Unmasking Hidden Conduction Delay with Extra-Stimuli

Answer: Yes – we can unmask “functional” substrate which is likely a major contributor to VT re-entry, acting as the critical slow limb of reentry.

 

The Basic Concepts:

• Activation maps with RV pacing (S1) may look normal, without obvious conduction delay or fragmentation.

  • ILAM can highlight deceleration zones, but a negative ILAM does not exclude arrhythmogenic substrate.

 

• Substrate analysis including MRI/CT(ADAS) can suggest channels of slow conduction.

  • However, pre-procedural planning is critical, and may not always be possible.

 

• Mapping & analyzing EGMs during Extra-stimuli (S3) may unmask hidden zones of conduction delay and abnormal EGMs, revealing a possible arrhythmogenic channel that can be targeted for ablation.

 

Case Study Example:

This case study showcases the different modalities of substrate analysis, and how unmasking functional substrate can offer a critical target for ablation when conventional modalities fail.

 

1. Activation Map during RV pacing (S1):

• No pathological conduction was evident.

 

• Local EGMs in the anterolateral basal wall appeared relatively normal – an absence of late, fractionated or low voltage EGMs.

 

2. ILAM Analysis:

• Isochronal Late Activation Mapping showed no pathological deceleration zones.

 

• This highlights that ILAM negativity does not rule out slow-conduction channels.

 

3. Substrate Mapping (ADAS):

• Pre procedural MRI analysis via ADAS software revealed a suspicious channel in the anterolateral basal wall, suggesting the presence of slow conduction.

 

4. Extra-Stimuli (S3):

• Pacing with S3 unmasked conduction delay in the same region.

 

• Local EGMs demonstrated prolongation and fragmentation, consistent with a possible arrhythmogenic channel.

 

Key Concept & Learning Points

• An apparently “normal” S1 activation map and ILAM do not exclude arrhythmogenic substrate.

 

• Substrate-guided strategies (ADAS) and extra-stimuli (S3) are essential to unmask hidden conduction delay.

 

• What looks normal under baseline pacing may reveal its arrhythmogenic potential when challenged with extra-stimuli, unmasking functional substrate for VT Reentry.

 

 

My Two Cents:

• Talking about a “critical isthmus” requires mapping during VT or entrainment criteria.

 

• In this case, what was identified is better described as a zone of slow conduction compatible with an arrhythmogenic channel, revealed only with S3.

 

Take-home: Do not dismiss a region because S1 and ILAM look normal. Combining substrate maps with extra-stimuli pacing can uncover clinically relevant hidden channels.

 

Authorship & Acknowledgements

• This BeatBox Blog post was written by EP in Practice member Dr Juan Ismael Almonte in collaboration with Dr Rodolfo San Antonio Dharandas, the lead Electrophysiologist of the above case.

 

CPiP thanks the authors for their expert insights, commitment to EP Education Excellence and their time and effort in generously contributing to the EP in Practice Community. We look forward to future collaboration.