Patent Publication Number: US-10786304-B2

Title: Temperature measurement in catheter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 14/642,135, filed Mar. 9, 2015, now U.S. Pat. No. 9,956,035, which claims priority to U.S. Provisional Patent Application Ser. No. 61/971,135, filed on Mar. 27, 2014, the entirety of these applications being incorporated herein by reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to invasive medical devices. More particularly, this invention relates to ablation of tissue using such devices. 
     2. Description of the Related Art 
     Ablation of body tissue using electrical energy is known in the art. The ablation is typically performed by applying alternating currents, for example radiofrequency energy, to the electrodes, at a sufficient power to destroy target tissue. Typically, the electrodes are mounted on the distal tip of a catheter, which is inserted into a subject. The distal tip may be tracked in a number of different ways known in the art, for example by measuring magnetic fields generated at the distal tip by coils external to the subject. 
     A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects, coagulum, and or steam pops due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure. 
     Self-regulating tissue ablators have been proposed to achieve the desired control. For example, PCT International Publication WO9600036 discusses ablation of body tissue in which ablating energy is conveyed individually to multiple emitters in a sequence of power pulses. The temperature of each emitter is periodically sensed and compared to a desired temperature established for all emitters to generate a signal individually for each emitter based upon the comparison. The power pulse to each emitter is individually varied, based upon the signal for that emitter to maintain the temperatures of all emitters essentially at the desired temperature during tissue ablation. 
     Commonly assigned U.S. Patent Application Publication No. 2012/0157890, which is herein incorporated by reference, discloses performing tissue ablation out by determining a measured temperature of the tissue and a measured power level of transmitted energy to a probe, and controlling the power output level responsively to a function of the measured temperature and the measured power level. 
     SUMMARY OF THE INVENTION 
     According to disclosed embodiments of the invention, temperature is measured according to the changes in impedance between a pair of irrigated electrodes on a catheter. The usual temperature sensor found on such catheters can be omitted. 
     There is provided according to embodiments of the invention a method of ablation, which is carried out by inserting a probe having an ablation electrode and a plurality of microelectrodes into a body of a living subject. The method is further carried out by establishing a contacting relationship between two of the microelectrodes and a target tissue, and energizing the ablation electrode. While the ablation electrode is energized the method is further carried out by measuring an impedance between the two microelectrodes, and responsively to the impedance adjusting the power level of the ablation electrode. 
     A further aspect of the method includes iteratively measuring the impedance, and estimating a tissue temperature from a change between two measurements of the impedance. 
     Yet another aspect of the method includes making a determination that the tissue temperature exceeds a predetermined limit, and responsively to the determination reducing the power of the ablation electrode. The power may be reduced to zero to deactivate the ablation electrode. 
     According to still another aspect of the method, measuring an impedance is performed by polling the microelectrodes to determine pairwise impedances therebetween. The pair of the selected microelectrodes may have the highest and the second highest measured impedance. 
     According to a further aspect of the method, measuring an impedance includes measuring a bipolar impedance between the selected pair of the microelectrodes. 
     According to an additional aspect of the method, establishing a contacting relationship includes determining a location and orientation of the tip of the probe with respect to the target tissue with six degrees of freedom. 
     According to another aspect of the method, measuring an impedance includes polling the microelectrodes to determine impedances between the microelectrodes and an indifferent electrode. 
     One aspect of the method includes deploying an inflatable balloon through a lumen of the probe, wherein the microelectrodes are disposed circumferentially about the longitudinal axis of the balloon on its exterior wall. 
     Another aspect of the method the balloon includes a subassembly comprising a plurality of strips extending longitudinally on the exterior wall of the balloon, and the microelectrodes are disposed on the strips. 
