The ECG was recorded from a 23 year old man with a history of palpitations since childhood
ECG. Rhythm: Sinus rhythm is present throughout.
Morphology: the PR interval is abnormally short at .10 seconds. Total QRS duration is prolonged at .16 seconds. The initial QRS deflection starts as a slurred delta wave particularly well seen in leads I, II, aVL and V4 – V6. Non specific ST depression and T wave flattening are present in leads I, II,V5andV6.
Comment. The EGG shows the typical pattern of ventricular pre-excitation seen in the Wolff Parkinson White syndrome (WPW). The cardinal feature of this condition is the combination of the typical EGG changes (a short PR interval and the presence of an initial delta wave) with the presence of episodes of paroxysmal tachycardia.
The abnormality in this condition is the presence of an anomalous bypass tract or accessory pathway that provides a route through which ventricular depolarization may occur. Ventricular depolarization may therefore occur through this pathway as well as by normal conduction through the AY node. The bypass tract (which consists of cardiac muscle cells) conducts the impulse more quickly than the AV node and causes premature ventricular depolarization, a process known as Ventricular pre-excitation’ or ‘accelerated atrioventricular conduction’. Several different types of bypass tract have been described; precise classification depending on the anatomical route and functional characteristics of the anomalous pathway. The most common type is that associated with the Wolff Parkinson White syndrome.
This EGG shows a superficial similarity to that of left bundle branch block. This occurs because the anomalous pathway responsible for ventricular pre-excitation is connected to the upper right side of the interventricular septum – a situation analogous to LBBB where septal activation is also initiated from the right side of the septum. The essential difference between the two is the presence of a short PR interval and the initial delta wave in the WPW syndrome. In both conditions septal depolarization occurs in the opposite direction to normal and no diagnosis that depends on changes in the morphology of the QRS complexes can be made in either the presence of ventricular pre-excitation or LBBB. WPW type B (with a right sided AV nodal bypass tract) is considerably more common than type A (left sided bypass tract) which is illustrated later.
During sinus rhythm the QRS complex results from a fusion of the initial accelerated conduction through the accessory pathway with the later normal conduction through the AV node. It is the initial depolarization of the ventricular myocardium which gives rise to the delta wave, this is frequently directed superiorly giving rise to Q waves in the inferior leads; the presence of Q or QS waves in the presence of pre-excitation must not be taken to indicate myocardial infarction.