Wenckebach point and atrio-ventricular node ERP

• what is Wenckebach point and decremental properties of AV node
• what is the consequent of those physiological features
• why it is important to assess them
• what to expect when you get unusual response

Antegrade Wenckebach point (WP) occurs when 1:1 conduction over the AV node ceases during atrial incremental stimulation.

Wenckebach cycle length (CL) is a pacing cycle length at which the Wenckebach point occurs. It depends on the patient's adrenergic status and administered drugs.

Atrial incremental pacing involves stimulation from a high right atrium catheter or coronary sinus catheter at a CL slightly shorter than the patient's rhythm. Start with 600 ms decreasing in small steps of 10 ms. Observe when the atrial signal is not conducted to ventricle - you get the WP.

Have the His catheter in place. You can observe 2 types of blocks:

1. physiological supra-hisian block (AH interval prolongs in parallel to shorter pacing CL; HV interval is relatively constant),
2. infra-hisian block (at some point you may observe A signal followed by H singal with no V potential)

During atrial incremental pacing look for preexcitation. With fast pacing you may block AV conduction while signal goes through accessory pathway

Always measure WP during each EP study, especially before and after AVNRT ablation

When your incremental pacing is fast enough you will reach AV node effective refractory period. On the His catheter you will see A signal with no following H singal - you blocked the AV node.

How to section:

1. Pick HRA or CS channel - you need to have a nice atrial signal.

2. Measure patient’s baseline cycle length (BCL)

3. Set stimulation mode to incremental pacing with pacing CL lower than BCL (i.e. 600 msec) - you want to be faster than patient’s own rhythm.

4. Stimulate from HRA or CS catheter

5. Decrease pacing CL by 10 msec and observe when atrial beat is not followed by a ventricular response

6. You have reached the antegrade WP. Note the lowest value of paced CL at which signal was still conducted - this is Wenckebach CL