Patent Publication Number: US-8979835-B2

Title: Sensing contact of ablation catheter using differential temperature measurements

Description:
This application is a divisional of U.S. application Ser. No. 12/646,165 filed Dec. 23, 2009, the complete disclosure of which is hereby incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to tissue ablation systems. More particularly, this invention relates to monitoring of contact between an invasive probe and tissue within the body. 
     2. Description of the Related Art 
     Cardiac arrhythmia, such as atrial fibrillation, occurs when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. 
     Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. 
     SUMMARY OF THE INVENTION 
     During ablation, parts of the catheter that are in contact with tissue typically become considerably hotter than parts that are in contact only with blood. Embodiments of this invention take advantage of this phenomenon to verify proper electrode contact with the tissue. 
     An embodiment of the invention provides a medical apparatus including an insertion tube, at least one ablation electrode disposed on the distal portion thereof for conveying electromagnetic energy to a target tissue during an ablation procedure, a first temperature sensor disposed on the distal portion sufficiently proximate the ablation electrode to detect heat generated during the ablation procedure, and a second temperature sensor disposed on the distal portion sufficiently distant from the ablation electrode to be less able or even unable to detect the heat. Electronic logic circuitry linked to the first temperature sensor and the second temperature sensor is programmed to compute a temperature differential between respective temperatures sensed by the first temperature sensor and the second temperature sensor when conveying the electromagnetic energy, and to indicate a satisfactory contact status between the ablation electrode and the target tissue when the temperature differential exceeds a predetermined threshold. 
     According to an aspect of the apparatus, the electronic logic circuitry is programmed to indicate an unsatisfactory contact status between the ablation electrode and the target tissue when the temperature differential fails to exceed the predetermined threshold. 
     According to still another aspect of the apparatus, the insertion tube is a lasso catheter. 
     According to one aspect of the apparatus, the electronic logic circuitry is programmed to compute a first temperature differential and a second temperature differential when relatively high and low flows of coolant are applied to the ablation electrode, respectively, wherein the electronic logic circuitry is programmed to indicate an unsatisfactory contact status between the ablation electrode and the target tissue when a difference between the first temperature differential and the second temperature differential fails to exceed the predetermined threshold. 
     According to an aspect of the apparatus, the first temperature sensor and the second temperature sensor are thermocouples. 
     A further aspect of the apparatus includes an ablation power generator coupled to the ablation electrode to supply the energy thereto. 
     According to yet another aspect of the apparatus, the insertion tube is configured for insertion through a blood vessel into a heart of a subject for ablation of myocardial tissue in the heart. 
     Other embodiments of the invention provide methods for carrying out the operations performed by the apparatus. 
    
    
     
       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 detecting areas of abnormal electrical activity and performing ablative procedures in a heart of a living subject in accordance with a disclosed embodiment of the invention; 
         FIG. 2  is an elevation of a lasso catheter that is constructed and operative in accordance with a disclosed embodiment of the invention; 
         FIG. 3  is a sectional view through the loop segment of the catheter shown in  FIG. 2  taken through line A-A, in accordance with a disclosed embodiment of the invention; 
         FIG. 4  is a fragmentary sectional view through the loop segment of the catheter shown in  FIG. 2  taken through line B-B, in accordance with a disclosed embodiment of the invention; and 
         FIG. 5  is a flow chart of a method of cardiac ablation in accordance with a disclosed 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 always 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 in accordance with a disclosed embodiment of the invention. The system comprises a catheter  14 , such as a lasso catheter, which is percutaneously inserted by an operator  16 , who is typically a physician, through the patient&#39;s vascular system into a chamber or vascular structure of the heart. The operator  16  brings the catheter&#39;s distal tip  18  into contact with the heart wall at a target site that is to be evaluated. Electrical activation maps are then prepared, according to the methods disclosed in the above-noted U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosure is 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 about 50° 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, and to mapping in sinus rhythm, and when many different cardiac arrhythmias are present. 
