Estimation of Electrode-Tissue Contact Using a Shaft Electrode and an Edge Electrode

Abstract

The invention is titled "Estimation of Electrode-Tissue Contact Using a Shaft Electrode and a Peripheral Electrode". The invention discloses a system that includes an expandable frame and a processor. The expandable frame includes (i) one or more ablation electrodes disposed on an outer surface of the frame and configured to be placed in contact with wall tissue of a patient's cavity, and (ii) a shaft electrode and a peripheral electrode coupled to only the proximal and only the distal sides of a balloon, respectively. The processor is configured to: (a) measure one or more first impedances between one or more of the ablation electrodes and the shaft electrode, (b) measure one or more second impedances between one or more of the ablation electrodes and the peripheral electrode, and (c) based on the first impedance and the second impedance, determine, for at least one of the one or more ablation electrodes, whether the ablation electrode is in physical contact with the wall tissue.

Description

Technical Field

[0001] The present invention generally relates to medical probes and, more particularly, to multi-electrode cardiac ablation catheters. Background Art

[0002] Various techniques for verifying contact of the electrodes of a catheter with cardiac tissue have been proposed in the patent literature. For example, U.S. Patent Application Publication 2007 / 0255162 describes a method and system for providing tissue contact assessment by providing a catheter having a shaft with a plurality of electrodes, positioning the catheter at a tissue treatment site, applying a current between at least two of the plurality of electrodes, measuring an impedance voltage between at least two of the plurality of electrodes, and processing the measured impedance voltage caused by the applied current to provide a contact assessment.

[0003] As another example, U.S. Patent Application Publication 2012 / 0143179 describes a number of catheter-based ablation device embodiments that include a balloon catheter that treats several regions of atrial target tissue and is characterized by firm and consistent ablation element-tissue contact to allow for the creation of effective continuous lesions. In one embodiment, energy can be applied to a distal annular electrode of the balloon catheter and a reference electrode positioned on the balloon catheter shaft only proximal to the balloon to measure the conductance coefficient on the balloon. If the balloon tightly seals the PV, the impedance increases and the measurement can also be used to verify PV closure. Summary of the Invention

[0004] Embodiments of the present invention provide a system that includes an expandable frame and a processor. The expandable frame is coupled to a distal end of a shaft for insertion into a lumen of a patient's organ and includes one or more ablation electrodes disposed on an outer surface of the frame, wherein the one or more ablation electrodes are configured to be placed in contact with wall tissue of the lumen. The expandable balloon also includes a rod electrode coupled to the distal end of the shaft proximal to the balloon and a rim electrode coupled to the distal end of the shaft distal to the balloon. The processor is configured to: (a) measure one or more first impedances between one or more of the ablation electrodes and the rod electrode, (b) measure one or more second impedances between one or more of the ablation electrodes and the rim electrode, and (c) based on the first impedance and the second impedance, determine, for at least one of the one or more ablation electrodes, whether the ablation electrode is in physical contact with the wall tissue.

[0005] In some embodiments, the processor is configured to determine that the ablation electrode is in physical contact with the tissue by determining that a measured first impedance or second impedance is greater than a pre-specified impedance by at least a pre-specified minimum value.

[0006] In some embodiments, the ablation electrode in contact with blood is utilized to measure the pre-specified impedance.

[0007] In one embodiment, the pre-specified minimum value is stored in a look-up table.

[0008] In another embodiment, the system further includes a relay configured to switch between two or more of the following configurations under the control of the processor: (i) a first configuration for measuring the impedance between the ablation electrode and the shaft electrode and the edge electrode, (ii) a second configuration for measuring the impedance between the ablation electrode and one or more body surface electrodes, and (iii) a third configuration for performing ablation by driving an electrical signal between the ablation electrode and the back patch electrode.

[0009] In some embodiments, the expandable frame includes an expandable balloon, and the outer surface of the frame includes the outer surface of the membrane of the balloon.

