Abstract:
A method, including inserting a flexible probe into a living subject and positioning a distal end of the probe in a heart of the subject, the distal end including a position sensor configured to generate position signals indicative of a position of the distal end, and an electrode configured to convey electrical signals from the heart. The method further includes formulating, in response to the position signals, a first indication of a change in a mean position of the heart within the living subject and deriving a second indication of a change in the electrical signals. The method also includes determining, in response to the first and second indications, a new mean position of the heart.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to electrocardiography, and specifically to tracking a probe used in an electrocardiography procedure. 
       BACKGROUND OF THE INVENTION 
       [0002]    A critical need during electrocardiography, wherein probes are positioned within the heart, is accurate tracking of the probes. Any method to improve the tracking is advantageous. 
         [0003]    Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
       SUMMARY OF THE INVENTION 
       [0004]    An embodiment of the present invention provides a method, including: 
         [0005]    inserting a flexible probe into a living subject; 
         [0006]    positioning a distal end of the probe in a heart of the subject, the distal end including a position sensor configured to generate position signals indicative of a position of the distal end, and an electrode configured to convey electrical signals from the heart; 
         [0007]    formulating, in response to the position signals, a first indication of a change in a mean position of the heart within the living subject; 
         [0008]    deriving a second indication of a change in the electrical signals; and 
         [0009]    determining, in response to the first and second indications, a new mean position of the heart. 
         [0010]    Typically the change in the mean position of the heart is responsive to a change in the position of the distal end. 
         [0011]    In a disclosed embodiment the position of the distal end includes a mean position of the distal end measured during a preset number of heart beats of the heart. Typically, formulating the first indication includes determining that the position of the distal end does not correspond to the mean position. 
         [0012]    In a further disclosed embodiment the second indication is derived in response to determining that the change in the mean position is a non-zero change. 
         [0013]    In a yet further disclosed embodiment the second indication is indicative of no change in the electrical signals. 
         [0014]    In an alternative embodiment the electrode consists of a plurality of electrodes attached to the distal end, and the plurality of electrodes are configured to convey respective electrical signals from respective sites in the heart. The method may include formulating a mapping between respective parameters of the respective electrical signals and respective positions of the respective sites. The method may also include positioning a further distal end of a further probe in the heart, and determining a further position of the further distal end in response to the mapping. 
         [0015]    The respective parameters may include respective local activation times of the respective electrical signals, and the method may include determining a change in the position of the distal end in response to the mapping. 
         [0016]    In a further alternative embodiment the method includes positioning a further distal end of a further probe in the heart, and determining a further position of the further distal end in response to the new mean position of the heart. 
         [0017]    There is further provided, according to an embodiment of the present invention, apparatus, including: 
         [0018]    a flexible probe, having a distal end, configured to be inserted into a living subject; 
         [0019]    a position sensor included in the distal end, configured to generate position signals indicative of a position of the distal end in a heart of the subject; 
         [0020]    an electrode included in the distal end, configured to convey electrical signals from the heart; and 
         [0021]    a processor, configured to: 
         [0022]    formulate, in response to the position signals, a first indication of a change in a mean position of the heart within the living subject, 
         [0023]    derive a second indication of a change in the electrical signals, and 
         [0024]    determine, in response to the first and second indications, a new mean position of the heart. 
         [0025]    The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic illustration of a heart movement compensation system, according to an embodiment of the present invention; 
           [0027]      FIG. 2  is a schematic illustration of a distal end of a probe used in the system, according to an embodiment of the present invention; 
           [0028]      FIG. 3A  is a schematic graph illustrating the motion of a reference distal end of the probe, and  FIGS. 3B and 3C  are schematic graphs illustrating signals derived from an electrode on the distal end, according to embodiments of the present invention; 
           [0029]      FIGS. 4A-4D  are schematic graphs of intrabody electrical signals, according to embodiments of the present invention; 
           [0030]      FIG. 5A  is a schematic graph illustrating the motion of the reference distal end, and  FIGS. 5B and 5C  are schematic graphs illustrating signals derived from the electrode on the distal end, according to alternative embodiments of the present invention; and 
           [0031]      FIG. 6  is a flowchart showing steps performed in operating the system, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
       [0032]    An embodiment of the present invention provides a system and method for tracking a distal end of a reference probe within the heart of a living subject. Typically the tracked distal end may be used as a reference for tracking distal ends of other probes in the heart. 