     The catheter  14  typically comprises a handle  20 , having suitable controls 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 system  26  that measures location and orientation coordinates of the catheter  14 . Throughout this patent application, the term “location” refers to the spatial coordinates of the catheter, and the term “orientation” refers to its angular coordinates. The term “position” refers to the full positional information of the catheter, comprising both location and orientation coordinates. 
     In one embodiment, the positioning system  26  comprises a magnetic position tracking system that determines the position of the catheter  14 . The positioning system  26  generates magnetic fields in a predefined working volume its vicinity and senses these fields at the catheter. The positioning system  26  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 coils  28  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 systems 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 system  26  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. The positioning system  26  may be realized as the CARTO XP EP Navigation and Ablation System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. 
     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 sensors (not shown) and a plurality of sensing electrodes  36 . 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 is 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 the figures 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, so as to provide an ECG synchronization signal to the console  24 . As mentioned above, 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 . By comparing the position of the catheter  14  to that of the reference catheter, the coordinates of catheter  14  are determined relative to the heart  12 , irrespective of heart motion. Alternatively, any other suitable method may be used to compensate for heart motion. Nevertheless, the positioning system  26  cannot guarantee that an energy-conveying component of the catheter  14  is in actual contact with the tissue to be ablated. 
     The Catheter 
     Reference is now made to  FIG. 2 , which is an elevation of a lasso catheter  38  that is constructed and operative in accordance with a disclosed embodiment of the invention. It is to be emphasized that in the following discussion, a lasso catheter is presented by way of example. The principles of the invention may equally be applied to ablation catheters having many configurations. For example, temperature sensors may be placed on opposing sides of other sorts of catheters and other ablation devices. In a further example, the temperature sensors may be fixed on opposite sides along the length of a multielectrode catheter that is used for linear ablation. 
     The catheter  38  is a steerable device. Its handle, control and steering mechanisms (not shown) are conventional and are omitted from  FIG. 2  for simplicity. The catheter  38  features a base segment  40 , which is bendable responsively to forces applied by the steering mechanisms. A distal loop segment  42  completes the lasso configuration. The loop segment  42  is joined to the base segment  40  by a range-restricted angle α at a joint  44 . The angle a between the loop segment  42  and the base segment  40  optimally is about 90 degrees. The joint  44  may define a point where two initially-separate members (base segment  40 ; loop segment  42 ) are joined, or, alternatively, the joint  44  may define a point on the catheter  38  where a single member is bent, so as to form the base segment  40  and the loop segment  42 . The loop segment  42  is of a known fixed length, having a curvature dimensioned to a particular medical application. The curvature may be adjustable using the steering and control mechanisms (not shown) of the catheter. A radius  46 , adjustable between 7-15 mm, is suitable for cardiac applications. However, the radius  46  may vary up to 25 mm in some applications. In any case, the loop segment  42  may be dimensioned so as to conform to structures such as the ostia of pulmonary veins or the coronary sinus. 
     The loop segment  42  is constructed of a material that preferably is twistable but not stretchable when subjected to typical forces encountered in medical practice. Preferably, the loop segment  42  is sufficiently resilient so as to assume a predetermined curved form, i.e., an open circular or semicircular form when no force is applied thereto, and to be deflected from the predetermined curved form when a force is applied thereto. Preferably, the loop segment  42  has an elasticity that is generally constant over at least a portion of its length, for example, because of internal reinforcement of the curved section with a resilient longitudinal member, as is known in the art. The loop segment  42  may be made from polyurethane and be at least one mm in diameter. 
     One or more electrodes, indicated representatively as electrodes  36 ,  48 ,  50 , are disposed on the loop segment  42 , and may be assigned ablation and mapping functions in many combinations. Ablation electrodes  36 ,  48  are associated with temperature sensors  52 ,  54 . The sensors are conventional miniature thermocouples. American Wire Gauge (AWG)  46  is suitable. Coolant is delivered conventionally, e.g., via accessory ports (not shown), to the ablation electrodes at a flow during operation of about 30-40 ml/min. 
