ECG tutorial: Preexcitation syndromes
- Jordan M Prutkin, MD, MHS, FHRS
Jordan M Prutkin, MD, MHS, FHRS
- Associate Professor of Medicine, Division of Cardiology, Electrophysiology Section
- University of Washington
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 reentrant tachycardia or atrial fibrillation/flutter.
A short PR interval, <0.12 sec, with a narrow 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, though, since ventricular activation is via the normal conduction pathway (His Purkinje system).
The Wolff-Parkinson-White (WPW) pattern results from an accessory pathway, the Kent bundle, which directly links the atria to the ventricles, bypassing the atrioventricular (AV) node. The ventricular myocardium is activated early as a result of this bypass tract, 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.