A 35-year-old man with recurrent palpitations undergoes electrophysiology study. During tachycardia, the earliest retrograde atrial activation is at the coronary sinus ostium, and the VA interval is 80 ms. Which single feature best distinguishes orthodromic AVRT (using the accessory pathway anterogradely) from antidromic AVRT (using the accessory pathway anterogradely)?

Explanation

## Orthodromic vs. Antidromic AVRT ### Core Pathophysiological Difference **Key Point:** The critical distinction lies in which pathway conducts anterogradely (toward the ventricles) during tachycardia: - **Orthodromic AVRT:** AV node → ventricles (narrow QRS), accessory pathway ← atria (retrograde) - **Antidromic AVRT:** Accessory pathway → ventricles (wide QRS), AV node ← atria (retrograde) ### QRS Morphology as the Discriminator | Feature | Orthodromic AVRT | Antidromic AVRT | |---------|------------------|------------------| | **Antergrade conduction** | Via AV node | Via accessory pathway | | **QRS width** | **Narrow (< 120 ms)** | **Wide (> 120 ms)** | | **QRS morphology** | Normal, resembles sinus rhythm | Fully pre-excited; mimics VT | | **Frequency** | ~95% of AVRT cases | ~5% of AVRT cases | | **Baseline ECG** | May show subtle pre-excitation or normal | Obvious delta wave + short PR | | **VA interval** | Variable (depends on AP location) | Variable (depends on AP location) | | **Adenosine response** | Terminates via AV block | May not terminate if AP conducts faster than AV node | ### Why QRS Width is the Best Discriminator **High-Yield:** QRS width during tachycardia directly reflects which tissue conducts the ventricular depolarization: - Conduction via the AV node (orthodromic) uses the normal His-Purkinje system → narrow QRS - Conduction via the accessory pathway (antidromic) bypasses the normal conduction system → wide, pre-excited QRS **Clinical Pearl:** Antidromic AVRT is rare and can be mistaken for ventricular tachycardia on ECG. The key clue is that it is regular, terminates with adenosine (if the AV node is involved in the retrograde limb), and occurs in a patient with known or suspected WPW syndrome. ### Mnemonic **NARROW = Nodal anterogradely (Orthodromic); WIDE = Pathway anterogradely (antidromic)** ### Why VA Interval Is Not the Discriminator **Warning:** The VA interval depends on the location of the accessory pathway, not on which pathway conducts anterogradely. Both orthodromic and antidromic AVRT can have variable VA intervals (50–150 ms) depending on where the AP is located (posteroseptal, right free wall, left free wall, anteroseptal).