[0010] According to an embodiment of the present invention, a method is also provided, the method including inserting an expandable balloon coupled to the distal end of a shaft into a lumen of a patient's organ, the expandable balloon including one or more ablation electrodes disposed on the outer surface of the membrane of the balloon, a shaft electrode proximally coupled to the distal end of the shaft to the balloon, and an edge electrode distally coupled to the distal end of the shaft to the balloon. Placing one or more of the ablation electrodes in contact with the wall tissue of the lumen. Measuring one or more first impedances between one or more of the ablation electrodes and the shaft electrode. Measuring one or more second impedances between one or more of the ablation electrodes and the edge electrode. Based on the first impedance and the second impedance, determining, for at least one of the one or more ablation electrodes, whether the ablation electrode is in physical contact with the wall tissue.

[0011] In conjunction with the accompanying drawings, the present invention will be more fully understood through the following detailed description of embodiments of the present invention, wherein: Brief Description of the Drawings

[0012] Figure 1 Is a schematic diagram of a catheter-based positioning-tracking and ablation system according to an embodiment of the present invention, the system including an expandable ablation catheter in the form of a balloon;

[0013] Figure 2 Is a schematic diagram of Figure 1 a balloon catheter in physical contact with the wall tissue of the lumen according to an embodiment of the present invention;

[0014] Figure 3A and Figure 3B are a schematic circuit diagram of an ablation electrode coupled to a marginal electrode when the ablation electrode is in contact with and fully in contact with a tissue portion, respectively, according to an embodiment of the present invention; and

[0015] Figure 4 is a flowchart schematically showing a method and algorithm for determining an ablation electrode in full contact with tissue according to an embodiment of the present invention. DETAILED DESCRIPTION

[0016] Overview <Q

[0017] Multi-electrode ablation catheters such as balloon ablation catheters or basket catheters typically include an expandable frame (e.g., an expandable balloon) coupled to the distal end of a shaft for insertion into a lumen of a patient's organ. To achieve optimal results of ablation therapy, a physician may need to determine that each ablation electrode disposed on the frame (e.g., the balloon) is in physical contact with the wall tissue of the lumen to be ablated. For example, when a balloon catheter having multiple ablation electrodes is used to ablate the ostium of a pulmonary vein (PV), typically all of the ablation electrodes of the catheter should be positioned such that they are in full contact with the PV tissue.

[0018] However, many times, some of the ablation electrodes may not be in full contact with the tissue, and instead, different portions of some of the ablation electrodes may be immersed in blood. For these electrodes that are not ablating tissue, the applied electrical power may cause undesirable side effects such as clot formation.

[0019] The embodiments of the present invention described below provide systems and methods capable of determining whether an ablation electrode is in full contact with tissue (e.g., fully covered by tissue). In some embodiments, a balloon ablation catheter is provided that includes: (i) at least one ablation electrode, (ii) an electrode disposed on the distal end of the shaft only proximal to the balloon (hereinafter referred to as a "shaft electrode"), and (iii) an electrode disposed on the distal end of the shaft only distal to the balloon (hereinafter referred to as a "marginal electrode"). Using the impedance measurement results between each ablation electrode and the shaft electrode and the marginal electrode, a processor of the ablation system determines for each ablation electrode whether the ablation electrode is in full contact with tissue.

[0020] In some embodiments, when the ablation electrode is at least partially exposed to blood, the processor of the system compares the measured in-situ impedance between the ablation electrode intended to contact tissue and the shaft electrode and the edge electrode with the same measured impedance. In the case of full contact, the impedance measured in-situ should be at least a pre-specified minimum value greater than the impedance measured using the ablation electrode located in the blood. Different minimum values of the impedance difference can be pre-specified depending on, for example, the number of electrodes that have made full contact with the tissue. The pre-specified minimum value can be stored, for example, in a look-up table.