         [0033]    The reference distal end comprises a position sensor and an electrode attached to the distal end. After insertion of the reference distal end to a reference site of the heart, typically comprising the coronary sinus, a processor records position signals from the sensor and intrabody electrical signals from the electrode. The processor monitors the position signals as the heart beats. Providing the signals are repetitive over a preset number of heart beats, so that there is substantially no change in a mean position of the reference distal end, the processor uses the mean position as a mean position of the heart. 
         [0034]    If the position signals register a change in the position of the reference distal end, the processor waits until the signals again become repetitive, and uses the new mean position of the distal end as a new mean position of the heart. 
         [0035]    Typically, the change registered by the position signals may be caused by a change in a body frame of reference used to measure the positions, and/or by a movement of the reference distal end with respect to its reference site. Embodiments of the present invention correct for both causes. Changes in the body frame of reference are corrected for using the mean position measurements described above. Changes because of reference distal end movement may be corrected for by generating a mapping between the intrabody electrical signals and different reference sites to where the distal end may move. In the event of distal end movement, the mapping may be used to estimate a position of a new reference site. 
         [0036]    The position of the reference distal end may be used as a reference for other probe distal ends positioned in the heart. Such a use averts recalibration of the body frame of reference, which in prior art systems is necessary if sensors defining the frame have changed. The use also corrects for overall movement of the heart in the thoracic cavity, such as may occur during defibrillation of the heart. 
       System Description 
       [0037]    Reference is now made to  FIG. 1 , which is a schematic illustration of a heart movement compensation system  20 , and to  FIG. 2 , which is a schematic illustration of a distal end of a probe  22  in system  20 , according to embodiments of the present invention. For simplicity and clarity, the following description assumes that system  20  operates while a medical procedure is performed on a heart  24 , herein assumed to comprise a human heart, using probe  22 . System  20  includes facilities for tracking probe  22 , as well as for receiving electrical signals, also herein termed intrabody electrocardiograph (ECG) signals, detected by the probe. System  20  typically includes other facilities used during the medical procedure, such as a facility for ablating one or more regions of heart  24 . 
         [0038]    Probe  22  comprises a catheter which is inserted into the body of a subject  26  during the medical procedure. The medical procedure is performed by a user  28  of system  20 , and in the description herein user  28  is assumed, by way of example, to be a medical professional. A distal end  30  of the probe comprises a plurality of generally similar electrodes  32 A,  32 B,  32 C, . . . , collectively referred to herein as electrodes  32 . Electrodes  32 A,  32 B,  32 C, . . . , receive electrical signals from respective sites  33 A,  33 B,  33 C, . . . in the heart of the subject, and the signals are analyzed by system  20 , as described herein. Distal end  30  is assumed to be positioned within a blood vessel  34  of heart  24 . 
         [0039]    Distal end  30  comprises a position sensor  36 , which is assumed herein to comprise one or more coils  37  providing signals which vary according to the position, i.e., the location and orientation, of the distal end. The operation of sensor  36  is described in more detail below. 
         [0040]    In addition to probe  22 , professional  28  also uses probes  23 A,  23 B, . . . collectively referred to herein as probes  23 , during the medical procedure. Probes  23  are generally similar in construction and operation to probe  22 , and have respective distal ends  31 A,  31 B, . . . collectively referred to herein as distal ends  31 . For simplicity only probe  23 A and distal end  31 A are shown in  FIG. 1 . 
