The use of RF pre-coagulation to reduce blood loss during hepatic resections has become increasingly widespread with two specialized devices now commercially available. Although the reduction in mean blood loss reported by previous studies is encouraging, the techniques and devices employed require several individual applications of RF or microwave energy, with correspondingly long procedural times. In addition, standard RF devices are not able to fully coagulate vessels larger than 3 mm in diameter in the resection plane. To address these limitations, we developed a device capable of thermally coagulating a typical hepatic resection plane in 3 to 6 min, including full coagulation of all blood vessels 14
The most straightforward method for pre-coagulating a plane of tissue with an electrode array would involve the insertion of the array along the desired plane and subsequent application of RF energy in the monopolar mode to all electrodes simultaneously. This method has several disadvantages. First, commercial clinical RF generators are incapable of supplying enough power to heat the tissue around a large number of electrodes simultaneously. More importantly, prior modeling results have shown that when multiple electrodes are placed in tissue in close proximity to each other and simultaneously energized, little current flows in the region between the electrodes, and thus little heating occurs there 14,16
For these reasons, we used the bipolar mode of RF power application in our device. This mode has two advantages for this device. First, no ground pads are required since at any time, the RF power ground is connected to one of the electrodes in the tissue, not an electrode on the skin. Ground pad skin burns are already a common complication of clinical monopolar RF ablations (which use a maximum power of 200 – 250 W), and since rapid coagulation of a large tissue slice requires much higher power (up to ~400 W), skin burns below ground pads would be likely if the monopolar mode were used with this device. Secondly, previous computer modeling experiments have shown that bipolar ablation produces higher, more uniform temperatures between adjacent electrodes than monopolar ablation 14,17
Initially, we created computer models where we applied bipolar power to all of the electrodes in the array simultaneously; i.e. the electrodes were all energized simultaneously with alternating polarity (+ polarity, − polarity, +, −, etc.). However, the resulting temperature profile was not uniform between each electrode pair. In addition, this method did not allow independent power regulation between each pair to account for variations in tissue thickness and the presence or absence of heat-sinking blood vessels.
As a result, we investigated the application of bipolar RF power to a single pair of electrodes at a time, rapidly switching the energy delivery to each pair of electrodes in the array in a repeating sequence (). This technique takes advantage of the relatively slow time constant of conductive heat transfer in tissue (typically in the range of seconds). If power application is switched sufficiently quickly between electrode pairs, little heat is transferred away from the target tissue during the time when each pair is not energized, and the whole tissue plane can be virtually heated simultaneously. This rapid switching technique has previously been used with monopolar RF energy to create large ablation zones in ex vivo tissue 18
Computer models of this switched bipolar power application algorithm in combination with blade shaped electrodes showed excellent concentration of current density between the electrodes and correspondingly high, as well as uniform, temperatures (see ) 14
. Additionally, since power is delivered to only a single electrode pair at a time, the applied power can be controlled individually for each pair, allowing more uniform heating of the entire tissue plane. In our prior study, power was switched between each pair every 600 ms 14
. This time period did not prove to be sufficient to fully coagulate all blood vessels in the resection plane, so in this study we modified the control software to switch the power between the electrode pairs every 150 ms. Since we were able to completely coagulate all blood vessels (up to 4.5 mm diameter) in this study, we believe that this switching period is sufficiently short for the device to be effective in this regard. However, as the number of active electrode pairs is increased (i.e. more than our current prototype’s 3 pairs), the amount of time that a given pair will not have power applied to it will increase correspondingly, and the switching time period may need to be reduced to compensate.
Temperature distribution after 3 min ablation in bipolar mode between needle electrodes (left), and blade electrodes (right). Blade electrodes produce significantly higher, and more uniform temperatures. Reproduced with permission from (14).
To determine the ideal electrode design and spacing for this device, we relied on our previously reported computer modeling results demonstrating that bipolar RF ablation using rectangular, blade-shaped, electrodes produced significantly higher temperatures in the region between adjacent electrodes than standard needle electrodes14
. Additionally, further modeling demonstrated that a 1.5 cm blade separation creates a more uniform ablation zone width than a 2 cm blade separation. The same models suggested that a 1.5 cm blade separation creates a similar ablation zone width as a 1 cm blade separation. Both of these modeling results were verified in preliminary ex-vivo
experiments. We used the 1.5 cm separation in this study instead of 1 cm because it required fewer electrodes to span a given resection plane (e.g. for a 6 cm long resection plane, 5 electrodes would be required at 1.5 cm spacing, or 7 electrodes with 1 cm spacing), resulting in less tissue trauma during electrode insertion.
We have in a previous study used an earlier prototype to perform partial kidney resection 15
. In the current study we applied the device to liver resection in a porcine model. We were able to provide a coagulation plane within 3 min for resection planes that required 4 electrodes or less, and within 6 min (2×3 min) if more than 4 electrodes were required. This is due to a limitation of the current prototype that allows switching between a maximum of four electrodes; a future design may circumvent this limitation. In addition, we were able to coagulate all blood vessels in all seven trials in this study, which were up to 4.5 mm in diameter as measured by IOUS. This is close to the maximum blood vessel diameter expected in human liver resection (~5 mm). We used liver inflow occlusion during this study to ensure coagulation of all vessels, but currently have no data on performance of our device when occlusion is omitted.
While in initial studies tumor ablation electrodes were used to sequentially coagulate small tissue volumes along the hepatic resection plane 8–11
, recently two commercial devices have become available for this purpose. The Tissuelink device (Tissuelink Medical, Dover, NH) uses a small spherical electrode that is assisted by saline infusion; the electrode is manually moved along the resection plane to dissect tissue while coagulating 19
. This is a lengthy procedure and does not allow coagulation of vessels larger than 3mm in diameter even when occlusion is used12
. A second device, the Habib4x (Rita Medical Systems, Fremont, CA) uses two pairs of needle electrodes with 1 cm spacing to precoagulate the resection plane with multiple sequential ablations - typically with blood inflow occlusion. While there is currently no extensive data available on performance of this device, the large number of required applications to created a coagulation plane results in significantly lengthier procedures than the device described in this study13