Patent Publication Number: US-9883918-B2

Title: Method for mapping ventricular/atrial premature beats during sinus rhythm

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a divisional of application Ser. No. 14/024,859, filed Sep. 12, 2013 and incorporated here by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to invasive medical devices. More particularly, this invention relates to identifying the anatomical origin of infrequent premature contractions of the heart chambers using an invasive probe. 
     Description of the Related Art 
     The meanings of certain acronyms and abbreviations used herein are given in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Acronyms and Abbreviations 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 CL 
                 Cycle Length 
               
               
                   
                 IS 
                 Induced Signal 
               
               
                   
                 LAT 
                 Local Activation Time 
               
               
                   
                 PM 
                 Pace Mapped 
               
               
                   
                 PVC 
                 Premature Ventricular Contraction 
               
               
                   
                 SR 
                 Sinus Rhythm 
               
               
                   
                 VT 
                 Ventricular Tachycardia 
               
               
                   
                   
               
            
           
         
       
     
     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. 
     Mapping of electrical potentials in the heart is now commonly performed, using cardiac catheters comprising electrophysiological sensors for mapping the electrical activity of the heart. Typically, time-varying electrical potentials in the endocardium are sensed and recorded as a function of position inside the heart, and then used to map a local electrogram or local activation time. Activation time differs from point to point in the endocardium due to the time required for conduction of electrical impulses through the heart muscle. The direction of this electrical conduction at any point in the heart is conventionally represented by an activation vector, which is normal to an isoelectric activation front, both of which may be derived from a map of activation time. The rate of propagation of the activation front through any point in the endocardium may be represented as a velocity vector. 
     Mapping the activation front and conduction fields aids the physician in identifying and diagnosing abnormalities, such as ventricular and atrial tachycardia and ventricular and atrial fibrillation, which result from areas of impaired electrical propagation in the heart tissue. 
     Localized defects in the heart&#39;s conduction of activation signals may be identified by observing phenomena such as multiple activation fronts, abnormal concentrations of activation vectors, or changes in the velocity vector or deviation of the vector from normal values. Examples of such defects include re-entrant areas, which may be associated with signal patterns known as complex fractionated electrograms. Once a defect is located by such mapping, it may be ablated (if it is functioning abnormally) or otherwise treated to restore the normal function of the heart insofar as is possible. 
     Mapping of the electrical activation time in the heart muscle requires that the location of the sensor within the heart be known at the time of each measurement. In the past, such mapping was performed using a single movable electrode sensor inside the heart, which sensor measured activation time relative to a fixed external reference electrode. This technique, however, requires calibration, for example impedance calibrations with adjustments for impedance unrelated to that of the body. Mapping of electrical activation time using a single electrode was, furthermore, a lengthy procedure, generally performed under fluoroscopic imaging, and thereby exposing the patient to undesirable ionizing radiation. Furthermore, in an arrhythmic heart, activation times at a single location may change between consecutive beats. 
     Because of the drawbacks of single-electrode mapping, a number of inventors have taught the use of multiple electrodes to measure electrical potentials simultaneously at different locations in the endocardium, thereby allowing activation time to be mapped more rapidly and conveniently, as described. 
     A reentrant circuit that produces sinus PVC&#39;s is one pathological condition amenable to ablative treatment. Identifying the optimum point of line of ablation is a practical difficulty, even with modern electroanatomic mapping equipment. U.S. Pat. No. 7,245,962 to Ciaccio et al. proposes a method and system for identifying and localizing a reentrant circuit isthmus in a heart of a subject during sinus rhythm. The method may include (a) receiving electrogram signals from the heart during sinus rhythm via electrodes, (b) creating a map based on the electrogram signals, (c) determining, based on the map, a location of the reentrant circuit isthmus in the heart, and (d) displaying the location of the reentrant circuit isthmus. 
