Patent Description:
Various techniques for verifying contact of an electrode of a catheter with cardiac tissue have been proposed in the patent literature. For example, <CIT> describes methods and systems for providing tissue contact assessment, by providing a catheter having a shaft having a plurality of electrodes, positioning the catheter at a tissue treatment site, applying an electrical current between at least two of the plurality of electrodes, measuring impedance voltage between the at least two of the plurality of electrodes and, processing the measured impedance voltage caused by the applied electrical current to provide contact assessment.

As another example, <CIT> describes a plurality of catheter-based ablation apparatus embodiments, including balloon catheters, that address several areas of atrial target tissue and which feature firm and consistent ablation element to tissue contact, to enable the creation of effective continuous lesions. In an embodiment, energy may be applied to a distal ring electrode of a balloon catheter, together with a reference electrode positioned on the balloon catheter shaft just proximal to the balloon, to measure the conductance across the balloon. If the balloon solidly occludes the PV, the impedance rises and the measurement can also be used to verify PV occlusion.

<CIT> discloses devices for therapeutic nasal neuromodulation and associated systems and methods are disclosed herein. A system for therapeutic neuromodulation in a nasal region configured in accordance with embodiments can include, for example, a shaft and a therapeutic element at a distal portion of the shaft. The shaft can locate the distal portion intraluminally at a target site inferior to a patient's sphenopalatine foramen. The therapeutic element can include an energy delivery element configured to therapeutically modulate postganglionic parasympathetic nerves at microforamina of a palatine bone of the human patient for the treatment of rhinitis or other indications. In other embodiments, the therapeutic element can be configured to therapeutically modulate nerves that innervate the frontal, ethmoidal, sphenoidal, and maxillary sinuses for the treatment of chronic sinusitis.

The methods described here below are not part of the claimed invention.

An embodiment of the present invention provides a system including an expandable frame and a processor. The expandable frame is coupled to a distal end of a shaft for insertion into a cavity of an organ of a patient and includes one or more ablation electrodes disposed over an external surface of the frame, wherein the one or more ablation electrodes are configured to be placed in contact with wall tissue of the cavity. The expandable frame further includes a stem electrode coupled to the distal end of the shaft proximally to the frame, and an edge electrode coupled to the distal end of the shaft distally to the frame. The processor is configured to: (a) measure one or more first impedances between one or more of the ablation electrodes and the stem electrode, (b) measure one or more second impedances between one or more of the ablation electrodes and the edge electrode, and (c) based on the first and second impedances, determine, for at least an ablation electrode from among the one or more ablation electrodes, whether the ablation electrode is in physical contact with the wall tissue.

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 or second impedance is larger than a prespecified impedance by at least a prespecified minimal value.

In some embodiments, the prespecified impedance is measured with the ablation electrode being in contact with blood.

In an embodiment, the prespecified minimal value is stored in a look-up table.

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

In some embodiments, the expandable frame includes an expandable balloon and the external surface of the frame includes an external surface of a membrane of the balloon.

A multi-electrode ablation catheter, such as a balloon ablation catheter or a basket catheter, typically comprises an expandable frame (e.g., an inflatable balloon) that is coupled to the distal end of a shaft for insertion into a cavity of an organ of a patient. For the best outcome of ablation treatment, a physician may need to determine that each of the ablating electrodes disposed over the frame (e.g., balloon) is in physical contact with cavity wall tissue to be ablated. For example, when a balloon catheter with multiple ablation electrodes is used to ablate an ostium of a pulmonary ventricle (PV), typically all of the ablation electrodes of the catheter should be positioned so they are in full contact with the PV tissue.

Many times, however, some of the ablation electrodes may not be in full contact with tissue, but, instead, different portions of some of the ablation electrodes are immersed in blood. For these electrodes, rather than ablating tissue, the applied electrical power may cause unwanted side effects such as clot formation.

Embodiments of the present invention that are described hereinafter provide a system capable of determining if an ablation electrode is in full contact with tissue (e.g., being entirely covered by tissue). In some embodiments, a balloon ablation catheter is provided that comprises (i) at least one ablation electrode, (ii) an electrode disposed on the distal end of the shaft just proximally to the balloon (named hereinafter "stem electrode"), and (iii) an electrode disposed on the distal end of the shaft just distally to the balloon (named hereinafter "edge electrode"). Using impedance measurements between each ablation electrode and the stem and edge electrodes, a processor of the ablation system determines, for each ablation electrode, if the ablation electrode is in full contact with tissue.

