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Not all patients who suffer from atrioventricular reciprocating tachycardia have a delta wave on their resting ECG. Concealed pathways only conduct retrogradely, while in latent pre-excitation the accessory pathway has a net conduction time to the ventricle (that is, intra-atrial conduction plus AP conduction times) greater than the net conduction time via the atrioventricular (AV) node. Latent conduction may be unmasked by blocking AV nodal conduction with adenosine (“adenosine testing”) but a third possibility, which can also be unmasked by this technique, is intermittent pre-excitation.
A 26 year old man with intermittent palpitations and a normal resting ECG received 18 mg of adenosine. Initially (1) there is progressive lengthening of the PR interval (vertical arrow = P waves), and the sixth P wave is an echo beat (oblique arrow). Theaccessory pathway then (2) begins to conduct (the PR interval is short, proving intermittent rather than latent conduction), but only alternate beats are transmitted because of a long refractory period. The very broad QRS suggests that the AV node is completely blocked at this stage. Accessory pathway refractory period shortens (3) (either because of enhanced sympathetic tone or another direct effect of adenosine), and AV nodal conduction recovers (open arrow = fusion beat). Finally (4), 1:1 AV conduction returns and the typical ECG appearances of the Wolff-Parkinson-White syndrome is seen. Accessory pathway conduction disappeared soon thereafter.