     U.S. Patent Application Publication No. 2009/0099468 to Thiagalingam et al. proposes locating a region of interest to ablate by recording electrogram data and corresponding spatial location data of an electrode that records the electrogram data; defining at least one reference channel containing a reference beat for determining temporal locations and against which beats of the recorded electrogram data are compared; examining the recorded electrogram data; defining a temporal location for each beat of the recorded electrogram data, and creating and analyzing an index of the temporal locations and other information of the beats within the recorded electrogram. 
     SUMMARY OF THE INVENTION 
     There is provided according to embodiments of the invention a method of ablation, which is carried out by inserting a probe into a heart of a living subject, urging the mapping electrode of the probe into a contacting relationship with a target tissue in a region of interest of the heart, and while detecting a cardiac arrhythmia, using the mapping electrode to associate a local activation time with a first location in the region of interest. The method is further carried out while detecting an absence of the cardiac arrhythmia and maintaining the contacting relationship, by associating the local activation time with a second location in the heart, assigning electrical data of the first location to the second location, and generating an electroanatomic map of the heart by including at least the second location for display thereof using the assigned electrical data of the first location. 
     According to an aspect of the method, detecting a cardiac arrhythmia and detecting an absence of the cardiac arrhythmia comprise obtaining electrocardiographic signals via the mapping electrode, holding a series of the electrocardiographic signals in a buffer, selecting a first signal from the buffer as indicative of the cardiac arrhythmia, and selecting a second signal from the buffer as indicative of the absence of the cardiac arrhythmia. 
     An additional aspect of the method includes navigating an ablation electrode to the assigned electrical data of the second location for ablation thereof referencing the electroanatomic map while navigating the ablation electrode. 
     Another aspect of the method includes displaying the first location on the electroanatomic map and excluding the first location from computations in generating the electroanatomic map. 
     According to one aspect of the method, detecting a cardiac arrhythmia includes identifying premature ventricular contractions wherein a cycle length thereof is within a predetermined range. 
     In a further aspect of the method, the probe has multiple mapping electrodes and the method is carried out by acquiring multiple instances of the first location and the second location using respective ones of the mapping electrodes. 
     Yet another aspect of the method is carried out by pace mapping and determining at respective locations a pace map correlation between signals produced during ventricular tachycardia and signals produced by pace mapping and wherein assigning electrical data includes assigning the pace map correlation of the first location to the second location. 
     There is further provided according to embodiments of the invention a medical apparatus, including a probe, adapted for insertion into a heart, the probe including an elongated body, and a mapping electrode disposed on a distal portion of the body, a memory having programs stored therein, a display, and a processor linked to the display that is coupled to access the memory to execute the programs. The processor is connectable to receive an input provided by the mapping electrode, wherein the programs cause the processor to perform the steps of obtaining electrocardiographic signals from a target in the heart via the mapping electrode, holding a series of the electrocardiographic signals in a buffer, selecting a first signal from the buffer as indicative of a cardiac arrhythmia, the first signal occurring at a first point in time, and selecting a second signal from the buffer as indicative of an absence of the cardiac arrhythmia, the second signal occurring at a second point in time, associating a first local activation time of the target at a first location of the mapping electrode at the first point in time, associating a second local activation time of the target at a second location of the mapping electrode at the second point in time, assigning electrical data of the first location to the second location, generating an electroanatomic map based on the assigned electrical data at the second location, and presenting the electroanatomic map on the display, the electroanatomic map showing the first location and the second location. 
    
    
     
