Atrioventricular re-entrant tachycardia: accessory pathways and ECG findings
THE CONTEXT
An accessory band of conducting tissue connects the atria and ventricles. Its direction of conduction determines whether it alters the electrocardiogram (ECG) during sinus rhythm.
Accessory pathway properties
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The accessory pathway contains rapidly conducting fibres resembling Purkinje tissue and is rich in sodium channels.
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In about 50% of cases, the pathway conducts only retrogradely, from the ventricles to the atria. This concealed accessory pathway does not alter the ECG in sinus rhythm.
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In the remaining cases, the pathway also conducts antegradely, from the atria to the ventricles. During sinus rhythm, conduction then occurs through both the atrioventricular (AV) node and the accessory pathway.
Manifest pathways and ventricular activation
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A manifest accessory pathway causes premature ventricular activation, shortening the PR interval and producing a slurred initial QRS deflection called a delta wave.
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In Wolff–Parkinson–White syndrome, accessory conducting tissue allows electrical activity to bypass the AV node and reach the ventricles rapidly without delay.
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Ventricular depolarisation through the AV node produces a normal ECG, whereas depolarisation through accessory tissue produces a very short PR interval and a broad QRS complex.
AV nodal re-entry mechanism shown for comparison
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AV nodal re-entrant tachycardia involves slow and fast right atrial AV nodal input pathways.
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Antegrade conduction occurs through the slow pathway; the wavefront passes through the AV node into the ventricles while re-entering the atria through the fast pathway.