[0021] The above-mentioned pre-specified minimum impedance difference is determined at a typical RF frequency of a few kHz, at which the impedance of cardiac tissue is usually several times that of blood (in some cases, about 300 Ω in tissue compared to about 100 Ω in blood). Additional information on the tissue impedance relative to the blood impedance as a function of the RF frequency can be obtained, for example, from "Medical Instrumentation: Application and Design", edited by Webster, 3rd edition, John Wiley & Sons, Inc., New York, 1998.

[0022] The measurement geometry disclosed in the present invention involves comparable path lengths in blood and tissue, so the measured impedance changes mainly due to different tissue properties. This feature of the technology disclosed in the present invention provides a high degree of certainty for the processor to make a distinction based on the measurement results between blood contact and tissue contact.

[0023] To verify that full physical contact with the tissue has been achieved from both ends (i.e., proximal and distal) of the elongated ablation electrode, measurements relative to the shaft electrode and the edge electrode need to be performed.

[0024] If full physical contact has not been achieved for all ablation electrodes, the physician can manipulate the balloon catheter to establish more complete contact between the ablation electrode and the tissue, and use the technology disclosed in the present invention to check the adequacy of the contact again.

[0025] In some embodiments, to measure the position of the balloon catheter within an organ, the ablation system includes a positioning and tracking subsystem that measures the impedance between the ablation electrode and a surface electrode. The method further described below is sometimes referred to as Advanced Catheter Localization (ACL). The system can use relays to switch the electrical connections between the ablation electrode and the surface electrode and between the ablation electrode and the shaft electrode and the edge electrode of the balloon catheter, so that the electrode position and the degree of contact of the electrode with the tissue at that position can be measured interchangeably.

[0026] In addition, the system can use a relay to switch the electrical connection between the ablation electrode and the shaft electrode and the edge electrode (for evaluating contact) or the surface electrode (for measuring position) to the back patch electrode for performing ablation by driving an electrical signal between the ablation electrode and the back patch electrode.

[0027] Typically, the processor is programmed with software that includes a specific algorithm that enables the processor to perform each of the processor-related steps and functions listed below.

[0028] By determining in real time which ablation electrodes are in full contact with the tissue and which ablation electrodes are not in full contact with the tissue, the techniques disclosed in the present invention can improve the safety and effectiveness of multi-electrode ablation therapy.

[0029] System Description

[0030] As used herein, the term "about" or "approximately" in reference to any numerical value or range indicates a suitable dimensional tolerance that allows a portion or collection of components to achieve its intended purpose as described herein. More specifically, "about" or "approximately" can refer to a range of values of ±20% of the recited value. For example, "about 90%" can refer to a range of values from 71% to 99%. Additionally, as used herein, the terms "patient," "host," "user," and "subject" refer to any human or animal subject, and are not intended to limit the system or method to human use, although the use of the subject invention in human patients represents a preferred embodiment. Figure 1 FIG. 14 is a schematic illustration of a catheter-based positioning-tracking and ablation system 20 according to an embodiment of the present invention. The system includes an ablation balloon catheter 40. Typically, the balloon catheter 40 is used for therapeutic procedures, such as ablating cardiac tissue at the left atrium, for example. The system 20 is used to determine the position of the balloon catheter 40 coupled to the distal end of the shaft 22 shown in inset 25. The system 20 is also used to determine, for example, whether each of the ablation electrodes 50 of the balloon catheter 40 is in contact with the tissue before performing ablation.

[0031] The physician 30 navigates the balloon catheter 40 to a target location in the heart 26 of the patient 28 by using a manipulator 32 near the proximal end of the catheter to manipulate the shaft 22 and / or deflect from the sheath 23. The balloon catheter 40 is inserted through the sheath 23 in a collapsed configuration, and only after the balloon is retracted from the sheath 23 does the balloon catheter 40 resume its intended functional shape. By including the balloon catheter 40 in a collapsed configuration, the sheath 23 also serves to minimize vascular trauma along its path to the target location.