In some embodiments, the processor of the system compares measured in-situ impedances between an ablation electrode intended to have contact with tissue, and the stem and edge electrodes when the ablation electrode is at least partially exposed to blood, to the same measured impedances. In the case of full contact, the impedances measured in-situ should be larger than the impedances measured with an ablation electrode in blood by at least a prespecified minimal value. Depending on, for example, the number of electrodes already in full contact with tissue, different minimal values of impedance-difference may be prespecified. The prespecified minimal values can be stored, for example, in a look-up table.

The above-mentioned prespecified minimal impedance-difference values are determined at a typical RF frequency of a few kHz, at which cardiac tissue impedance is typically several times higher than that of blood (in some cases, approximately <NUM> S2 in tissue vs. approximately <NUM>Ω in blood). Further information on tissue vs blood impedances as a function of RF frequencies is available, for example, in "<NPL>.

The disclosed measurement geometry involves comparable path lengths in blood and tissue, so the measured impedances mainly change due to different tissue properties. This characteristic of the disclosed technique gives a high degree of certainty to the distinction made by the processor based on the measurements between blood contact and tissue contact.

In order to verify that full physical contact with tissue has been achieved from both ends of the elongated ablation electrodes (i.e., proximal and distal), it is required to perform the measurements relative to the stem and edge electrodes.

If full physical contact is not achieved for all ablation electrodes, the physician may maneuver the balloon catheter to establish more complete contact of the ablation electrodes with tissue, and again check the sufficiency of contact using the disclosed technique.

In some embodiments, in order to measure a balloon catheter position inside the organ, the ablation system includes a position tracking sub-system that measures impedances between the ablation electrodes and surface electrodes. The method, which is further described below, is sometimes called Advanced Catheter Location (ACL). Using a relay, the system can switch electrical connections between the ablation electrodes and surface electrodes and between the ablation electrodes the stem and edge electrodes of the balloon catheter in order to interchangeably measure electrode position and degree of electrode contact with tissue at the location.

Furthermore, using the relay, the system can switch electrical connections between the ablation electrodes and either the stem and edge electrodes (for assessing contact) or the surface electrodes (for measuring positions) to a back patch electrode, for performing ablation by driving electrical signal between the ablation electrodes and the back patch electrode.

Typically, the processor is programmed in software containing a particular algorithm that enables the processor to conduct each of the processor-related steps and functions outlined hereinafter.

By determining, in real-time, which ablation electrode is in full contact with tissue and which is not, the disclosed technique may increase the safety and effectiveness of multi-electrode ablation treatments.

<FIG> is a schematic pictorial illustration of a catheter-based position-tracking and ablation system <NUM> comprising an ablation balloon catheter <NUM>, in accordance with an embodiment of the present invention. Typically, balloon catheter <NUM> is used for therapeutic treatment, such as ablating cardiac tissue, for example at the left atrium. System <NUM> is used to determine the position of balloon catheter <NUM>, seen in an inset <NUM> coupled to a distal end of a shaft <NUM>. System <NUM> is further used to determine, e.g., prior to performing an ablation, whether each of ablation electrodes <NUM> of balloon catheter <NUM> is in contact with tissue.

Physician <NUM> navigates balloon catheter <NUM> to a target location in a heart <NUM> of a patient <NUM> by manipulating shaft <NUM> using a manipulator <NUM> near the proximal end of the catheter and/or deflection from a sheath <NUM>. Balloon catheter <NUM> is inserted, in a folded configuration, through sheath <NUM>, and only after the balloon is retracted from the sheath <NUM> does balloon catheter <NUM> regain its intended functional shape. By containing balloon catheter <NUM> in a folded configuration, sheath <NUM> also serves to minimize vascular trauma on its way to the target location.

Balloon catheter <NUM> comprises elongated and large area ablation electrodes <NUM>, which are disposed on an outer surface of the balloon membrane. A stem electrode <NUM> is disposed on a distal end of shaft <NUM> just proximally to the balloon. An edge electrode <NUM> is disposed on the distal end of shaft <NUM> just distally to the balloon. Electrodes <NUM> and <NUM> are used to determine whether each of ablation electrodes <NUM> is in full contact with tissue or at least partially immersed in blood.

Ablation electrodes <NUM>, stem electrode <NUM>, and edge electrode <NUM> are connected by wires running through shaft <NUM> to interface circuits <NUM> in a console <NUM>. A detailed view of balloon catheter <NUM> with ablation electrodes <NUM>, stem electrode <NUM>, and edge electrode <NUM> is shown in <FIG>.