     There is further provided according to embodiments of the invention an apparatus for carrying out the above-described method. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
         FIG. 1  is a pictorial illustration of a system for performing ablative procedures, which is constructed and operative in accordance with a disclosed embodiment of the invention; 
         FIG. 2  is a schematic diagram of a distal portion of a catheter, in accordance with an embodiment of the invention; 
         FIG. 3  is a sectional view through line  3 - 3  of  FIG. 2 , in accordance with an embodiment of the invention; 
         FIG. 4  is an electrical schematic of circuitry for impedance measurement during ablation, in accordance with an embodiment of the invention; 
         FIG. 5  is a schematic diagram of a distal portion of a catheter, in accordance with an embodiment of the invention&#39; 
         FIG. 6  is a sectional view through line  6 - 6  of  FIG. 5 , in accordance with an embodiment of the invention; 
         FIG. 7  is a schematic sectional view of a portion of an ablation electrode, in accordance with an embodiment of the invention; 
         FIG. 8  is a pictorial view of a balloon assembly for a cardiac catheter in accordance with an alternate embodiment of the invention; 
         FIG. 9  is a tracing of bipolar impedance measured between two microelectrodes of a catheter, in accordance with an embodiment of the invention; and 
         FIG. 10  is a flow chart of a method of tissue temperature determination during a catheterization procedure, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
     Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for performing ablative procedures on a heart  12  of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention. The system comprises a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart. The operator  16 , who is typically a physician, brings the catheter&#39;s distal tip  18  into contact with the heart wall at an ablation target site. Electrical activation maps may then be prepared, according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. Although the embodiment described with respect to  FIG. 1  is concerned primarily with cardiac ablation, the principles of the invention may be applied, mutatis mutandis, to other catheters and probes and to body tissues other than the heart. 
     Areas determined to be abnormal by evaluation of the electrical activation maps can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip  18 , which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically above 60° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. Alternatively, other known methods of applying ablative energy can be used, e.g., ultrasound energy, as disclosed in U.S. Patent Application Publication No. 2004/0102769, whose disclosure is herein incorporated by reference. The principles of the invention can be applied to different heart chambers, when many different cardiac arrhythmias are present. 
     The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , the distal portion of the catheter  14  contains position sensors (not shown) that provide signals to a positioning processor  22 , located in a console  24 . The console  24  typically contains an ablation power generator  25 . The catheter  14  may be adapted to conduct ablative energy to the heart using any known ablation technique, e.g., radiofrequency energy, ultrasound energy, and laser energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference. 
     The positioning processor  22  is an element of a positioning subsystem of the system  10  that measures location and orientation coordinates of the catheter  14 . 
     In one embodiment, the positioning sub-system comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume and sensing these fields at the catheter. The magnetic position tracking arrangement typically comprises a set of external radiators, such as field generating coils  28 , which are located in fixed, known positions external to the patient. The field generating coils  28  are driven by field generators (not shown), which are typically located in the console  24 , and generate fields, typically electromagnetic fields, in the vicinity of the heart  12 . 
     In an alternative embodiment, a radiator in the catheter  14 , such as a coil, generates electromagnetic fields, which are received by sensors (not shown) outside the patient&#39;s body. 
     Some position tracking techniques that may be used for this purpose are described, for example, in the above-noted U.S. Pat. No. 6,690,963, and in commonly assigned U.S. Pat. Nos. 6,618,612 and 6,332,089, and U.S. Patent Application Publications 2004/0147920, and 2004/0068178, whose disclosures are all incorporated herein by reference. Although the positioning sub-system shown in  FIG. 1  uses magnetic fields, the methods described below may be implemented using any other suitable positioning system, such as systems based on electromagnetic fields, acoustic or ultrasonic measurements. 
     As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . Console  24  includes a processor, preferably a computer with appropriate signal processing circuits. The processor is coupled to drive a monitor  30 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals generated by the above-noted sensors and a plurality of sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received and used by the console  24  to compute the position and orientation of the catheter  14  and to analyze the electrical signals from the electrodes. The information derived from this analysis may be used to generate an electrophysiological map of at least a portion of the heart  12  or structures such as the pulmonary venous ostia, for diagnostic purposes such as locating an arrhythmogenic area in the heart or to facilitate therapeutic ablation. 
     Typically, the system  10  includes other elements, which are not shown in  FIG. 1  for the sake of simplicity. For example, the system  10  may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, to provide an ECG synchronization signal to the console  24 . The system  10  typically also includes a reference position sensor, either on an externally-applied reference patch attached to the exterior of the subject&#39;s body, or on an internally-placed catheter, which is inserted into the heart  12  maintained in a fixed position relative to the heart  12 . Conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. 
