Filling the lumen with saline during third-space endoscopy improves visualization and hemostasis, but profoundly alters electrosurgical behavior by collapsing tissue impedance. How these changes translate into collateral tissue damage according to electrode diameter remains insufficiently quantified.

In an ex-vivo porcine gastric model, three commonly used knives with different tip diameters (1.5 mm HybridKnife T-type, 0.5 mm HybridKnife flex I-type, 0.4 mm DualKnife J) were tested in air and under 0.9% saline using seven endoscopic waveforms (two cutting, three coagulation, two dissection modes). A total of 252 standardized 10-mm cuts were performed. Impedance, peak voltage, effective power, cutting success, and lateral thermal spread were recorded. Linear regression was used to identify independent predictors of lateral spread.

Transition from air to saline reduced impedance by 55% with thin knives versus 92% with the thick knife, while effective power increased by 4.9-fold and 19-fold, respectively. Peak voltage rose slightly with thin knives (+1.7%), but fell by about one-third (–30.2%) with the thick knife. In air, lateral spread remained <1.0 mm across all knife–mode combinations (0.31–0.91 mm). Under saline, spread increased by 70.8% with thin electrodes and by 255.5% with the thick electrode. Mean spread with the thick knife expanded from 0.56 to 1.35 mm, frequently exceeding 1.0 mm, whereas both thin knives remained ≤0.8 mm across cutting and dissection modes. In multivariable analysis (adjusted R² = 0.546), electrode diameter was the strongest predictor of lateral spread (standardized β = 0.469), increasing it by approximately 0.36 mm per additional millimeter. This was followed by saline immersion (β = 0.353), effective power, and impedance, while higher peak voltage independently reduced spread (β = –0.387).

In saline-immersion third-space electrosurgery, electrode diameter is the primary determinant of lateral thermal spread. and strongly modulates the impact of the conductive medium on voltage and power delivery. Thin electrodes, particularly when combined with appropriately selected modes, maintain lateral spread below 1.0 mm while preserving cutting performance, whereas thick electrodes in saline are prone to excessive collateral heating.