Additionally, using the aforementioned ACL method, ablation electrodes <NUM> can be used to measure a position of balloon catheter <NUM> inside heart <NUM>, by sensing impedances relative to surface electrodes <NUM>, which are seen in the exemplified system as attached by wires running through a cable <NUM> to the chest of patient <NUM>. The ACL method for tracking the positions of electrodes <NUM> is implemented in various medical applications, for example in the CARTO™ system, produced by Biosense-Webster Inc. (Irvine, California) and is described in detail in <CIT>, <CIT>,<CIT>, and <CIT>. Console <NUM> drives a display <NUM>, which shows the tracked position of balloon catheter <NUM> inside heart <NUM>.

Console <NUM> comprises a processor <NUM>, typically a general-purpose computer and a suitable front end and interface circuits <NUM> for transmitting and receiving signals, such as RF signals and position signals, respectively. Interface circuits <NUM> may also receive electrocardiograms from surface electrodes <NUM> and/or from any electrode disposed on the catheter.

In some embodiments, processor <NUM> controls a relay <NUM> in system <NUM> to switch electrical connections between two or more of: (i) a first configuration having a connection (<NUM>) between the ablation electrodes and surface electrodes <NUM> for measuring impedances between the ablation electrodes and one or more body-surface electrodes, (ii) a second configuration having a connection (<NUM>) between the ablation electrodes and the stem and edge electrodes of the balloon catheter for measuring impedances between the ablation electrodes and the stem and edge electrodes, where connections <NUM> and <NUM> are used in order to interchangeably measure electrode position and degree of electrode contact with tissue at the location, and (iii) a connection (<NUM>) between the ablation electrodes and a back patch electrode (not shown) in order to perform ablation by driving electrical signal between the ablation electrodes and the back patch electrode.

Processor <NUM> is typically programmed in software to carry out the functions described herein. The software may be downloaded to the computer in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. In particular, processor <NUM> runs a dedicated algorithm as disclosed herein, including in <FIG>, that enables processor <NUM> to perform the disclosed steps, as further described below.

<FIG> shows only elements related to the disclosed techniques, for the sake of simplicity and clarity. System <NUM> typically comprises additional modules and elements that are not directly related to the disclosed techniques, such as irrigation and temperature modules, and thus are intentionally omitted from <FIG> and from the corresponding description.

<FIG> is a schematic pictorial illustration of balloon catheter <NUM> of <FIG> in physical contact with cavity wall tissue <NUM>, in accordance with an embodiment of the present invention. Balloon catheter <NUM> comprises ablation electrodes <NUM> that are disposed over a membrane <NUM> of the balloon. Stem electrode <NUM> and edge electrode <NUM> are disposed on the distal end of shaft <NUM> and are immersed in blood <NUM>.

As seen, an ablation electrode <NUM> at the top of the balloon is in full contact with tissue, i.e., over an entire area of the electrode. An electrode <NUM> at the bottom, on the other hand, has a distal area 50a that is immersed in blood <NUM>. Correspondingly, different measured impedance values between the top and bottom ablation electrodes and edge electrode <NUM>, which are indicative of the full and partial contact of the top and bottom ablation electrodes with tissue, respectively, are described in <FIG>.

<FIG> is brought by way of example, and is simplified for clarity of presentation. For example, balloon elements not relevant to the embodied invention, such as temperature sensors and irrigation holes, are omitted for simplicity.

<FIG> are schematic electrical diagrams of an ablation electrode <NUM> coupled to an edge electrode <NUM> while ablation electrode <NUM> is in partial contact and in full contact with tissue <NUM>, respectively, in accordance with an embodiment of the present invention. The diagram of <FIG> describes a case of ablation electrode <NUM> having a distal area, such as area 50a seen in <FIG>, that is immersed in blood <NUM>, resulting in electrode <NUM> having insufficient tissue contact. As seen, the impedance between ablation electrode <NUM> and edge electrode <NUM> equals that of blood, RB, in parallel to a shunt resistance RS that might result from blood and/or tissue and/or other electrically conductive intra-body channel. In brief notation this is represented as |Z_insufficient|=RB∥RS. A minimal value of Z_insufficient is about RB/<NUM>, in case that a balloon mostly immersed in blood so that the shunt resistivity is dominated by blood resistivity. A maximal value is RB in case of infinite shunt resistivity. For typical blood resistivity value of approximately <NUM> Ohms, Z_insufficient falls in the range of <NUM> -<NUM> Ohms.

