Parkinson- White syndrome shows a widened QRS complex which is not maximally preexcited. Although the delta wave and QRS are positive in lead I, the negative delta wave in lead aVL and the right bundle branch block morphology localize the accessory pathway to the left lateral mitral annulus. Graphic 50322 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 32/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a right posteroseptal accessory AV pathway Electrocardiogram in sinus rhythm from a patient with Wolff-Parkinson-White syndrome shows a maximally preexcited QRS complex. There is early transition across the precordial leads, with R/S ratio of 1 occurring in lead V2. The delta waves are positive in the lateral leads I, aVL, and V6, and negative in leads III and aVF, localizing the pathway to the posteroseptal area. The positive delta wave in lead II and in the lateral leads localize the pathway to the right side. Graphic 69988 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 33/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a right posteroseptal accessory AV pathway Electrocardiogram during rapid atrial pacing from a patient with Wolff- Parkinson-White syndrome. The QRS complex is maximally excited; the PR interval is short, and there is a left bundle branch morphology, left axis deviation, and early transition across the anterior precordial leads. The delta waves are positive in the lateral leads I, aVL, and V6 and negative in the inferior leads, localizing the pathway to the posteroseptal region. The negative delta wave in lead II places the pathway on the right, in the area between the coronary sinus os and middle cardiac vein. Graphic 57211 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 34/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a right posteroseptal accessory AV pathway The 12 lead ECG during rapid atrial pacing shows QRS complexes that are maximally preexcited; the complexes have a pattern suggesting an AV accessory pathway located in the right posteroseptal area. At the time of ablation, the pathway was localized to the midseptal region. Graphic 59055 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 35/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a left posteroseptal accessory AV pathway Electrocardiogram during right atrial pacing in a patient with Wolff- Parkinson-White syndrome shows a long stimulus to the P wave with a short PR interval, the appearance of both right and left bundle branch block morphologies, and a negative QRS morphology and delta wave in the inferior leads. This localizes the pathway to the posteroseptal area. The presence of a positive delta wave in lead I, RS ratio in lead V1 of approximately 1, and a negative delta wave in lead II places the pathway on the left side, inside the coronary sinus os or along the middle cardiac vein. This type of accessory pathway may be related to cardiac venous anomalies and aneurysms. Graphic 71868 Version 4.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 36/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a right lateral accessory AV pathway Electrocardiogram in sinus rhythm from a patient with Wolff-Parkinson-White syndrome shows classic preexcitation with a short PR interval, delta wave, and widened QRS complex with a left bundle branch block morphology. The delta wave is positive in the lateral leads 1, aVL, and V6, it is negative in lead III and isoelectric in lead aVF. The QRS axis of <30L distinguishes this pathway from an anteroseptal accessory AV pathway. Courtesy of Morton F Arnsdorf, MD. Graphic 77218 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 37/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a left anterior lateral accessory pathway Electrocardiogram during atrial pacing in a patient with the Wolff- Parkinson-White syndrome shows QRS complexes that are maximally preexcited. The PR interval is short, there is a right bundle branch morphology, and the axis is rightward. The delta wave is positive in the lateral leads I, avL, and V6, localizing the pathway to the left anterior lateral region. Courtesy of Morton F Arnsdorf, MD. Graphic 69928 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 38/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate 12-lead electrocardiogram (ECG) of a midseptal accessory AV pathway Electrocardiogram from a patient with Wolff-Parkinson-White syndrome. The ECG criteria for a midseptal pathway are intermediate between those for posteroseptal and anteroseptal connections within the triangle of Koch. These pathways have been difficult to predict from the surface ECG and have usually been grouped as septal pathways. The ECG shows positive delta waves in the lateral leads I, aVL, and V6, with early transition and a tall R wave in lead V2. The delta wave is positive in leads II and aVF and negative in lead III, and the QRS axis is <0 , localizing the pathway in the midseptum. Graphic 51236 Version 3.0 https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 39/40 7/5/23, 10:45 AM Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome - UpToDate Contributor Disclosures Bradley P Knight, MD, FACC Grant/Research/Clinical Trial Support: Abbott [Electrophysiology]; Atricure [Electrophysiology]; Biosense Webster [Electrophysiology]; Biotronik [Electrophysiology]; Boston Scientific [Electrophysiology]; BSCI [Electrophysiology]; MDT [Electrophysiology]; Philips [Electrophysiology]. Consultant/Advisory Boards: Abbott [Electrophysiology]; Atricure [Electrophysiology]; Biosense Webster [Electrophysiology]; Biotronik [Electrophysiology]; Boston Scientific [Electrophysiology]; BSCI [Electrophysiology]; CVRx [Heart failure]; MDT [Electrophysiology]; Philips [Electrophysiology]; Sanofi [Arrhythmias]. Speaker's Bureau: Abbott [Electrophysiology]; Biosense Webster [Electrophysiology]; Biotronik [Electrophysiology]; Boston Scientific [Transeptal catheterization]; BSCI [Electrophysiology]; MDT [Electrophysiology]. All of the relevant financial relationships listed have been mitigated. Peter J Zimetbaum, MD Consultant/Advisory Boards: Abbott Medical [Lead extraction]. All of the relevant financial relationships listed have been mitigated. Susan B Yeon, MD, JD, FACC No relevant financial relationship(s) with ineligible companies to disclose. Contributor disclosures are reviewed for conflicts of interest by the editorial group. When found, these are addressed by vetting through a multi-level review process, and through requirements for references to be provided to support the content. Appropriately referenced content is required of all authors and must conform to UpToDate standards of evidence. Conflict of interest policy https://www.uptodate.com/contents/anatomy-pathophysiology-and-localization-of-accessory-pathways-in-the-preexcitation-syndrome/print 40/40