[0032] The balloon catheter 40 includes an elongated and large-area ablation electrode 50 disposed on the outer surface of the balloon membrane. The shaft electrode 51 is disposed only proximally of the balloon on the distal end of the shaft 22. The edge electrode 52 is disposed only distally of the balloon on the distal end of the shaft 22. The electrodes 51 and 52 are used to determine whether each ablation electrode in the ablation electrode 50 is in full contact with the tissue or at least partially immersed in blood.

[0033] The ablation electrode 50, the shaft electrode 51, and the edge electrode 52 are connected to an interface circuit 44 in the console 24 by wires extending through the shaft 22. A detailed view of the balloon catheter 40 with the ablation electrode 50, the shaft electrode 51, and the edge electrode 52 is shown in Figure 2 in.

[0034] Additionally, using the aforementioned ACL method, the ablation electrode 50 can be used to measure the position of the balloon catheter 40 within the heart 26 by sensing the impedance relative to the surface electrode 49, which in an exemplary system is shown attached to the chest of the patient 28 by a wire extending through the cable 39. The ACL method for tracking the position of the electrode 50 is implemented in various medical applications, such as in the CARTO TM system manufactured by Biosense-Webster Inc. (Irvine, California), and is described in detail in U.S. Patents 7,756,576, 7,869,865, 7,848,787, and 8,456,182, the disclosures of which are incorporated herein by reference. The console 24 drives the display 27, which shows the tracked position of the balloon catheter 40 within the heart 26.

[0035] The console 24 includes a processor 41 (commonly a general-purpose computer) and suitable front-end and interface circuits 44 for transmitting and receiving signals, such as RF signals and position signals, respectively. The interface circuit 44 can also receive an electrocardiogram from the surface electrode 49 and / or from any electrode disposed on the catheter.

[0036] In some embodiments, the processor 41 controls the relay 60 in the system 20 to switch the electrical connection between two or more of the following configurations: (i) a first configuration having a connection (62) between the ablation electrode and the surface electrode 49 for measuring the impedance between the ablation electrode and one or more body surface electrodes, (ii) a second configuration having a connection (64) between the ablation electrode and the shaft electrode and the edge electrode of the balloon catheter for measuring the impedance between the ablation electrode and the shaft electrode and the edge electrode, wherein the connections 62 and 64 are used to interchangeably measure the electrode position and the degree of contact of the electrode with the tissue at that position, and (iii) a connection (66) between the ablation electrode and a back patch electrode (not shown) for performing ablation by driving an electrical signal between the ablation electrode and the back patch electrode.

[0037] The processor 41 is typically programmed in software to perform the functions described herein. The software can be downloaded electronically to the computer via a network, for example, or alternatively or additionally it can be set up and / or stored on a non-transitory tangible medium (such as magnetic memory, optical memory, or electronic memory). Specifically, the processor 41 runs a dedicated algorithm as disclosed herein, which is included in Figure 4 and enables the processor 41 to perform the steps disclosed by the present invention, as further described below.

[0038] For simplicity and clarity, Figure 1 only the elements related to the technology disclosed by the present invention are shown. The system 20 generally includes additional modules and elements such as a flushing and temperature module, which are not directly related to the technology disclosed by the present invention and are therefore intentionally omitted from Figure 1 and the corresponding description.

[0039] Estimation of Balloon Electrode-Tissue Contact Using a Shaft Electrode and an Edge Electrode

[0040] Figure 2 A schematic diagram of the balloon catheter 40 in physical contact with the wall tissue 48 according to an embodiment of the present invention. The balloon catheter 40 includes an ablation electrode 50 disposed on the membrane 46 of the balloon. The shaft electrode 51 and the edge electrode 52 are disposed at the distal end of the shaft 22 and immersed in the blood 55. Figure 1 As shown, the ablation electrode 50 at the top of the balloon is in full contact with the tissue, i.e., over the entire area of the electrode. On the other hand, the electrode 50 at the bottom has a distal region 50a immersed in the blood 55. Correspondingly,

[0041] and Figure 3A and Figure 3B describe different measured impedance values between the top ablation electrode and the bottom ablation electrode and the edge electrode 52 indicating partial contact and full contact of the top ablation electrode and the bottom ablation electrode with the tissue, respectively.