     Mapping electrode  36  is not associated with temperature sensor. Electrodes  36 ,  48 ,  50  are shown as having a circular configuration, but this is not critical. Many different types of electrodes can be used in various combinations, e.g., a tip ablation electrode, ring electrodes, or coil electrodes, so long as the ablation electrodes are sufficiently close to one temperature sensor, enabling the one sensor to detect increased temperature in the target tissue during ablation and sufficiently spaced apart from another temperature sensor, such that the other temperature sensor does not detect the increased temperature in the target tissue or detects it to a lesser extent than the one electrode. The other temperature sensor is sometimes referred to as a “reference temperature sensor”. 
     Reference is now made to  FIG. 3 , which is a sectional view of the catheter  14  through the loop segment  42  of the catheter  38  ( FIG. 2 ) taken through line A-A. Electrode  50  diametrically opposes sensor  56 , which is not seen on  FIG. 2 . The sensors  52 ,  56  are connected by wires  58  to a cable  60  that extends proximally and conveys signals to the console  24  ( FIG. 1 ). It will be apparent that when the sensor  52  is in contact with endocardium  62 , the sensor  56  is not in contact with the tissue, but is irrigated by blood. The temperature sensors  52 ,  56  can be disposed in different configurations, and need not necessarily be diametrically opposed as shown in  FIG. 3 . However, when one or more of the ablation electrodes  36 ,  48  is operationally engaged with target tissue, the sensor  52  is intended to respond to heat generated by ablation and the sensor  56  is not intended to so respond. Therefore, sensors  52 ,  56  should be sufficiently spaced apart such that when temperature readings are taken during a normally proceeding ablation procedure, the sensor  56  is less responsive to heat generated at the ablation site than the sensor  52 . The sensor  56  should read at least 2-3° C. lower than the sensor  54 . 
     Reference is now made to  FIG. 4 , which is a fragmentary sectional view of the catheter  14  through the loop segment  42  of the catheter  38  ( FIG. 2 ) taken through line B-B. Electrode  50  is shown in relation to sensor  52 . A wire  64  conducts ablation currents from the console  24  ( FIG. 1 ) to the electrode  50  via the cable  60 . 
     The catheter  38  may be adapted to ablate a circular path within one of the pulmonary veins. The loop segment  42  ( FIG. 2 ) fits into the ostium of the vein, so that ablation electrodes disposed on the outside of the lasso contact the inner circumference of the vein. The inside of the lasso does not contact tissue, but is rather irrigated by blood flow through the vein. As best seen in the example of  FIG. 4 , ablation energy is conducted through the electrode  50  into the target tissue  62 , e.g., endocardium or the intima of a pulmonary vein. If the catheter  38  is in contact with the target tissue  62 , the sensor  52  is preferably also in contact or near contact with the target tissue  62 . In any case, the sensor  52  should be disposed sufficiently close to the electrode  50  such that heat generated in the target tissue  62  is detected by the sensor  52 . It is recommended to mount the sensor  52  on the internal side of the electrode. The sensor  56  is located away from the electrode  50 , and does not detect the heat, but acts as a reference temperature sensor. The sensor  52  reports a higher temperature reading than the sensor  56  to the console  24  ( FIG. 1 ). A threshold temperature differential exceeding 10-15 degrees C. is an indication to the operator that the catheter  38  is properly positioned in contact with the target tissue  62 . Typically the sensor  56  is diametrically opposite the sensor  52  on the shaft of the catheter  38 , as shown in  FIG. 3 . An arc of at least 90 degrees should separate the sensor  52  from the sensor  56  to assure that the sensor  56  does not sense the heat generated in the target tissue  62 . 
     Contrarily, a temperature differential less than the threshold is an indication to the operator that the catheter  38  is not properly positioned in contact with the target tissue  62 . In such a case, the ablation current may be turned off, and the lasso may be repositioned before continuing the procedure. 