     One system that embodies the above-described features of the system  10  is the CARTO® 3 System, available from Biosense Webster, Inc., 33 Technology Drive, Irvine, Calif. 92618. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
     Reference is now made to  FIG. 2 , which is a schematic diagram of a distal portion of a catheter  32 , in accordance with an embodiment of the invention, which is suitable for use in the system  10  ( FIG. 1 ). An ablation electrode  34  is disposed at the tip of the catheter  32 . A hydraulic line  36  supplies irrigation fluid to cool an ablation site when the ablation electrode  34  is active. Pores  38  provide egress for the irrigation fluid. While the pores  38  may be placed through the ablation electrode  34 , this is not essential, so long as the irrigation fluid exiting the pores  38  is able to bathe the ablation site. Mapping electrodes  40  may be provided for purpose of conventional electrophysiological mapping. 
     A series of microelectrodes  42  are positioned distally on the external surface of the catheter  32 , They are disposed circumferentially its longitudinal axis  44  and close to the ablation electrode  34  such that at least two of the microelectrodes  42  and the ablation electrode  34  can be concurrently in firm contact with the target tissue when ablation is carried out. The inventors have found that measurements of bipolar impedance between the two contacting microelectrodes  42  is useful in determining the temperature of the target tissue. 
     One way of identifying a pair of contacting microelectrodes  42  is to determine their pairwise impedances, e.g., by polling. Either or both the magnitude and the phase of the impedance can be used. An additional way, due to the microelectrodes&#39; small size, is to measure the impedance between a microelectrode and a back patch (indifferent electrode) to identify contact. Alternatively, the identification of a contacting pair of microelectrodes  42  can be achieved by exploiting the ability of a position tracking system ( FIG. 1 ) such as the aforementioned CARTO system to determine the position and orientation of the catheter  32  with six degrees of freedom. Contact between a particular pair of the microelectrodes  42  can be determined by reference to the location and orientation of the tip of the catheter with respect to the target tissue. 
     Reference is now made to  FIG. 3 , which is a sectional view through line  3 - 3  of  FIG. 2 , in accordance with an embodiment of the invention. The microelectrodes  42  are distributed generally evenly in perforations distributed about the circumference of the catheter  32 . They microelectrodes  42  may be bonded within the perforations by suitable glues or bonding material. A flat profile of the outer surface exposed to the tissue is shown in this example. However, the profile of the microelectrodes  42  may be convex or sinusoidal. The profile of the microelectrodes  42  may be level with or raised above the external surface of the catheter  32 . Wires  46  electrically connect the microelectrodes  42  to impedance measuring circuitry (not shown) via a cable  48 . Hydraulic conduit  50  conducts irrigation fluid to the pores  38  ( FIG. 1 ). 
     The microelectrodes  42  are composed of an electrically conductive material, such as platinum, palladium, gold, stainless steel, silver or silver chloride, all of which tend to maximize the coupling between the microelectrodes and the target tissue. The microelectrodes  42  are substantially solid, but may include a bore  52  that can receive and assure electrical connection between the wires  46  and the microelectrodes  42 . The wires  46  may be secured to the microelectrodes  42  e.g., by solder  54 , glue, or other convenient methods. Further details of the manufacture of the microelectrodes  42  are shown in U.S. Patent Application Publication No. 2014/0058375 and U.S. Pat. No. 8,414,579, the disclosures of which are herein incorporated by reference. 
     The microelectrodes  42  are dimensioned such that a desired number of them can be accommodated about the circumference of the catheter  32 . The diameter of the microelectrodes  42  should be no greater than half the length of the ablation electrode  34 , preferably no greater than one-fourth the length of the ablation electrode  34 . The microelectrodes  42  should be spaced apart from one another by no more than one-half the diameter of the microelectrodes  42  (or one-half the shortest dimension in the case of non-circular embodiments). 