[0042] Figure 2 Given by way of example and simplified for clarity of presentation. For example, for simplicity, balloon elements unrelated to the implemented invention such as temperature sensors and flushing holes are omitted.

[0043] Figure 3A and Figure 3B are schematic circuit diagrams of the ablation electrode 50 connected to the edge electrode 52 when the ablation electrode 50 is in partial contact and full contact with the tissue 48, respectively, according to an embodiment of the present invention. Figure 3A illustrates the case of the ablation electrode 50, which has a distal region (such as Figure 2 the region 50a shown) immersed in the blood 55, resulting in insufficient tissue contact of the electrode 50. As shown, the impedance between the ablation electrode 50 and the edge electrode 52 is equal to the parallel value of the blood impedance R B and the bypass resistance R S , which may originate from channels in the blood and / or tissue and / or other conductors. In short, this is expressed as |Z_insufficient| = R B ║R S . In the case where most of the balloon is immersed in the blood such that the bypass resistivity is determined by the blood resistivity, the minimum value of Z_insufficient is approximately R B / 2. In the case where the bypass resistivity is infinite, the maximum value is R B . For a typical blood resistivity value of about 100 ohms, Z_insufficient lies in the range of 50 ohms to 100 ohms.

[0044] Figure 3B illustrates the case of the ablation electrode 50 being fully contacted by the tissue (i.e., fully covered by the tissue). As shown, the impedance between the ablation electrode 50 and the edge electrode 52 is the parallel value of the series impedance of the blood and tissue R B +R T and the bypass resistance R S . In short, this is expressed as |Z_sufficient| = (R B +R T )║R S . As described above, since the tissue impedance is significantly greater than the blood impedance, the measured "sufficient" impedance is typically large enough, for example, at least a few ohms, larger than the "insufficient" impedance, and thus the method disclosed in the present invention can utilize, for example, a calibrated threshold impedance value to distinguish between these two cases.

[0045] In the case where most of the balloon is immersed in the blood such that the bypass resistivity is determined by the blood resistivity, the minimum value of Z_sufficient is approximately R B, in this case, repositioning of the balloon is required due to low bypass resistivity. Where bypass resistivity is mainly via tissue formation, the practical threshold for Z_sufficient is R T . For typical blood resistivity values of about 100 ohms and tissue resistivity values of 300 ohms, Z_sufficient is higher than 150 ohms. However, a lower limit value still higher than about 100 ohms can be used as the threshold for Z_sufficient, depending on, for example, measurement reproducibility.

[0046] In one embodiment, the processor 41 is configured to determine physical contact of the ablation electrode with tissue by determining that a measured first impedance or second impedance is at least a pre-specified minimum value greater than a pre-specified impedance, the pre-specified minimum value being given in a look-up table which, by way of example, has the form of Table I:

[0047] Contact Level Pre-Specified Minimum Value Lowest Adequate Approximately 110 Ohms Adequate Approximately 130 Ohms Good Approximately 150 Ohms Excellent Approximately 200 Ohms

[0048] Table I

[0049] Figure 3A and Figure 3B is fully applicable to the rod electrode 51. By measuring the impedance between the ablation electrode 50 and both the rod electrode 51 and the edge electrode 52, the techniques disclosed in the present invention verify that full physical contact with tissue has been achieved from both ends of the elongated ablation electrode.

[0050] Figure 3A and Figure 3B The circuit diagrams shown are highly simplified for the purpose of presenting the concept. The actual values can be determined empirically or by a more complex electrical model.

[0051] Figure 4 is a flowchart schematically showing a method and algorithm for determining full contact of an ablation electrode with tissue according to an embodiment of the present invention. The algorithm according to this embodiment implements the following process, which begins at a balloon positioning step 80 where the physician 30 positions a partially inflated balloon catheter 40 at a target location within a heart chamber 26 of the heart 26 (such as at the ostium of a pulmonary vein). Next, in a balloon inflation step 82, the physician 30 inflates the balloon to bring the ablation electrode 50 into full contact with the tissue. Next, at an impedance measurement step 74, the system 20 measures the impedance between each ablation electrode in the ablation electrode 50 and the rod electrode (51) and the edge electrode (52).