         [0041]    System  20  is typically controlled by a system processor  38  which may be realized as a general purpose computer. The system processor comprises a processing unit  40  communicating with a memory  42 . Processor  38  may be mounted in a console  44 , comprising operating controls  46  that typically include a keypad and a pointing device such as a mouse or trackball that professional  28  uses to interact with the processor. Results of the operations performed by processor  38  are provided to the professional on a screen  47  which displays a diagram  48  of results generated by system  20 . The screen typically displays other items of auxiliary information related to the heart while the heart is being investigated, such as the positions of distal end  30 , and the positions of other catheters used by professional  28 . Screen  47  typically also presents a graphic user interface to the professional. Professional  28  is able to use controls  46  to input values of parameters used by processor  38  in the operation of system  20 . 
         [0042]    Processor  38  uses computer software, including a probe tracker module  50 , to operate system  20 . The software may be downloaded to processor  38  in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible computer-readable media, such as magnetic, optical, or electronic memory. 
         [0043]    Probe tracker module  50  tracks distal end  30  while the probe is within subject  26 . The tracker module typically tracks both the location and the orientation of the distal end of the probe, within the heart of subject  26 . In some embodiments module  50  tracks other sections of the probe. While the tracker module may use any method for tracking probes known in the art, using an appropriate position sensor, in the present description for clarity and simplicity module  50  is assumed to comprise a magnetic tracker, such as the Carto® system produced by Biosense Webster, of Diamond Bar, CA. Module operates magnetic field transmitters  52  in the vicinity of subject  26 , so that magnetic fields from the transmitters interact with tracking coils  37  located in distal end  30 . 
         [0044]    The coils interacting with the magnetic fields generate signals which are transmitted to the module, and the module analyzes the signals to determine a location and orientation of distal end  30 . Alternatively or additionally, tracker module  50  may track the distal end of probe  22  by measuring impedances between one or more of electrodes  32  and electrodes on the skin of subject  26 . (In this case electrodes  32  also act as the position sensor, being used for both intrabody ECG detection and for tracking.) The Carto3® system produced by Biosense Webster uses both magnetic field transmitters and impedance measurements for tracking. U.S. Pat. No. 7,848,789, to Govari et al., whose disclosure is incorporated herein by reference, describes using both magnetic fields and impedance measurements for probe tracking. 
         [0045]    By generally similar methods as those used for tracking distal end  30 , probe tracker module  50  also tracks the locations and orientations of distal ends  31 . As explained further below, distal end  30  of probe  22  is typically used as a reference for tracking of distal ends  31 , so that probe  22  is also referred to herein as reference probe  22 , and distal end  30  is also referred to herein as reference distal end  30 . 
         [0046]    Transmitters  52  are fixed, and define a transmitter frame of reference in terms of a first set of orthogonal axes x T , y T , z T  which are fixed with respect to the transmitters. However, while the signals derived from sensor  36  and the sensors of probes  23  provide coordinates of the location and orientation of distal end  30  and distal ends  31  with respect to the transmitter frame of reference, professional  28  typically requires knowledge of the location and orientation of the distal ends with respect to subject  26 . In order to provide this latter location and orientation, a set of patient position sensors  54 , generally similar to sensor  36 , are attached to the skin of subject  26 . Typically sensors  54  are attached to known locations on the back of the subject. Module  50  receives signals from sensors  54 , and uses the signals to define a body coordinate frame of reference in terms of a second set of body coordinate orthogonal axes x B , y B , z B  which are fixed with respect to subject  26 . As described below, during a procedure processor  38  registers the two frames of reference, and so is able to generate the location and orientation of the distal ends with respect to the subject. 
         [0047]      FIG. 3A  is a schematic graph illustrating the motion of reference distal end  30 , and  FIGS. 3B and 3C  are schematic graphs illustrating signals derived from one of the electrodes on the distal end, according to embodiments of the present invention. A graph  100  illustrates the motion plotted spatially on body coordinate axes x B , y B , z B . As stated above, probe  22  is assumed to be used as a reference probe, so that professional  28  places reference distal end  30  of the probe in a known position within heart  24 , herein assumed to be within the coronary sinus of the heart. Once placed, professional  28  does not move distal end  30 , and except as described hereinbelow, the reference distal end is assumed to remain substantially fixed within the coronary sinus. It will be understood that system  20  may use positions of distal end  30  to determine positions of heart  24 , so that, for example, a mean position of the distal end corresponds to a mean position of the heart. 
