A short PR interval combined with a slurred initial upstroke of the QRS (a delta wave) reflects early ventricular activation through an accessory pathway bypassing the normal AV nodal delay; recognizing this pattern matters procedurally because AV-nodal-blocking drugs (calcium channel blockers, in some cases adenosine) can be dangerous if the patient develops pre-excited atrial fibrillation, since blocking the AV node preferentially can promote even faster conduction down the accessory pathway.
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