Pathology
The refractory period of the atrioventricular node normally prevents a supraventricular focus from driving the ventricles at greater than about 150 bpm. However in the presence of an alternate AV conduction pathway with a short refractory period, the ventricular response may exceed 200 bpm:
- patients with Wolf-Parkinson-White syndrome and Lown-Ganong-Levine syndrome have alternate atrio- ventricular pathways which result in aberrant conduction of the myocardial depolarisation
- re-entry tachycardias result from anterograde conduction through the AVN and retrograde conduction up the aberrant pathway
- the process may be initiated by an ectopic beat
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