Which observation during ventricular pacing would most strongly suggest a bypass tract?

Which observation during ventricular pacing would most strongly suggest a bypass tract?

• A. Eccentric activation pattern
• B. Concentric activation pattern
• C. Delayed activation of the His bundle
• D. No change in activation pattern

Answer: (A)

The main idea here is that a bypass tract (an accessory pathway) provides an alternate route for ventricular activation that can bypass the normal His–Purkinje system. During ventricular pacing, if such a pathway is present and able to conduct, the ventricles can be activated via this accessory pathway in a region that is not the pacing site. This causes the earliest ventricular activation to occur away from where you paced, producing an eccentric activation pattern on the ECG. That eccentric pattern is the strongest clue to a bypass tract because it shows preexcitation through a pathway outside the normal conduction system. In contrast, a concentric activation pattern means activation spreads outward from the pacing site via the usual His–Purkinje network, with no early activation from an accessory route. Delayed activation of the His bundle would not specifically point to a bypass tract, and no change in activation pattern would argue against an accessory pathway altogether. So, observing an eccentric activation pattern during ventricular pacing best indicates the presence of a bypass tract.

The main idea here is that a bypass tract (an accessory pathway) provides an alternate route for ventricular activation that can bypass the normal His–Purkinje system. During ventricular pacing, if such a pathway is present and able to conduct, the ventricles can be activated via this accessory pathway in a region that is not the pacing site. This causes the earliest ventricular activation to occur away from where you paced, producing an eccentric activation pattern on the ECG.

That eccentric pattern is the strongest clue to a bypass tract because it shows preexcitation through a pathway outside the normal conduction system. In contrast, a concentric activation pattern means activation spreads outward from the pacing site via the usual His–Purkinje network, with no early activation from an accessory route. Delayed activation of the His bundle would not specifically point to a bypass tract, and no change in activation pattern would argue against an accessory pathway altogether.

So, observing an eccentric activation pattern during ventricular pacing best indicates the presence of a bypass tract.