INTRODUCTION — The presence of a short PR interval, frequently with a delta wave, defines the preexcitation syndrome. While no clear arrhythmia is associated with Lown-Ganong-Levine syndrome, patients with Wolff-Parkinson-White syndrome may have atrioventricular (AV) reentrant tachycardia or atrial fibrillation/flutter.
LOWN-GANONG-LEVINE SYNDROME — A short PR interval, <0.12 sec, with a narrow (normal) complex QRS and palpitations or supraventricular tachycardia has been termed the Lown-Ganong-Levine (LGL) syndrome (waveform 1). The exact mechanism of LGL has not been completely described but is due to one of three possibilities. The most likely is due to faster atrioventricular (AV) nodal conduction, possibly due to rapidly conducting fibers within the AV node. There may also be atrial-His connections (Brechenmacher fibers) that bypass the AV node and lead to a short PR interval. Lastly, the classic description is due to the James bundle, which is an accessory pathway that links the atrium to the low AV node. James fibers are present in all hearts and are likely part of the usual AV node anatomy. (See "General principles of asynchronous activation and preexcitation" and "Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome" and "Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction".)
Conduction occurs more rapidly than normal from the atria to the ventricles, explaining the short PR. The QRS complex is normal in duration and morphology since ventricular activation is via the normal conduction pathway (His Purkinje system).
WOLFF-PARKINSON-WHITE SYNDROME — The Wolff-Parkinson-White (WPW) pattern results from an accessory pathway, the Kent bundle, which directly links the atria to the ventricles, bypassing the AV node. The ventricular myocardium is activated early as a result of this bypass pathway, prior to activation via the normal AV node/His-Purkinje pathway (see "Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome"). WPW syndrome is defined by the WPW pattern in addition to arrhythmias as described below.
The electrocardiogram (ECG) demonstrates a short PR interval (<0.12 sec) and a delta wave (slurred and broad upstroke of the QRS complex), representing early ventricular activation via the abnormal accessory pathway (waveform 2 and waveform 3A-B). The QRS complex is wide (>0.12 sec) and bizarre appearing. This is due to myocardial activation directly through the ventricular myocardium fusing with myocardial activation using the His-Purkinje system. Thus, the QRS complex in WPW represents a fusion beat; the initial part results from slow ventricular activation via the accessory pathway, while the terminal portion of ventricular activation is via the normal conduction system. Often there are associated ST segment and T wave abnormalities reflecting abnormal ventricular repolarization.
The extent of preexcitation may be labile depending upon the refractory period and conduction characteristics of the accessory pathway and AV node; the latter in turn depends upon sympathetic tone. There may be episodic, random, and sporadic changes in the width of the delta wave. Thus, the PR interval and QRS duration may be variable, unassociated with changes in heart rate; this has been termed a "concertina effect."
Older ECG schema described two types of QRS patterns identified in patients with WPW:
●Type A due to a left sided bypass pathway, in which there is a tall R wave in leads V1 to V3 (eg, a positive or upward delta wave).
●Type B due to a right sided bypass pathway in which there are QS complexes in leads V1 to V3 (eg, a negative or downward delta wave).
Bypass pathways may be located anywhere along the AV ring (groove) or in the septum. The most frequent locations are left lateral (46 to 60 percent), posteroseptal (25 percent), right lateral (13 to 21 percent), and anteroseptal (2 percent). Various algorithms have been developed to help determine pathway location [1-4].(See "Anatomy, pathophysiology, and localization of accessory pathways in the preexcitation syndrome".)
There are other rare types of bypass pathways that may give abnormal ECG appearances. These include atriofascicular pathways, which directly insert from the atrium into the right bundle branch and can be associated with atrioventricular reentrant tachycardia (AVRT); and fasciculoventricular pathways, which connect the right bundle branch and the ventricular myocardium, but have not been associated with arrhythmias. Atriofascicular pathways usually have a precordial transition at or after V4, R in I, rS in V1, and left axis deviation. Fasciculoventricular pathways usually have a small delta wave but normal PR interval. In addition, nodofascicular and nodoventricular pathways, connecting the AV node and one of the fascicles or ventricular tissue, respectively, may be manifest on ECG, though usually do not show significant pre-excitation. (See "Atriofascicular ("Mahaim") pathway tachycardia".)
Atrioventricular reentrant tachycardia — The presence of a bypass pathway may lead to a type of supraventricular tachycardia (SVT) called AVRT. (See "Atrioventricular reentrant tachycardia (AVRT) associated with an accessory pathway" and "ECG tutorial: Atrial and atrioventricular nodal (supraventricular) arrhythmias".)
The circuit includes the accessory pathway, AV node, and His Purkinje system, linked proximally within the atrial myocardium and distally within the ventricular myocardium. (See "Reentry and the development of cardiac arrhythmias".)
Most frequently, the QRS complexes during AVRT are narrow, called orthodromic AVRT (figure 1 and waveform 4 and waveform 5). This occurs when antegrade conduction from the atria to the ventricles is via the normal conduction system, while retrograde conduction from ventricles to atria is via the accessory pathway.
In contrast, during antidromic AVRT, the QRS complexes during the SVT are wide, bizarre appearing, and frequently confused for ventricular tachycardia. Antegrade conduction occurs from the atria to the ventricles via the accessory pathway and retrograde conduction from the ventricles to the atria is via the normal His Purkinje and AV node system (figure 2 and waveform 6 and waveform 7). Rarely, if two or more bypass pathways are present, the retrograde limb may be a second pathway.
Atrial fibrillation or flutter — The risk of atrial fibrillation or flutter is increased in patients with WPW (waveform 8A-C). (See "Atrioventricular reentrant tachycardia (AVRT) associated with an accessory pathway".)
There is usually variable QRS morphology, depending on how much conduction is through the AV node and how much is through the bypass pathway. The QRS is narrower if the refractory period of the accessory pathway is relatively long and antegrade conduction to the ventricles is via the AV node. If the refractory period of the accessory pathway is short, however, antegrade conduction to the ventricles is predominantly via the accessory pathway, bypassing the AV node, and the QRS complexes are relatively wide and bizarre in appearance. There is a possibility that the ventricular rate will be very rapid (>300 beats per minute) due to 1:1 conduction from the atrium to the ventricles and can degenerate into ventricular fibrillation.
SUMMARY
●Lown-Ganong-Levine syndrome is defined by a short PR interval with a normal QRS complex.
●The Wolff-Parkinson-White (WPW) pattern is defined by a short PR interval, with a wide QRS and a delta wave. When there are arrhythmias, such as atrioventricular reentrant tachycardia or atrial fibrillation/flutter, it is termed the WPW syndrome.
