Thread by @ZiadIssaMD

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Impedance and RF ablation:
Part 2: How does RF ablation affect impedance?
 
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As tissue temperature rises during RF energy application, ions within the tissue being heated become more mobile, resulting in a decrease in impedance to current flow.

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There are currently 2 methods to measure impedance: Generator Impedance (GI) & Local Impedance (LI).

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Lack of impedance drop during RF energy application can reflect inefficient energy delivery to the tissue due to poor tissue contact, lack of catheter stability, or inadequate power delivery.

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LI is measured at the distal electrode of the catheter and represents near-field impedance at the ablation electrode-tissue interface.

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LI drop rate and the magnitude of drop closely correlate to the intra-mural tissue temperature (2-4 mm below the tissue surface) during standard and high-power RF ablation and can thereby act as an intramural thermometer.

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LI drop of 10-20-Ω is observed in clinically effective & safe (intramural temperature 55-90°C) RF applications.
Larger LI drops (>35 Ω) often reflect excessive tissue heating (intramural peak temperature nearing 100°C), indicating a risk for steam pops/char formation.

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LI offers 4-time greater working range than GI, which may permit more precise titration of energy delivery, especially during high-power RF ablation when tissue heating occurs rapidly.

Compared to GI, LI offers more sensitive & more specific measure of intramural tissue heating, as well as larger and more rapid impedance drops during RF application, allowing LI to be used as a real-time monitor of RF lesion formation & to guide titration of RF energy delivery.