Finding the sweet spot

• how to map PVC
• how to interpret the uni polar signal
• what are the best signals for a successful ablation

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The key to successful ablation is meticulous mapping. Before starting ablation, you should be absolutely sure you are in a good spot. To verify this use following techniques:

• Activation mapping: since you want to ablate the origin of the PVC your local signal at this spot should precede all other signals. You can access this by comparing onset of the local signal to a stable reference (begin of QRS or another ventricular signal i.e. RVA). Remember that everything is relative and you need to map points around your target region in order to be sure that the adjacent point is not earlier. A good spot for ablation in RVOT should be about 35-40ms before onset of QRS. In case of LVOT you can be satisfied with 15-20ms measurement.
• Signal characteristics: the best bipolar signal should not only by early in comparisons to QRS but also frequently begins with low voltage fractionated potentials. Very important is also the unipolar signal characteristics. Since the unipolar catheter works as an antenna receiving signals form the large region of myocardium, it can indicate the wave front direction of local activation. It shows a positive slope when the wave front is traveling towards the catheter and a negative slope then it is moving away from the catheter. Thus, at the origin of the signal only negative slope should be recorded (QS-morphology of unipolar signal).
• Pace mapping as described in previous lesson can be also used for additional verification. Using this method, you should remember that pacing might cause sudden change in catheter position due to abnormal contraction and a signal verification just before ablation is recommended.
• Spontaneous induction of PVC with the identical morphology caused by ablation (i.e. firing) is a good indication of a correct ablation side. An induction of different PVCs should discourage from further ablation in this position.

Power setting for effective and safe ablation depends on type of catheter, localization, underlying substrate and operator experience.