The diagram of <FIG> describes the case of an ablation electrode <NUM> that is completely in contact (i.e., covered in its entirety) by tissue. As seen, the impedance between ablation electrode <NUM> and edge electrode <NUM> is of blood in series with tissue, RB+RT, in parallel to the shunt resistance RS. In brief notation this is represented as |Z_sufficient|=(RB+RT)∥RS. As tissue impedance is considerably larger than that of blood, as described above, a "sufficient" impedance can typically be larger than an "insufficient" impedance by a value large enough to be measured, e.g., at least several ohms, and thus the disclosed method can differentiate between the two cases, using, for example, a calibrated threshold impedance value.

A minimal value of Z_sufficient is about RB, in case that a balloon mostly immersed in blood so that the shunt resistivity is dominated by blood resistivity, in which case repositioning of the balloon is required due to low shunt resistivity. A practical threshold value for Z_sufficient is RT in case of a shunt resistivity is mainly via tissue. For typical blood resistivity value of approximately <NUM> Ohms and tissue resistivity value of <NUM> Ohams, Z_sufficient falls above <NUM> Ohms. Yet, lower number, which is still above approximately <NUM> ohms, can be used as a threshold for Z_sufficient, depending, for example, on measurement repeatability.

In an embodiment, processor <NUM> is configured to determine that the ablation electrode is in physical contact with the tissue by determining that a measured first or second impedance is larger than a prespecified impedance by at least a prespecified minimal value given in a look-up table having, by way of example the form of Table I:.

<FIG> are fully applicable to stem electrode <NUM>. By measuring the impedance between ablation electrodes <NUM> and both stem electrode <NUM> and edge electrode <NUM>, the disclosed technique verifies that full physical contact with tissue has been achieved from both ends of the elongated ablation electrodes.

The electrical diagrams shown in <FIG> are highly simplified, with the aim of presenting the concept. Actual values may be determined empirically or by a more elaborate electrical model.

<FIG> is a flow chart that schematically illustrates a method and algorithm for determining full contact of ablation electrode with tissue, in accordance with an embodiment of the present invention. The algorithm according to the present embodiment carries out a process that begins with physician <NUM> positioning a partially expanded balloon catheter <NUM> at a target location inside a cardiac cavity of heart <NUM>, such as at an ostium of a pulmonary vein, at a balloon positioning step <NUM>. Next, physician <NUM> expands the balloon to bring ablation electrode <NUM> into full contact with tissue, in a balloon expansion step <NUM>. Next, at a impedances measurement step <NUM>, system <NUM> measures impedances between each of ablation electrodes <NUM> and stem (<NUM>) and edge (<NUM>) electrodes.

At a physical contact determination step <NUM>, based on the measured impedances, processor <NUM> determines, for each ablation electrode <NUM>, whether the electrode is in full contact with tissue, as defined above. If, at a contact checking step <NUM>, the processor determines that all ablation electrodes <NUM> are in full contact with tissue, the process continues to perform ablation, at an ablation step <NUM>. If, on the other hand, one or more electrodes are determined by processor <NUM> to have insufficient contact with tissue (due to insufficient impedance (Table I) as measured by the electrode(s)), physician <NUM> then repositions balloon catheter <NUM> in an attempt to improve contact, and the process loops back to step <NUM>, to reassess sufficiency of contact.

The example flow chart shown in <FIG> is chosen purely for the sake of conceptual clarity. The present embodiment also comprises additional steps of the algorithm, such as acquiring intra-cardiac electrocardiograms, which have been omitted from the disclosure herein purposely on order to provide a more simplified flow chart. In addition, other steps, such as temperature measurements and applying irrigation, are omitted for clarity of presentation.

Although the embodiments described herein mainly address cardiac applications, the methods and systems described herein can also be used in other applications, such as in renal denervation.

Claim 1:
A system (<NUM>) for determining full physical contact between one or more ablation electrodes and wall tissue of a cavity, comprising:
an expandable frame coupled to a distal end of a shaft (<NUM>) for insertion into a cavity of an organ (<NUM>) of a patient (<NUM>), the expandable frame comprising one or more ablation electrodes (<NUM>) disposed over an external surface of the frame, wherein the one or more ablation electrodes are configured to be placed in contact with wall tissue of the cavity;
a stem electrode (<NUM>) coupled to the distal end of the shaft (<NUM>) proximally to the expandable frame, and an edge electrode (<NUM>) coupled to the distal end of the shaft (<NUM>) distally to the expandable frame; and
a processor (<NUM>), which is configured to:
measure one or more first impedances between one or more of the ablation electrodes and the stem electrode;
measure one or more second impedances between one or more of the ablation electrodes and the edge electrode; and
based on the first and second impedances, determine, for at least an ablation electrode from among the one or more ablation electrodes, whether the ablation electrode is in full physical contact with the wall tissue.