The ECG was recorded from a 59 year old man admitted for routine surgery.
ECG. Rhythm: Sinus rhythm is present throughout.
Morphology. The PR interval is short (0.12 seconds). There is a delta wave present in all leads but this is particularly well seen in leads I, II, and V1-V6. Total QRS duration is prolonged (0.14 seconds). There is a tall R wave in the right sided precordial leads and non-specific ST segment/T wave changes accompany this.
Comment. It will be seen that the QRS morphology is different from the previous example (B27) of WPW type B; there is a superficial resemblance to right bundle branch block and this occurs because the accessory pathway connects to the left ventricle and activation of the heart begins in an analagous fashion to that in RBBB. This pattern of pre-excitation occurring with paroxysmal tachycardia is known as Type A Wolff Parkinson White.
The significance of the accessory pathway arises from the fact that it creates an additional pathway for the conduction of impulses between atria and ventricles. An anatomical circuit therefore exists whereby an impulse may be conducted to the ventrical by one pathway (usually the AV node), spread. through the ventricular myocardium, and be conducted retrogradely through the other pathway to depolarise the atria again. The resulting atrial depolarisation may then re-enter the first pathway and cause ventricular activation. If this sequence becomes established a sustained tachycardia will result, dependent on the circus movement of the depolarisation between atria and ventricles. This is the mechanism that causes the tachycardia in the WPW syndrome; it is often termed an atrio-ventricular re-entrant through one of two separate pathways with different refractory periods lies behind many other types of tachycardia.
In sinus rhythm the QRS complex is a fusion beat; ventricular activation occurring through two routes, the accessory pathway and through the AV node and His Purkinje system. The resulting QRS complex shows the characteristic appearance illustrated with a slurred upstroke. During re-entrant tachycardia however, the ventricles are depolarised through one pathway only and when this is the AV node the delta wave will disappear and the resulting QRS complexes will be entirely normal. Should ventricular activation during a re-entrant tachycardia occur through the accessory pathway the slurred upstroke will remain giving rise to a wide QRS complex tachycardia. This can occur rarely; retrograde conduction of the atria through the AV node. If atrial fibrillation occurs in the presence of an accessory pathway atrial impulses may be conducted to the ventricles at extremely fast rates. A bizarre broad QRS complex tachycardia will result and there is a risk of ventricular fibrillation developing.