Which of the following SVTs is most commonly associated with an accessory pathway and exhibits a characteristic delta wave on the baseline ECG?

Explanation

## Accessory Pathway-Mediated SVTs **Key Point:** WPW syndrome is characterized by the presence of an accessory pathway (Bundle of Kent) that bypasses the AV node, allowing pre-excitation and generating a delta wave on the baseline ECG. ### Delta Wave Pathophysiology The delta wave represents early ventricular depolarization via the accessory pathway before normal AV nodal conduction reaches the ventricle. This creates a slurred upstroke at the beginning of the QRS complex. ### SVT Mechanisms in WPW | Feature | AVNRT | AVRT (WPW) | Atrial Flutter | Atrial Tachycardia | | --- | --- | --- | --- | --- | | Accessory pathway required | No | Yes | No | No | | Delta wave at baseline | Absent | Present | Absent | Absent | | Reentry circuit | AV node (slow + fast pathway) | AP + AV node | Atrial macro-reentry | Ectopic focus | | Most common SVT | Yes (60% of SVTs) | Yes (30% of SVTs) | No | No | **High-Yield:** WPW is the ONLY SVT with a delta wave visible on the baseline (sinus rhythm) ECG. This is pathognomonic for an accessory pathway. ### Orthodromic vs. Antidromic AVRT - **Orthodromic AVRT** (90% of AVRT): conduction down AV node → up accessory pathway; narrow QRS - **Antidromic AVRT** (10% of AVRT): conduction down accessory pathway → up AV node; wide QRS with delta wave during tachycardia **Clinical Pearl:** During orthodromic AVRT in WPW, the baseline delta wave may disappear during tachycardia because the accessory pathway is being used for retrograde conduction, not antegrade conduction.