Purpose
Both 5% dextrose in water (D5W) and normal saline provide a thermal buffer and displacement of surrounding structures during RF ablation. However, the degree of protection afforded with saline is less than with D5W, likely due to saline’s high electrical conductivity. The purpose of this study was to elucidate the mechanisms leading to the greater protective effect of D5W.
Material and methods
RF ablations were performed in rectangular gel phantoms designed to mimic liver tissue (5% agar, 3% sucrose, 0.3% NaCl) with a fluid layer (3, 6 or 9 mm wide) of either D5W or saline. A constant 2.0 A of RF current was passed through a single Cool-tip™ electrode (Valleylab; Boulder, CO) placed 0.5 cm from the fluid layer with ground pads at both ends of the phantom. Thus, two current paths were created: one through gel only and one through the fluid layer and gel. This two-path setup was designed to mimic clinical situations, where several current paths exist in the body, and allowed us to demonstrate how the fluid layer affects local current flow. Thermosensors were used to measure temperature rises in the gel and fluid layer. We also measured current flow through each of the ground pads.
Results
RESULTS: Impedances measured through D5W were significantly higher and increased with fluid layer width (93, 162 and 186 Ω at 3, 6 and 9 mm widths, respectively) when compared to impedances measured through saline (62 Ω for all widths). D5W significantly decreased the amount of current traveling through the fluid layer (1.3, 0.9 and 0.5 A at 3, 6 and 9 mm widths, respectively) when compared to saline (1.7-1.9 A for all widths). Correspondingly, temperature increases on the “protected” side of the fluid layer were less for the D5W group (5-23 °C) than for the saline group (10-29 °C).
Conclusion
D5W is more effective at providing both electrical and thermal insulation than saline. The electrical insulation effect of D5W is proportional to the thickness of the fluid layer, whereas additional saline provides no electrical benefit and may actually increase the likelihood of current flow through the “protected” structure. Therefore, D5W is better-suited for protecting perihepatic structures during RF ablation than saline.