Radiofrequency Ablation Devices
. Radiofrequency ablation devices work by sending alternating current through the tissue. This creates increased intracellular temperatures and localized interstitial heat. When temperatures exceed 60°C, cell proteins rapidly denature and coagulate, resulting in a lesion. The lesion can be used to resect and remove the tissue or to simply destroy the tissue, leaving the ablated tissue in place. [Image: Covidien Cool-tipTM RF Ablation System.]
Due to adverse effects such as charring, as well as perceived limitations on the use of RF devices, improvements have been developed and marketed which are designed to better control and direct the high temperatures produced at the tip of the RF device. These include so-called temperature controlled and fluid (saline or water) cooled versions.
Temperature Controlled. One way to control the temperature at the tip of the RF electrode is to build in a feedback mechanism between the tip of the device and the RF generator. For example, in a device such as Covidien’s Cool-tip system, the RF generator was designed with a feedback algorithm. This algorithm senses tissue impedance and automatically delivers the best amount of radiofrequency energy for the situation. The Cool-tip’s electrode design serves further to eliminate tissue charring while allowing delivery of the maximum amount of energy, resulting in the ability to ablate a larger zone of tissue more quickly.
Fluid Cooled. In the highly-popular fluid-cooled devices, the electrode’s internal circulation of water cools the tissue adjacent to the exposed electrode, maintaining low impedance during the treatment cycle. Low impedance permits maximum energy deposition for a larger ablation volume. It also decreases the risk of charring, and allows the surgeon to work faster.
Catheter Manipulation Systems. Another recent development is the use of routine RF electrophysiology catheters in conjunction with a robotic catheter manipulation system, for both diagnostic and ablative EP applications. Examples of such systems include Catheter Robotics’ Remote Catheter Manipulation System, Hansen Medical’s Artisan Control Catheter/Sensei Robotic Catheter System, and Stereotaxis’ Magnetic Navigation System.
Benefits of such systems include use of a catheter which is already familiar to the EP; the ability to manipulate the catheter (once it has been inserted into the heart) via a system from outside of the zone of radiation, and thus eliminate the need to wear the lead aprons; and the EP can then guide the procedure while watching the EP monitors and x-ray images.
Hansen Medical’s system has been used in the ablation of AF. The Stereotaxis Magnetic Navigation System utilizes two magnets, external to the patient, to create a magnetic field. This generates sufficient force to steer magnetically tipped, FDA-approved catheters and guidewires. The system has been used in various EP procedures, including mapping, pacing, and ablation.
It is probably fair to say that there is more media noise about RF ablation than about all of the other modalities put together. This is due to the (primarily off-label) use of RF ablation devices to treat atrial arrhythmias. Since February 2009, there has been an RF catheter FDA cleared for AF ablation, BioSense Webster’s NaviStar ThermoCool saline irrigated radio-frequency ablation catheter and the EZ Steer ThermoCool Nav. All other RF catheters used for AF ablation in the USA are being used off label. BioSense Webster is a Johnson & Johnson company.