Purpose
The size of coagulation zones that can be achieved with a single ablation have been steadily increasing from 1.5 cm when RF tumor ablation was introduced in 1992, to approximately 4-6 cm in diameter with state-of-the-art generators and electrodes. Over this period of time, RF generator power has increased from 25W to 250 W. Future devices will likely continue the trend towards higher generator power and multiple electrodes to increase the size of coagulation zones and decrease treatment times. One major obstacle in employing higher power RF generators and enabling increased coagulation zones is ground pad related heating, which can lead to 3rd degree skin burns in severe cases. The current incidence of ground pad skin burns ranges from 0.1 – 3.2% for severe skin burns (2nd or 3rd degree), with mild skin burns ranging between 5 – 33%. Placement pads in addition to the currently up to 4 pads only provides minimal improvement since RF current travels preferentially to the closest pads.We investigated whether sequential activation of multiple ground pads reduces ground pad related skin heating.
Material and methods
We performed ex vivo experiments with three ground pads (5 x 5 cm) separated by 4 cm placed on a tissue phantom made of Agar-Water gel. We applied 100W of power for 12 min between the pads, and an RF electrode placed 35 cm distant from the front pad. We measured leading edge temperature below each pad, and placed temperature-sensitive LCD-paper on each pad to visualize temperature distribution. We compared conventional operation (i.e. simultaneous connection of all three pads) to sequential activation of the pads where each pad is only active for ~0.5 s. The timing during sequential activation was adjusted to keep leading edge temperature equal between the pads.
Results
Temperature rise below the leading edge for closest, middle and far ground pad was 10.7±1.04, 1.0±0.15 and 0.3±0.07 ºC for conventional operation, and 4.8±0.16, 4.4±0.20 and 4.5±0.35 ºC for sequentially activated operation. The leading edge temperature was significantly different between each pad for conventional operation (p<0.001), and approached significance for sequentially activated operation (p=0.06). The maximum leading edge temperature rise was about twice as high for conventional compared to switched operation (p<0.001).
Conclusion
Sequential activation of multiple ground pads resulted in reduced maximum leading edge temperature, and allows control of each pad such that leading edge temperature of all pads is equal. This may reduce the incidence of ground pad burns by allowing each pad to reach same temperatures independent of location, and may allow higher power RF generators due to reduces ground pad related heating. Furthermore, currently tedious groundpad placing procedure will become simplified, as the pads don’t need to be equidistant from the RF electrode.