     Reference is now made to  FIG. 4 , which is an electrical schematic of circuitry  56  for impedance measurement during ablation for temperature determination, in accordance with an embodiment of the invention. Multiple microelectrodes  58  are connected by respective lead wires  60  via the catheter handle (not shown). A signal generator  62  (SG) sends a high frequency test signal, e.g., an alternating current (AC) signal at about 2 μamps, in the frequency range of about 10 kHz to about 100 kHz, preferably about 50 kHz, to a multiplexer  64  via a high output impedance buffer  66  (IB). 
     The multiplexer  64  has multiple channels  68 , each of which is in communication one of the microelectrodes  58 , which receive the same current. 
     A return electrode  70  is also driven by the signal generator  102 . The signal to the return electrode  70  is first inverted in phase by an inverter  72  and conditioned by high output impedance buffer  74 . 
     Impedance measurement circuitry  76  (IMC) measures the impedance of each of the microelectrodes  58  as an indicator of the extent of its respective tissue contact and the condition of the tissue being ablated. The impedance measurement circuitry  76  includes a differential amplifier  78  (DA), an amplifier  80  (AMP) and a synchronous detector  82  (SD). The differential amplifier  78  measures a difference signal, specifically the voltage across a selected microelectrode  58  and the return electrode  70 . The difference signal is further amplified by the amplifier  80  whose output is sent to the synchronous detector  82 , which transforms the AC signal into a direct current (DC) signal and decreases the sensitivity of the circuitry  56  to external noise. The signal from the synchronous detector  82  is then used by a microcontroller  84  to control the multiplexer  64 . To that end, the microcontroller  84  continuously stores in a memory  86  a plurality of different impedance signals from the synchronous detector  82  that equals the plurality of channels  68  in the multiplexer  64  (which is at least the plurality of microelectrodes  58  on the catheter), along with identification information on the channels  68  associated with each impedance value stored. 
     As such, the microcontroller  84  is at any time capable of identifying the channels  68  (and hence the microelectrodes  58 ) exhibiting the highest impedance value, which should be the microelectrode with the greatest tissue contact. Further details of the circuitry  56  are found in commonly assigned U.S. Patent Application Publication No. 2011/0106075, which is herein incorporated by reference. 
     Appropriate bipolar impedances between two microelectrodes can then be measured. This may be done by selecting the microelectrodes with the highest and second highest impedance, and providing signals from the microcontroller  84  to configure one of the two microelectrodes as the return electrode  70 . 
     First Alternate Embodiment 
     Reference is now made to  FIG. 5 , which is a schematic diagram of a distal portion of a catheter  88 , in accordance with an embodiment of the invention. Mounted on an ablation electrode  90  is a series of microelectrodes  92 . The microelectrodes  92  are elongated in the longitudinal direction of the catheter  88 , which allows a larger number to be accommodated than is the case with round microelectrodes having the same surface area. The elongated configuration is not essential, and, other configurations of the microelectrodes may be used. The microelectrodes  92  are thermally and electrically isolated from the ablation electrode  90  by an insulation layer  94 . 
     Reference is now made to  FIG. 6 , which is a sectional view through line  6 - 6  of  FIG. 5 , in accordance with an embodiment of the invention. The microelectrodes  92  are disposed within perforations through the ablation electrode  90 . The insulation layer  94  surrounds the microelectrodes  92  and separates the microelectrodes  92  from the ablation electrode  90 . As described in the above-noted U.S. Patent Application Publication No. 2014/0058375, the insulation layer  94  may be composed of the suitable electrically and thermally insulative material, such as a high temperature thermoset plastic with high dielectric properties, e.g., polyimide or plastics from the phenolic group, such as Bakelite® or Ultem® plastics. The insulation layer  94  and microelectrodes  92  may be bonded within the perforations using a suitable bonding material, such as epoxy. 
     Second Alternate Embodiment 
     This embodiment is similar to the embodiments of  FIGS. 5 and 6 , except that it is unnecessary to place large perforations in ablation electrode. Reference is now made to  FIG. 7 , which is a schematic sectional view of a portion of an ablation electrode  96 , in accordance with an embodiment of the invention. A microelectrode  98  is embedded in a recess  100  formed in the wall of the ablation electrode  96  and separated from the ablation electrode  96  by a thermally and electrically insulative layer  102 . A relatively small perforation  104  extending from the base of the recess  100  through the wall of the ablation electrode  96  carries a wire  106  into the interior of the catheter to ultimately connect to impedance measuring circuitry (not shown). 
