During retrograde conduction via the AV node, the VA interval lengthens.

During retrograde conduction via the AV node, the VA interval lengthens.

• A. Lengthen
• B. Shorten
• C. Remain constant
• D. Unpredictable

Answer: (A)

The key idea is that the AV node conducts retrograde impulses with decremental conduction. When a beat activates the ventricles and then travels back to the atria through the AV node, faster rates or repeated retrograde activations make the AV node more refractory and slow its conduction in the retrograde direction. That slower retrograde conduction takes longer, so the time from ventricular activation to atrial activation—the VA interval—becomes longer. This rate-dependent slowing is a hallmark of AV nodal retrograde conduction. By contrast, pathways outside the AV node tend to conduct retrogradely in a more consistent, nondecremental manner, which is why the VA interval through an accessory pathway would typically be shorter and less rate-dependent.

The key idea is that the AV node conducts retrograde impulses with decremental conduction. When a beat activates the ventricles and then travels back to the atria through the AV node, faster rates or repeated retrograde activations make the AV node more refractory and slow its conduction in the retrograde direction. That slower retrograde conduction takes longer, so the time from ventricular activation to atrial activation—the VA interval—becomes longer. This rate-dependent slowing is a hallmark of AV nodal retrograde conduction. By contrast, pathways outside the AV node tend to conduct retrogradely in a more consistent, nondecremental manner, which is why the VA interval through an accessory pathway would typically be shorter and less rate-dependent.