All 30watt lesions are equal. But some are more equal than others.

Answer: Baseline impedance due to indifferent electrode placement on the leg vs on the chest

• Where you place the indifferent electrode, as well as how many indifferent electrodes you place on the body can have a significant impact on baseline impedance, and subsequently impact on lesion size and volume.

• RF Ablation passes current between the ablation catheter tip and the RF return electrode/patch. Current flow is inversely related to impedance. High baseline body impedance acts as an impediment to current delivery to tissue (at a given power setting on your generator), limiting lesion formation.

• Placing an indifferent electrode close to the heart minimizes the amount of tissue current must travel through to complete the circuit, minimizing circuit resistance and, therefore, encouraging greater current delivery to tissue for a given power setting. Placing two return electrodes helps to further lower the body impedance, encouraging more current to travel through tissue (hence you deliver more current to tissue for the same power, promoting larger lesions).

My two cents:

• Knowing this, we now know that not all 30watt lesions are made equal. A 30 watt lesion delivered in the setting of a lower baseline impedance (100ohms) will result in significantly more current delivery, heating and lesion volume, compared to a 30watt lesion delivered in the setting of a high baseline impedance (200 ohms).

• So next time, think about that when deciding where to place the indifferent/return electrode. If you place it further away from the heart (on the leg), you are committing to a high baseline impedance and smaller lesions. If you place it closer to the heart, or put two electrodes on, you will probably have a lower baseline impedance and relatively larger lesions.

Cheers
Mitch & CPP Team

p.s Sorry for the bad drawings. 

Barkagan M, Rottmann M, Leshem E, Shen C, Buxton AE, Anter E. Effect of Baseline Impedance on Ablation Lesion Dimensions: A Multimodality Concept Validation From Physics to Clinical Experience. Circ Arrhythm Electrophysiol. 2018 Oct;11(10):e006690.