The basic abnormality in ventricular pre-excitation is that the depolarisation wave, after passing through the atrial myocardium, activates the ventricles earlier than would be expected if the impulse travelled normally from atria to ventricles via the AV node and the bundle of His.
Hence, this is also known as accelerated AV conduction. In its most common form, there is the congenital presence of an "accessory" AV conduction pathway, which results in a rapid bypass of the normal slow route.
Wolff-Parkinson-White (WPW) syndrome
The ventricular pre-excitation syndrome, Wolff-Parkinson-White (WPW) syndrome, comprises two ECG criteria plus a clinical criterion:
1) A short PR interval
2) Widened QRS complex and
3) Episodes of paroxysmal tachycardia.
The PR interval is shortened by the rapid transmission of depolarisation from atrial to ventricular myocardium.
The accessory pathway passes first to the upper part of the right side of the interventricular septum, and this is the first part of the myocardium to be activated (normally, the left side of the upper part of the interventricular septum is the first part of the ventricles to be activated). This changes the initial direction of the QRS deflection.
The initial part of the QRS complex is slurred and this premature, slurred initial portion is termed the delta wave.
The normal pathways of intraventricular conduction are not followed and hence the QRS complex becomes distorted in shape and prolonged in duration.
Since ventricular depolarisation is abnormal, ventricular repolarisation is also abnormal and ST-segment depression and/or T wave inversion may be seen.
The combination of a short PR interval and an abnormally long QRS complex comprises the ECG diagnosis of ventricular pre-excitation.
The presence of two pathways for AV conduction results in cyclical, repetitive entries of the depolarisation wave between the atria and the ventricles, giving rise to paroxysmal tachycardia or atrial flutter.
Diagnostic criteria for WPW-type pre-excitation
All of the following three criteria must be fulfilled for a diagnosis to be made:
1) P-R interval <0.12 s (i.e. 0.11 s or less) in the presence of sinus rhythm.
2) Abnormally wide QRS complex >0.10 s (i.e. 0.11 s or more).
3) Presence of initial slurring (in the first 0.03 to 0.05 s) of the QRS complex.
Note the short PR and the subtle "delta" wave at the beginning of the QRS complexes. The delta wave represents early activation of the ventricles in the region where the AV bypass tract inserts. The rest of the QRS is derived from the normal activation sequence using the bundle branches.
The above ventricular pre-excitation syndrome must be distinguished from a complete left bundle branch block in which the P-R interval is normal, without any initial slurring of the QRS complex, but the similarity lies in the fact that the initial direction of QRS deflection is reversed and the total QRS duration is also prolonged.
Lown-Ganong-Levine (LGL) syndrome
This is the next most common type of ventricular pre-excitation syndrome. Here, the accessory pathway is believed to run from the atrial myocardium to the distal part of the AV node or to the beginning of the His bundle, thus short-circuiting the normal A-V nodal delay (short P-R interval). The intraventricular conduction (and hence the QRS complex) remains normal.
The ECG criteria for the LGL type of pre-excitation syndrome are:
1) P-R interval <0.12 s (i.e. 0.11 s or less).
2) Normal QRS duration with no delta wave.
Patients having the above criteria fulfilled on their ECG in addition to the clinical criterion of having had episodes of paroxysmal tachycardia have the LGL syndrome.