Sequential activation of ground pads reduces skin heating during radiofrequency tumor ablation: in vivo porcine results

Abstract

Skin burns below ground pads during monopolar RF ablation are increasingly prevalent, thereby hindering the development of higher power RF generators capable of creating larger tumor ablation zones in combination with multiple or new applicators. Our goal was to evaluate reduction in skin temperatures via additional ground pads in an in vivo porcine model. Three ground pads placed on the animal's abdomen were activated either simultaneously or sequentially, where activation timing was adjusted to equilibrate skin temperature below each pad. Thirteen RF ablations (n = 4 simultaneous at 300 W, n = 5 sequential at 300 W, and n = 4 sequential at 375 W) were performed for 12 min via two internally cooled cluster electrodes placed in the gluteus maximus of domestic swine. Temperature rise at each pad and burn degree as determined via histology were compared. Ablation zone size was determined via T2-weighted MRI. Maximum temperature rise was significantly higher with simultaneous activation than with either of the sequential activation group (21.4 degrees C versus 8.1 degrees C or 9.6 degrees C, p < 0.01). Ablation zone diameters during simultaneous (300 W) and sequential activations (300 and 375 W) were and 6.9 +/- 0.3, 5.6 +/- 0.3, and 7.5 +/- 0.6 cm, respectively. Sequential activation of multiple ground pads results in significantly lower skin temperatures and less severe burns, as measured by histological examination.

Figure 1

Typical electrode and ground pad placement. The internally cooled cluster electrodes (black arrows) were placed percutaneously in the center of the left and right gluteal muscles. The ground pads (white arrows) were placed collinearly ~25–30 cm from the cluster electrodes. Temperatures were measured at the center of the leading edge of each ground pad (black diamonds).

Figure 2

Diagram of pad placement arrangement. (A) Standard clinical placement with 2 equidistant pads. (B) Two additional pads are added on each thigh further from the electrode than the original pads (shown in A). (C) Pad placement used in this study, which would correspond to (B) in a clinical setting (i.e. can be translated into humans by using this arrangement on both thighs).

Figure 3

Timing diagram for sequential activation algorithm of ground pads; pads are labeled proximal, middle and distal according to location relative to RF electrode. Pads active during each time period t1, t2 and t3 are shaded. During power application, skin heating occurs primarily at the leading edge of the activated ground pad that is nearest to the active RF electrode (ie, the proximal pad during t1, the middle pad during t2, and the distal pad during t3). Ratios between t1, t2 and t3 were adjusted and total period (t1+t2+t3) was fixed at 2s. Typically, t1 > t2 > t3 as indicated on the time axis, since active total pad area is largest during t1 and smallest during t3 (see also [22]).

Figure 4

Diagram of ground pad activation control system. A control algorithm on the PC regulated the generator output power P. A digital multimeter/data acquisition device (DAQ) measured the applied voltage V and relayed it to the control algorithm, which then calculated the current and impedance. A second DAQ recorded the temperatures (T1, T2, T3) at the leading edge of each ground pad. During the group 1 procedures, all 3 ground pads were activated for the entire ablation. During the groups 2 and 3 ablations, the control algorithm adjusted the switching periods (t1, t2, and t3) via the digital multimeter/DAQ to keep leading edge temperature equal between pads in a way such that total period (t1+t2+t3) was kept constant at 2s.

Figure 5

MR image of ablation zone orthogonal to electrode axis. Location of three-electrode array insertion is visible at the center as three white dots. Dotted arrow represents maximum transverse ablation zone diameter (8.2 cm in this case), and is measured at the boundary of the hyperintense (i.e. bright) rim.

Figure 6

Temperature profiles for representative experiments in groups 1, 2 and 3. Temperature profiles for proximal, middle and distal pads are labeled separately for group 1, but not for groups 2 and 3 due to the small difference between pad temperatures.

Figure 7

Gross pathology of skin burns. (A) Animal from group 2: skin shows light reddening (black arrows). This lesion correlated to a score 1, or 1st degree burn. (B) Animal from group 1: blistering and charring of skin (black arrows) below proximal pad. This lesion correlated to a score 5, or 3rd degree burn.

Figure 8

Graph illustrates mean burn score at each pad for group 1 (simultaneous 300W), group 2 (sequential 300W), and group 3 (sequential 375W). There was a significant difference in the severity of the burn between groups 1 and 2 (p = 0.04) but not between groups 1 and 3 (p = 0.12). There was no significant difference in burn severity between the groups at the middle pad, but significantly more severe burns at the distal pad in group 3 than group 1 (p = 0.04). Comparing the burn scores at pad locations within each respective group, there are significant differences within groups 1 and 3 (p < 0.0001 and 0.04), and difference was approaching significance within group 2 (p = 0.06).

Figure 9

H&E stained skin samples from region under the ground pad edge. (A, left) Skin from animal in group 2. The epidermis is intact with mild perivascular cellular infiltrate within the upper dermis (white arrowhead). This lesion correlates to a score of 1, or 1st degree burn. (B, right) Skin below proximal pad from animal in group 1. The epidermis is necrotic and has separated from the dermis (star). Collagen is disrupted and denatured with a hyaline appearance (arrow). This lesion correlates to a score of 5, or 3rd degree burn. 20X magnification.