     Third Alternate Embodiment 
     In this embodiment electrodes of a lasso, or loop, catheter having capabilities for ablation may be configured for bipolar impedance measurement. Such a catheter is known, for example from commonly assigned U.S. Patent Application Publication No. 2010/0168548, which is hereby incorporated by reference. Electrodes in contact with the tissue may be determined as described above. 
     Fourth Alternate Embodiment 
     In this embodiment the microelectrodes are disposed on a flexible circuit substrate and adhered to the exterior of a balloon that can be inserted through a catheter and applied to the target as described in copending application Ser. No. 14/578,807, entitled Balloon for Ablation around Pulmonary Veins, which is herein incorporated by reference. Reference is now made to  FIG. 8 , which is a pictorial view of a balloon assembly for a cardiac catheter in accordance with an alternate embodiment of the invention. A subassembly, e.g., a flexible circuit board  101  is configured as multiple strips or bands radiating from shaft  103 , extending longitudinally and adhering to the exterior wall of balloon  105  Arrays of microelectrodes  107  are disposed on the circuit board  101 . 
     Operation. 
     Reference is now made to  FIG. 9 , which is a prospective example of a tracing  108  that indicates bipolar impedance measured between two microelectrodes of a catheter during an ablation procedure, in accordance with an embodiment of the invention. Prior to time T 0  the microelectrodes are out of contact with tissue, as evidenced by a relatively low impedance. At time T 0 , the electrodes have come into tissue contact, and the bipolar impedance rises. At time T 1 , the ablator is energized. Tissue temperature rises during the interval between times T 1 , T 2 , as evidenced by gradually decreasing bipolar impedance. At time T 2 , the ablator power is reduced, as the impedance is approaching a threshold indicated by broken line  110 . Nevertheless, during the time interval T 2 -T 3 , impedance continues to decrease, albeit at a slower rate than prior to time T 2 . At time T 3 , the threshold of line  110  has been reached, and the ablator is deactivated. Actual impedance values vary according to the surface area of the microelectrodes, and are typically in the order of several hundred Ohms. 
     Reference is now made to  FIG. 10 , which is a flow-chart of a method of tissue temperature determination during a catheterization procedure, in accordance with an embodiment of the invention. At initial step  112 , a catheter in accordance with any of the above embodiments is inserted into contact with target tissue of a subject. The target is typically the endocardial surface of a heart chamber. 
     Next, at step  114  two microelectrodes of the catheter are determined to be in contact with the target. This determination may be made, for example, using the position processor of the CARTO system as noted above, by polling the microelectrodes pairwise until an impedance level consistent with tissue contact is identified, or measuring the impedance between a microelectrode and a backpatch (indifferent electrode), or a combination of the above. 
     Next, at step  116  the ablator is energized and its power level set. Irrigation fluid is caused to flow onto the ablation electrode and the target tissue. 
     Next, at step  118 , while the ablator is active, bipolar impedance measurements are taken between the pair of electrodes identified in step  114 . 
     Next, at step  120 , tissue temperature is estimated based on the change in the impedance measurements, either absolute or as a percentage, and using, for example, empirical data from simulations that reveals a correlation similar to the plot in  FIG. 9 . 
     Next, at decision step  122 , it is determined if the temperature is too high for continued ablation. If the determination at decision step  122  is negative, then control returns to step  116 . 
     If the determination at decision step  122  is affirmative then control proceeds to final step  124 , where the power level of the ablator is lowered. The power level of the ablator may be adjusted manually or automatically by a controller in accordance with known algorithms, for example as taught in commonly assigned U.S. Patent Application Publication No. 2012/0157890, which is herein incorporated by reference. The process iterates until the time set for the ablation expires, at which the power to the ablator is reduced or the ablator deactivated entirely by reducing the power to zero. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.