[0052] At the physical contact determination step 86, based on the measured impedance, the processor 41 determines, for each ablation electrode 50, whether the electrode is in full contact with the tissue, as defined above. If, at the contact check step 88, the processor determines that all ablation electrodes 50 are in full contact with the tissue, the method proceeds to perform ablation at the ablation step 90. On the other hand, if the processor 41 determines that one or more electrodes have inadequate contact with the tissue (due to inadequate impedance (Table I), as measured by one or more electrodes), the physician 30 repositions the balloon catheter 40 in an attempt to improve contact, and the process loops back to step 84 to re-evaluate the adequacy of contact.

[0053] Figure 4 The exemplary flow chart shown is chosen solely for clarity of concept. This embodiment also includes additional steps of the algorithm, such as acquiring an intracardiac electrocardiogram, which have been intentionally omitted from the disclosure herein to provide a more simplified flow chart. In addition, other steps such as temperature measurement and application of irrigation have been omitted for purposes of clarity of presentation.

[0054] Although the embodiments described herein primarily relate to cardiac applications, the methods and systems described herein can also be used for other applications, such as renal denervation.

[0055] Accordingly, it should be understood that the embodiments described above are cited by way of example, and the present invention is not limited to what is specifically shown and described above. Rather, the scope of the present invention includes combinations and sub-combinations of the various features described above, as well as their variations and modifications, which will occur to those of ordinary skill in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference into this patent application are considered an integral part of this application, except that any terms defined in these incorporated documents that conflict with the definitions expressly or implicitly given in this specification shall be considered only in light of the definitions in this specification.

Claims

1. A system for providing tissue contact assessment, comprising: An inflatable frame coupled to a distal end of a shaft for insertion into a lumen of a patient's organ, the inflatable frame including one or more ablation electrodes disposed on an outer surface of the inflatable frame, wherein the one or more ablation electrodes are configured to be placed in contact with wall tissue of the lumen; A rod electrode and an edge electrode, the rod electrode being coupled to the distal end of the shaft proximal to the inflatable frame, the edge electrode being coupled to the distal end of the shaft distal to the inflatable frame; And A processor configured to: Measure one or more first impedances between the one or more ablation electrodes and the rod electrode; Measure one or more second impedances between the one or more ablation electrodes and the edge electrode; And Based on the one or more first impedances and the one or more second impedances, determine, for at least a first ablation electrode from the one or more ablation electrodes, whether the first ablation electrode is in physical contact with the wall tissue.

2. The system according to claim 1, wherein the processor is configured to determine that the first ablation electrode is in physical contact with the wall tissue by determining that one or more measured first impedances or one or more second impedances are greater than a pre-specified impedance by at least a pre-specified minimum value.

3. The system according to claim 2, wherein the pre-specified impedance is measured using the first ablation electrode in contact with blood.

4. The system according to claim 2, wherein the pre-specified minimum value is stored in a look-up table.

5. The system according to claim 1, and including a relay configured to switch, under control of the processor, between two or more of the following configurations: (i) a first configuration for measuring impedances between the one or more ablation electrodes and the rod electrode and the edge electrode, (ii) a second configuration for measuring impedances between the one or more ablation electrodes and one or more body surface electrodes, and (iii) a third configuration for performing ablation by driving an electrical signal between the one or more ablation electrodes and a back patch electrode.

6. The system according to claim 1, wherein the inflatable frame includes an inflatable balloon, and wherein the outer surface of the inflatable frame includes an outer surface of a membrane of the inflatable balloon.

7. The system according to claim 4, wherein the pre-specified impedance measured by the first ablation electrode is from about 100 ohms to about 130 ohms.

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