     Electronic logic circuitry, e.g., a computer, in the console  24  ( FIG. 1 ) is programmed to compute a temperature differential between respective temperatures sensed by the sensor  52 ,  56  when conveying electromagnetic energy to the electrode  50 , and to record a contact status between the electrode  50  and the target tissue  62  responsively to the temperature differential. 
     Alternate Embodiments 
     In the embodiment of  FIG. 2 , each ablation electrode is associated with a respective pair of temperature sensors—one closely located and the other more distantly located. In other embodiments, each ablation electrode continues to be associated with respective closely located temperature sensors. However the readings of these sensors are compared with a common reference temperature sensor, which is spaced apart from all the ablation electrodes. 
     Further alternatively, more than one reference temperature sensor may be employed, so long as none is close enough to an ablation electrode to produce a misleading reading caused by target tissue heating. In this embodiment the number of reference temperatures sensors is smaller than the number of ablation electrodes. 
     An advantage of using one reference temperature sensor or a small number of reference temperature sensors is cost and simplicity of construction. Use of a larger number of reference electrodes, while requiring a more complex construction, e.g., more internal wiring, permits detection of spurious readings in a reference temperature sensor. This can be dealt with by employing arbitration logic in the console  24  ( FIG. 1 ). 
     Operation 
     Reference is now made to  FIG. 5 , which is a flow chart of a method of cardiac ablation in accordance with a disclosed embodiment of the invention. At initial step  66  a catheter, constructed in accordance with one of the above-described embodiments, is introduced into the heart, and an ablation electrode, together with its associated temperature sensor, is positioned at a target site using the positioning system  26  ( FIG. 1 ). 
     Next, at step  68  ablation of the target is initiated by enabling the ablation power generator  25 . During ablation the ablation electrode is cooled conventionally using a flow of coolant in the range of 30-40 ml/min. 
     Temperature readings are taken twice from the temperature sensors. A delay is initiated prior to the first reading, at delay step  70  for a predetermined time interval, typically about 2-3 sec. During this delay interval, the flow of coolant is reduced to a minimal value, about 4 ml/min. This allows heat buildup to occur in the target tissue if the ablation electrode is in proper contact with the target tissue. 
     Next, at step  72  the first temperature readings are taken from the temperature sensor associated with the ablation electrode and from one or more reference temperature sensors. The readings may be averaged or arbitrated among the temperature sensors (when they disagree). In any case, the result is a temperature reading T 1 . When the first readings are completed, the coolant flow is restored 30-40ml/ml. 
     Next, at delay step  73 , lasting about 2-3 seconds, the temperature re-equilibrates. Then at step  74 , the temperature sensors are read a second time to obtain a temperature reading T 2 . 
     Next, at step  75  the temperature differential (T 2 −T 1 ) is computed. If ablation has been proceeding normally, i.e., there was sufficient contact between the ablation electrode and the ablation site, a temperature difference T 2 −T 1  of at least 10-15° C. is expected. However, If there were no ablation then the difference T 2 −T 1  would be insignificant. 
     If desired, the temperature readings T 2  and T 1  may be taken in reverse order, so long as one reading is taken with minimal cooling of the ablation electrode and the other with normal cooling. 
     Useful information can be gained even from the first reading in step  72 . If the temperature differential between the temperature sensors  52 ,  56  exceeds a threshold, it is possible to conclude that adequate contact exists between the electrode  50  and the ablation site. When relying on such a finding, delay step  73  and step  74  may be omitted. 
     Control now proceeds to decision step  76 , where it is determined if the temperature differential T 2 −T 1  exceeds the above-noted predetermined threshold. If the determination at decision step  76  is affirmative, then control proceeds to final step  78 . It is determined that the ablation electrode is in good contact with the target tissue. Success is reported to the operator, and ablation may continue. 
     If the determination at decision step  76  is negative then control proceeds to final step  80 . The operator is alerted that the ablation electrode may not be in proper contact with the target tissue, which may cause him to interrupt the ablation by disabling the ablation power generator  25  and reposition the catheter. 
     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 subcombinations 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.