       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 on a heart of a living subject, which is constructed and operative in accordance with an embodiment of the invention; 
         FIG. 2  is a flow chart of a method of mapping arrhythmogenic areas producing ventricular premature beats in accordance with an embodiment of the invention; 
         FIG. 3  is a screen display illustrating a representative local activation time map of a heart in accordance with an embodiment of the invention; 
         FIG. 4  is a screen display of a local activation time map  73  illustrating points of interest, in accordance with an embodiment of the invention; 
         FIG. 5  is a screen display illustrating a role of a spline catheter in the generation of a local activation time map in accordance with an embodiment of the invention; and 
         FIG. 6  is a flow chart of a method of mapping arrhythmogenic areas producing ventricular premature beats in accordance with an alternate 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. 
     Aspects of the present invention may be embodied in software programming code, which is typically maintained in permanent storage, such as a computer readable medium. In a client/server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known non-transitory media for use with a data processing system, such as a diskette, hard drive, electronic media or CD-ROM. The code may be distributed on such media, or may be distributed to users from the memory or storage of one computer system over a network of some type to storage devices on other computer systems for use by users of such other systems. 
     Definitions 
     The term “physical coordinates” of a point refers to coordinates of a point in the body of the subject that are determined with respect to fiducials or natural anatomic landmarks. 
     The term “map coordinates” of a point as used herein refers to coordinates or a point relative to a reference point on an electroanatomic map. 
     System Description 
     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  12 . 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. Optionally, Electrical activation maps, such as local activation time 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. One commercial product embodying elements of the system  10  is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
     Areas determined to be abnormal, for example 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. The principles of the invention can be applied to different heart chambers to treat many different cardiac arrhythmias. 
     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 . 
     Ablation energy and electrical signals can be conveyed to and from the heart  12  through one or more ablation electrodes  32  located at or near the distal tip  18  via cable  34  to the console  24 . Pacing signals and other control signals may be conveyed from the console  24  through the cable  34  and the electrodes  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24  are disposed between the ablation electrodes  32  and have connections to the cable  34 . 
     Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning subsystem. The electrodes  32  and the body surface electrodes  30  may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is herein incorporated by reference. A temperature sensor (not shown), typically a thermocouple or thermistor, may be mounted on or near each of the electrodes  32 . 
     The console  24  typically contains one or more ablation power generators  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-produced light 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 in the system  10  that measures location and orientation coordinates of the catheter  14 . 
     In one embodiment, the positioning subsystem 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, using field generating coils  28 . The positioning subsystem may employ impedance measurement, as taught, for example in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference, and in the above-noted U.S. Pat. No. 7,536,218. 
     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  29 . 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 location sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14 , and to analyze the electrical signals from the electrodes. 
     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, 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 . Conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. 
     Application of the system  10  to the ablation of regions causing transient arrhythmias, e.g., short runs of PVC&#39;s, presents certain technical difficulties. The medical procedure encompasses two stages, which may be accomplished in the same or different sessions. 
     In any case, in a first stage, one object is to identify a point having the shortest local activation time in an arrhythmogenic region of the heart, referred to herein as a “PVC point”. Typically, an electroanatomic map is generated during the first stage. Although aspects of the invention are described for convenience with respect to PVC&#39;s, its principles are equally applicable to premature atrial contractions. 
