A Heartfelt plea: Putting Ablation Impedance back at the Front of the bus

• Global (& local) impedance drop indicates tissue heating during ablation & correlates with lesion depth and volume.
  

  – Local impedance, measured specifically at the catheter tip, is likely more sensitive & likely has a greater predictive value of lesion depth compared to global impedance (as measured by the RF generator). Unfortunately, few catheters report local impedance.

• Larger (global or local) impedance fall during ablation correlates with greater tissue heating; implying that RF energy is modifying the tissue beneath the catheter tip. If no impedance fall occurs during ablation, this may indicate little to no tissue heating. This may indicate the ablation electrode is NOT in contact with tissue and attempts to verify catheter-tissue contact should be made. Alternatively, in the setting of good catheter-tissue contact, the presence of scar or fatty tissue can limit conductive heating and lesion formation, resulting in a smaller impedance fall reflecting the smaller lesion size being generated.

• Many workflows include titrating RF power & duration based on impedance drop. Power &/or duration may be increased to achieve a 10-ohm impedance drop in thick tissue such as LV septal VT ablation or LA roof ablation where deep lesions may be desirable. In other circumstances, RF power or duration may be reduced to minimise the impedance drop. For example, impedance falls of >10-ohms within the Triangle of Koch during slow pathway ablations may be undesirable since deep lesions may risk His Bundle injury or steam pops. Steam pops will be discussed in a future post. These biophysics are also discussed in detail in the “Intro to EP” Program 1 course.

My “two-cents” statement: At the end of the day, conventional factors such as catheter contact force, stability, power output and ablation duration are all “pre-ablation predictors” of lesion size. For an irrigated ablation (without temperature feedback), impedance fall is one of the few real-time feedback mechanisms that tell us whether or not we are actually modifying the tissue. That makes it worth paying attention to.

Thanks for tuning in :)
Cheers
Mitch & CPP Team

Iwakawa H, Takigawa M, Goya M, Iwata T, Martin CA, Anzai T, Takahashi K, Amemiya M, Yamamoto T, Sekigawa M, Shirai Y, Tao S, Hayashi T, Takahashi Y, Watanabe H, Sasa no T. Clinical implications of local impedance measurement using the IntellaNav MiFi OI ablation catheter: an ex vivo study. J Interv Card Electrophysiol. 2022 Jan;63(1):185-195.