         [0048]    Within the coronary sinus, (or within any other reference position within heart  24 ) distal end  30  moves according to the beating of heart  24 . During the procedure on heart  24 , processor  38  uses signals from position sensor  36  to measure positions, i.e., locations and orientations, of the distal end. Because of the beating of the heart, locations r and orientations φ of the reference distal end taken over a period of time repeat spatially, as illustrated schematically by overlapping graph lines  106 . The occurrence of a substantially repetitive nature of the reference distal end motion, typically over a preset number of heart beats, defines a valid system tracking state. During such a valid system tracking state distal end positions p are assumed to vary about a mean position p 1 . Equation (1) is an identity for mean position p 1 : 
         [0000]        p   1 ≡( r   1 ,φ 1 )  (1)
 
         [0049]    where r 1  is a mean location of the reference distal end in heart  24 , and 
         [0050]    φ 1  is a mean orientation of the reference distal end in the heart. 
         [0051]    The situation illustrated by lines  106  is after processor  38  has registered the body frame of reference with the transmitter frame of reference. The illustrated situation typically exists while the frames of reference registration is valid, and also while the valid system tracking state (i.e., that the reference distal end moves repetitively) exists. While a valid frames of reference registration exists, as illustrated by lines  106 , processor  38  may use distal end  30  as a reference for valid tracking of distal ends  31 , by evaluating relative position vectors between distal ends  31  and reference distal end  30 . (For simplicity, distal ends  31  and the relative position vectors to distal ends  31  from reference distal end  30  are not shown in the diagram.) 
         [0052]    The frames of reference registration exemplified by lines  106  typically continues to be valid unless there is a movement of position sensors  54  with respect to transmitters  52 . Even if the registration of the two frames of reference remains valid, other factors, such as movement of heart  24  within patient  26  (due, for example, to the patient being defibrillated), may invalidate the tracking of distal end  30 , and thus, since distal end  30  is used as a reference, the tracking of distal ends  31 . 
         [0053]    As described below, embodiments of the present invention check if the tracking of reference distal end  30  has changed, and provide a method for system  20  to correct for any such changes. 
         [0054]    Overlapping lines  108  depict a situation where the tracking of reference distal end  30  has changed, and a new valid tracking state of the distal end exists. The change may be because of a movement of the body frame of reference, so that the body frame of reference depicted in  FIG. 3A  is no longer applicable; alternatively or additionally, the change may be because of a movement of the heart within the thoracic cavity. In both cases, the mean position of the reference distal end changes, from its initial value p 1  to a new value p 2 , as illustrated in  FIG. 3A . Equation (2) is an identity for mean position p 2 : 
         [0000]        p   2 ≡( r   2 ,φ 2 )  (2)
 
         [0055]    where r 2  is a mean location of the reference distal end during the second valid tracking state, and 
         [0056]    φ 2  is a mean orientation of the reference distal end in the second state. 
         [0057]    The transfer between the two valid tracking states of the distal end is indicated schematically by a broken arrow  110 . 
         [0058]    A change from a valid tracking state may also occur because of a movement of reference distal end  30  within heart  24 . Such a movement may happen, even if professional  28  has not moved the proximal end of probe  22 , for example due to defibrillation. Embodiments of the present invention check if reference distal end  30  has moved within the heart. In the event of such movement, an embodiment of the present invention measures and provides a correction for the movement, as described below. 
         [0059]    While processor  38  measures the position of distal end  30  using sensor  36 , it also records the intrabody electrical signals received from electrodes  32 . Graphs  102  and  104  illustrate the signals received from single electrode  32 B. The signals are plotted as potential vs. time graphs, received respectively during the period within which lines  106  are generated (graph  102 ), and during the period within which lines  108  are generated (graph  104 ). 
         [0060]    The two graphs show the signals compared to a reference parameter, typically generated by the processor using skin ECG signals that are recorded simultaneously with the intra-cardiac signals from electrodes  32 . For simplicity and clarity, in the description herein the reference parameter is assumed to comprise a local activation time (LAT) of the intrabody signals measured with respect to a reference time t R . However, any other convenient reference parameter, such as a phase of the signals, a time of occurrence of the R peak of the QRS complex, an amplitude of the peak, or a combination of such parameters, may be used as a reference parameter. Processor  38  applies the reference parameter in order to quantitatively compare sequential intrabody electrical signals. 
