Surgery – Instruments – Electrical application
Reexamination Certificate
2001-04-04
2002-12-24
Peffley, Michael (Department: 3739)
Surgery
Instruments
Electrical application
C607S101000
Reexamination Certificate
active
06497704
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrosurgical apparatus for tissue ablation generally, and particularly to electrosurgical apparatus for radio frequency (RF) tissue ablation.
BACKGROUND OF THE INVENTION
Radio frequency (RF) tissue ablation is a well-known technique for making thermal lesions in the vicinity of an uninsulated tip of an electrode due to tissue coagulation caused by resistive heating. The electrode can be applied directly on superficial structures, surgically, endoscopically, laparascopically, or even via a transcatheter access such as a treatment for symptomatic cardiac arrhythmias. If the electrode is formed as a needle, then the electrode may be inserted interstitially, and guided by imaging.
As is known in the art, resistive heating is proportional to the square of the current density, the latter being inversely proportional to the square of the distance from the ablation electrode. Therefore, resistive heating decreases from the ablation electrode with the distance to the fourth power. In other words, significant resistive heating only occurs within a narrow rim (of a few millimeters) of tissue in direct contact with the ablation electrode. Deeper tissue heating occurs as a result of passive heat conduction from that rim.
A general problem in RF tissue ablation is limitation in lesion size. Increasing the power to the electrode or exposure time to the tissue increases the amount of energy delivered around the electrode and thereby increases the lesion size. However, at high temperatures (e.g., above 100° C.) at the electrode-tissue interface, the impedance increases significantly because of desiccation followed by charring around the electrode tip. This leads to an abrupt fall in lesion current (and delivered effect) and no further energy is delivered around the electrode, and no further tissue heating occurs. This phenomenon tends to limit lesion size in the transverse direction around the electrode. The longitudinal dimension of the lesion is basically dependent on the length of the uninsulated part of the electrode.
Tissue adhesion and limited lesion size are just two examples of problems associated with RF tissue ablation. Another problem occurs when attempting to ablate tissue in difficult-to-reach locations, such as the lower esophagus. It is difficult to control the depth of the ablation into the tissue and to obtain a uniform ablation over the entire surface area that needs to be ablated. Surface irregularities, folds into the tissue, and variations in the anatomical configurations of body cavities increase the difficulty of achieving uniform RF tissue ablation. As a result, only a portion of the tissue to be ablated may be destroyed, and in some areas, more tissue may be ablated than was intended.
Attempts have been made in the prior art to overcome the abovementioned problems. One known solution is that of “electrolyte-assisted” ablation. This form of ablation relies on contacting the tissue to be ablated with an electrolyte, such as a fluid or gel. Electrical energy is applied through the electrolyte to the tissue in contact with the electrolyte. Because the electrical resistance of the electrolyte-tissue interface is significantly high relative to the resistance of the electrolyte itself, most of the energy will be dissipated at this interface in the form of heat, leading to thermal ablation of the superficial tissue at this interface. Introduction of a conducting and/or cooling liquid into the treated area generally increases the coagulated volume and tends to reduce tissue adhesion. Electrolyte-assisted ablation is also effective in difficult-to-reach locations, because the liquid or gel electrolyte effectively bathes the entire surface area of the tissue that is to be ablated.
One example of electrolyte-assisted ablation is discussed in U.S. Pat. No. 6,112,123 to Kelleher et al., assigned to Endonetics, Inc., San Diego, Calif., USA, the disclosure of which is incorporated herein by reference. Kelleher et al. describes electrolyte-assisted tissue ablation of metaplasia in the esophagus by means of a shaft with an expandable barrier that is deployable from the shaft's distal end. An electrode and a temperature/impedance sensor are mounted on the shaft. The shaft may be a catheter formed with several lumens that are used independently for housing optical elements and for transferring fluid.
In the operation of the device, the distal end of the shaft or catheter is placed in the esophagus proximal the cardia. The barrier is then deployed to the cardia and expanded to seal the esophagus from the stomach. The esophageal volume between the barrier and the catheter is partially flooded with a conducting medium and the electrode is deployed into the conducting medium. The sensor is also deployed into contact with the tissue to be ablated. Using open-loop control, or using temperature, impedance, or visual monitoring for closed-loop control, the metaplasia is ablated by passing RF energy from the electrode and through the conducting medium for contact with the tissue being ablated.
Kelleher et al. limits the RF power to a moderate level by using the closed-loop control, wherein the output of the temperature sensors near the electrode is used to control power delivery. However, a disadvantage of Kelleher et al. is that enlargement of the coagulated volume is at the expense of slower coagulation speed and prolonged treatment time.
Another example of electrolyte-assisted ablation is described in U.S. Pat. No. 5,348,554 to Imran et al., assigned to Cardiac Pathways Corporation, Sunnyvale, Calif., USA, the disclosure of which is incorporated herein by reference. Imran et al. includes a catheter constructed of an elongate member having proximal and distal extremities. A metal conducting electrode is secured to the distal extremity of the elongate member and has a chamber formed therein. A conductor extends through the elongate member from the proximal to the distal extremity for supplying RF energy to the electrode. The elongate member has a lumen in the distal extremity, which is in communication with the chamber. A coolant is disposed in the chamber and in contact with the electrode for dissipating heat created in the electrode by the application of RF energy thereto. A disadvantage of Imran et al. is that the coagulated volume is limited due to the relatively small diameter of the electrode used.
Other examples of cooled electrodes include U.S. Pat. No. 5,100,388 to Behl et al., which describes a catheter having a conductive material delivery lumen and a distal tip heating element, suitable for hollow body organs, such as the gall bladder. U.S. Pat. No. 5,304,214 to DeFord et al., describes an RF ablation device specifically designed to selectively ablate prostatic tissue about the prostatic urethra. The disclosures of these patents are incorporated herein by reference. A disadvantage of Behl et al. or DeFord et al. is that the structure of these devices is custom-made to fit a particular anatomy and is not generally suitable for any other anatomy in the body. They also suffer from limited coagulated volume that is possible with the electrode.
In order to overcome the limitation of the small coagulated volume associated with a fine linear needle electrode, U.S. Pat. Nos. 5,431,649 and 6,016,809 to Mulier et al., assigned to Medtronic, Inc., Minneapolis, Minn., USA, the disclosures of which are incorporated herein by reference, propose a hollow helical electrode, with injection of a cooled conducting liquid.
In general, Mulier et al. is directed to treatment of tachyarrhythmias, wherein one or more chambers of the heart exhibit an excessively fast rhythm. In particular, Mulier et al. is directed to treatment of tachycardias, which are due to the presence of ectopic foci within the cardiac tissue or due to the presence of aberrant condition pathways within the cardiac tissue. In Mulier et al., a catheter is provided with a hollow, helical electrode, which is screwed into cardiac tissue at a desired ablation site and connected
Dekel Patent Ltd.
Klein David
Peffley Michael
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