     In a second stage, which occurs subsequent to the first stage, ablation of tissue in a region about the PVC point is performed. This is intended to interrupt automatic focal electrical activity and thereby prevent recurrence of the arrhythmia. In the second stage, the ablation electrode is navigated to the PVC point, which is a location on the electroanatomic map having the previously determined shortest local activation time. Often, by the time ablation has been accomplished, the patient has returned to sinus rhythm (SR). The term “sinus rhythm” is employed herein for convenience as an example of a stable cardiac rhythm. It will be recalled that in the case of a patient with infrequent premature ventricular contractions, the abnormal heart rate originating from the ventricle appears infrequently and in short bursts (1-3 beats). The heart is mostly in sinus rhythm, except that from time to time short PVC bursts appear. When creating a map consisting of PVC location points and their associated activation times, a discrepancy likely exists between the locations of the map&#39;s points (PVC geometry) and the navigational catheter, as the ablation catheter location is displayed mostly in sinus rhythm. As a result, the ablated area will not be optimum to correct the patient&#39;s arrhythmia. It is believed that the geometry of the heart differs during sinus rhythm, compared to its geometry during runs of ventricular tachycardia. In any event, it is desirable to accurately identify the physical location of the PVC point subsequent to its original determination. 
     In embodiments of the invention, the following procedure is suitable for recording signals during mapping: record approximately 2.5 seconds of each mapping electrode, and capture PVC complexes in a beat buffer, which holds and graphically displays the last few beats recorded by the electrocardiogram. Typically, 10 beats are held in the beat buffer. Inspection of the beat buffer allows abnormal beats such as PVCs to be identified automatically or automatically with operator assistance, or manually by the operator. This phase may be automated according to an operator-selected cycle length range, a procedure, which is available in the above-noted CARTO system. Alternatively, the operator can manually acquire a point and the related beat buffer whenever he notices a PVC occurrence. 
     Operation 
     Reference is now made to  FIG. 2 , which is a flow chart of a method of mapping arrhythmogenic areas producing ventricular premature beats while the subject is in sinus rhythm, in accordance with an embodiment of the invention. The figures described below are obtained using the CARTO  3  system, and are presented for convenience. However, the method may be performed with other imaging and mapping systems. 
     At initial step  37 , a cardiac catheter having mapping electrodes and location sensors is inserted into the heart of a subject, using known methods. Catheters such as the PentaRay® NAV or Navistar® Thermocool® catheters, available from Biosense Webster, are suitable for use in initial step  37 . The catheter is navigated to an abnormal area  39  ( FIG. 3 ). 
     It is assumed that the subject is experiencing transient PVCs, either spontaneously occurring, or induced as noted above. The appearance of a PVC is noted at step  41 . 
     Next, at step  43 , the beat buffer is filled with PVC and SR beats, and frozen to prevent its replacement by new beats. 
     The method continues with step  45 . A beat showing a qualifying electrical abnormality, e.g., a PVC, is identified on the beat buffer. This step may be performed automatically or by the operator. Next, at step  47 , a point in the area  39 , referred to for convenience as a “PVC point” is automatically acquired based on the beat in the beat buffer that was chosen in step  45 . Optionally, the PVC point is acquired only if the cycle length of the selected beat falls within a predetermined range. Acquisition of the point comprises (1) determining the location the point, typically by using a location sensor to determine the position of the mapping electrode; (2) obtaining electrical data regarding the point from the mapping electrode, specifically its LAT; and (3) displaying the point on a map, e.g., an LAT map. 
     When catheters having multiple mapping electrodes are used, e.g., spline or lasso catheters, many points may be acquired concurrently. The PVC point ideally has the earliest local activation time within the abnormal area; however this is not essential; points throughout the area  39  ( FIG. 3 ) may be acquired and displayed. 
     Next, at step  49 , a second beat (SR beat) is selected from the beat buffer. The second beat represents normal sinus rhythm for the patient. Then, at step  51 , a second point (SR point) is acquired and displayed on the same map as the PVC point. Generally, the locations of the PVC point and the SR point differ on the map, as explained above. 
     Next, at step  53 , electrical information, e.g., the local activation time (LAT) of the PVC point that was established in step  47  is associated with the SR point in a single representative display by assigning the information to the SR point. 
     Next, at step  55 , the PVC point is designated as “floating”, meaning that the location of the PVC point may be indicated on LAT maps to indicate the disparity in the geometry of the heart when it is experiencing PVCs and SR. However, the PVC point is excluded from computations in the generation of such maps, and does not contribute to the map geometry and its electrical information. In consequence, a LAT map generated during sinus rhythm reflects the geometry of the heart in sinus rhythm, including the location of the SR point. However, the map is adjusted to associate the electrical information of the PVC point at the location of the SR point. 
     Then, at step  57 , the LAT map is regenerated or adjusted to reflect the new data of the SR point. In other words, the SR map is presented with PVC electrical data. This version is referred to as a SR-PVC map. As explained above, the SR-PVC map reflects the geometry of the heart during sinus rhythm, but has the activation time of the heart while experiencing PVCs. 