         [0061]    The LAT of the intrabody electrical signals is dependent on the position of electrodes  32  in heart  24 . As is illustrated by graphs  102  and  104 , there is no change of LAT between the two graphs. Because there is no change of LAT between graphs  102  and  104 , distal end  30  has not moved within heart  24 , and so is at its initial site within coronary sinus of the heart, as placed by professional  28 . Thus, processor  38  is able to use the new reference distal end mean position value, p 2 , as a reference for the initial site within the coronary sinus. In addition, using the new mean position p 2  as a reference, the processor is able to continue to track the positions, i.e., the locations and orientations, of distal ends  31 , by applying the relative position vectors between distal ends  31  and reference distal end  30  recorded before the change in valid tracking states. 
         [0062]      FIGS. 4A-4D  are schematic graphs of intrabody electrical signals, according to embodiments of the present invention. Graphs  120 ,  122 , and  124  respectively illustrate the signals conveyed from electrodes  32 A,  32 B, and  32 C to processor  38  ( FIGS. 1 and 2 ). As stated above, the respective signals are generated by respective sites  33 A,  33 B,  33 C in heart  24 , and the sites are assumed to be within the coronary sinus. While the graphs illustrate that signals generated at the different sites are generally similar in morphology and in period, because of the different physical locations of the sites, there are differences, such as different levels of particular sections of the signals, and/or differences in phase. 
         [0063]    Processor  28  can record the signals from different sites, and can generate and store a mapping between the different signals and the different sites generating the signals. For clarity and simplicity in the following description, the mapping is assumed to comprise a mapping between respective LATs and site locations, and those having ordinary skill in the art will be able to modify the description, mutatis mutandis, for other types of mapping. Thus, as illustrated in the graphs, the LAT increases from site  33 A (graph  120 ), to site  33 B (graph  122 ) to site  33 C (graph  124 ). 
         [0064]    Processor  28  is able to use the mapped LATs to identify if distal end  30  has moved relative to sites  33 A,  33 B, and  33 C. If there has been no movement, then the electrode signals are generally unchanged, as exemplified above for graphs  102  and  104  ( FIGS. 3B ,  3 C). 
         [0065]    Graph  126  ( FIG. 4D ) is a graph of signals from electrode  32 B, assumed to be recorded at a time different from the time of recording of graph  122 . The LAT of the signals of graph  126  is larger than the LAT of graph  122 , indicating that electrode  32 B, and thus distal end  30 , has moved relative to sites  33 A,  33 B, and  33 C. Processor  28  quantifies the movement by using the stored mapping, described above. Thus, since the LAT of graph  126  lies between the LATs of graphs  122  and  124 , corresponding to regions  33 B and  33 C, processor  28  can determine by interpolation the new site location, between sites  33 B and  33 C, of electrode  33 B. 
         [0066]    In general, from the stored mapping between signals and electrode locations described above, processor  28  can use interpolation and/or extrapolation to determine a new location for a given electrode, and thus a new location of the electrode&#39;s distal end, from the new intrabody electrical signals generated at the electrode. Furthermore, while the description above has referred to determining a new location for the distal end, a similar method, which will be apparent to those having ordinary skill in the art, mutatis mutandis, may be used to determine a new orientation of the distal end from the new intrabody electrical signals. Thus, the new intrabody electrical signals may be used to find a new position, i.e., a new location and a new orientation, of the reference distal end. 