     Steps  41 - 57  may be performed concurrently using different mapping electrodes, for example in a spline or lasso catheter. Additionally or alternatively steps  41 - 57  may be iterated to present multiple SR points having reassigned PVC data on the map. 
     The procedure ends at final step  59 . If appropriate, ablation may be performed at the data-adjusted SR points with an ablation catheter displayed in SR locations. In such case, navigation of the ablation electrode may be guided by the SR-PVC map prepared in step  57 . 
     EXAMPLES 
     Reference is now made to  FIG. 3 , which is a screen display  61  illustrating a representative local activation time map  63  of a heart  65  taken during performance of step  43 , in accordance with an embodiment of the invention. The procedure for generating a local activation time map using a mapping catheter is known, and therefore its details are not discussed herein. Local activation times are typically coded using pseudocolor. In  FIG. 3 , local activation times information is coded using patterns in accordance with a key  67 . These patterns simulate the pseudocolors of an actual functional map. The area  39  has a relatively short LAT, and may be automatically flagged or selected by an operator as abnormal. The area  39  is circumscribed by an area  69  that has a relatively longer local activation time. 
     Reference is now made to  FIG. 4 , which is a screen display  71  of a local activation time map  73  illustrating points of interest, in accordance with an embodiment of the invention. The map  73  shows the locations of PVC point  75  and SR point  77  that were obtained according to the procedure described above in reference to  FIG. 2 . The points  75 ,  77  are enclosed by circles for improved visibility. The beats selected from the beat buffer in order to acquire the points  75 ,  77  are shown in right pane  79  with a window of interest interval  81 . The right pane  79  also shows the activation times of the PVC point  75  and SR point  77  on a scale  83  in the lower right portion. The beats selected are the SR and PVC points indicated by circles  85 ,  87 , respectively. In other words, isochronal points are acquired from an area of interest using the beat buffer during PVCs (step  45 ) and during sinus rhythm (step  49 ) are shown on the map  73  as the points  75 ,  77  respectively. The points  75 ,  77  were measured at different times, based on different beats in the beat buffer, and thus are associated with different time stamps. It is evident that they have different map locations. In particular, the LAT map displays the physical coordinates of an SR point whose associated activation time data was obtained at a different time. 
     Reference is now made to  FIG. 5 , which is a screen display  89  illustrating a role of a spline catheter in the generation of a local activation time map  91 , in accordance with an embodiment of the invention. 
     A spline catheter  93 , e.g., the above noted Pentaray NAV catheter has been navigated such that an electrode of one of the spline, spline  95  is in contact with a point on a colored area  97 , which has anomalous short activation times throughout. By appropriate repositioning of the spline catheter  93 , PVC and SR points may then be automatically acquired using one or more splines, and processed according to the method described above with reference to  FIG. 2 . 
     Alternate Embodiment 
     Another embodiment of the invention involves pace mapping, for example using PaSo™ software. 
     Pace mapping is a diagnostic technique used for identification of ventricular tachycardia foci. This involves pacing the chamber at the ventricular tachycardia rate, then comparing a body surface 12-lead ECG acquired during pacing to an ECG recorded during clinical arrhythmia, either induced or previously recorded. 
     In this embodiment, the mapping phase is used to identify a pace-map correlation, i.e., a correlation between signals produced during ventricular tachycardia and signals produced by pace mapping, referred to as pace mapped—induced signal (PM-IS) correlation, as taught in commonly assigned U.S. Pat. No. 7,907,994 to Stolarski et al., which is herein incorporated by reference. A paced rhythm point serves as the PVC point from which PVC&#39;s originate. A map produced during pacing functions in the same manner as the map produced during sinus rhythm in the previous embodiment. 
     Reference is now made to  FIG. 6 , which is a flow chart of a method of mapping arrhythmogenic areas producing ventricular premature beats while the subject is in sinus rhythm, in accordance with the preceding alternate embodiment of the invention. Some of the steps in  FIG. 6  are performed in the same manner as those of  FIG. 2 . Their description is not repeated in the interest of brevity. 
     After performing initial step  37 , pacing is conducted at step  99 . A point is then acquired at step  101 , and pacing is then discontinued at step  103 . 
     Next, at step  105 , the beat buffer is filled with pace-mapped (PM) beats and SR beats. 
     Next, at step  107 , a PM beat is selected from the beat buffer. 
     Then, after performing steps  47 ,  49  and  51  as described above, at step  109 , the pace-mapped correlation based on the PM point acquired in step  47  is assigned to the SR point location that was selected in step  51 . 
     Next, at step  111 , the PM point is presented as floating. 
     Next, at step  113 , the SR Map is presented with PM-IS correlation data, referred to as a SR-PM map. 
     Then, at final step  115 , ablation is performed according to the SR-PM map. 
     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.