         [0067]      FIG. 5A  is a schematic graph illustrating the motion of reference distal end  30 , and  FIGS. 5B and 5C  are schematic graphs illustrating signals derived from one of the electrodes on the distal end, according to alternative embodiments of the present invention. Apart from the differences described below, a graph  140  of  FIG. 5A  is generally similar to graph  100  ( FIG. 3A ), and elements indicated by the same reference numerals in the graphs are generally similar in property. Graphs  140  and  100  both illustrate the motion of reference distal end  30  plotted spatially on body coordinate axes x B , y B , z B . A graph  142  of  FIG. 5B  is generally similar to graph  102  ( FIG. 3B ). Both graphs illustrate intrabody electrical signals derived from one of the electrodes on the distal end, herein assumed to be electrode  32 B, during a first valid system tracking state illustrated by lines  106 . 
         [0068]    In contrast to the situation illustrated in  FIGS. 3A-3C , where there is no movement of distal end  30  within heart  24  as the end transfers between two valid system tracking states, in the situation illustrated in  FIGS. 5A-5C , there is movement of distal end  30  within heart  24 . As illustrated in  FIG. 5A , distal end  30  transfers from a first valid state, illustrated by overlapping lines  106  and having a mean position p 1  defined by equation (1). The transfer is to a new valid tracking state, illustrated by overlapping lines  148 , having a mean position p 3  defined by equation (3): 
         [0000]        P   3 =( r   3 ,φ 3 )  (3)
 
         [0069]    where r 3  is a mean location of the reference distal end during the new valid tracking state, and 
         [0070]    φ 3  is a mean orientation of the reference distal end in the new state. 
         [0071]    The transfer to the new valid tracking state is indicated schematically be a broken arrow  150 . 
         [0072]    Graph  144  is a graph of the signals from electrode  32 B taken during the new valid tracking state. As illustrated in graphs  142  and  144 , there is a change in LAT between the two sets of signals: the LAT of graph  144  is larger than the LAT of graph  142 . Thus, by measuring the LATs, processor  38  is able to deduce that distal end has moved. In addition, as explained above with reference to  FIGS. 4A-4D , from the values of the LATs of graphs  142  and  144 , the processor is able to quantify the movement using the stored mapping, based on signals from electrodes  32 A,  32 B,  32 C, . . . . 
         [0073]      FIG. 6  is a flowchart  200  showing steps performed in operating system  20 , according to an embodiment of the present invention. In a system setup step  202 , professional  28  attaches position sensors  54  to subject  26  and activates transmitters  52 . Processor  38  then uses module  50  to formulate a body coordinate frame of reference. Professional  28  may also select a value for the number of heart beats to be used as a preset value for system  20  to use in deciding if a valid system tracking state exists. A typical value is 10, although any other convenient number may be selected. In setup step  202  the professional typically sets other preset values to be used in the flowchart. In addition, the professional defines the parameter to be used in checking if signals from electrodes  32  have changed. For simplicity, in the description of the flowchart the parameter is assumed to be the LAT of the signals. 
         [0074]    In a probe insertion step  204 , professional  28  inserts reference probe  22  into subject  26  until reference distal end  30  is in a selected site within a desired reference region, herein assumed to comprise the coronary sinus. In addition, as required, the professional inserts other probes  23  into subject  26 , until the distal ends of the other probes are also in desired regions. The distal end positions are calculated by processor  38  and module  50 , and are typically displayed to the professional numerically and/or graphically on screen  47 . 
         [0075]    In a first recording step  206 , the processor records the positions of reference distal end  30  over a time interval. The processor uses the positions of the reference distal end to provide a position for heart  24 . During the same time interval the processor also records electrical signals from electrodes  32 . In addition, the processor records the positions of distal ends  31  of other probes  23 . The recordings are made for the preset number of heart beats. 
         [0076]    In a first decision step  208 , processor  38  checks if the positions of reference distal end  30  recorded in step  206  are repetitive, so that over the recording time interval the positions are substantially constant. The check for repetitiveness may be by any convenient means, for example by checking that the average position of the distal end, calculated for each heart beat, does not vary by more than a preset range. 
         [0077]    If the positions do repeat, then in a mapping step  209  the processor generates a mapping between the LATs of the electrical signals of electrodes  32 , and positions of the respective sites within the heart which the electrodes contact. The positions of the sites may be determined from known dimensions of reference distal end  30  and its electrodes, as well as from the position of the reference distal end as recorded in step  206 . The flowchart then proceeds to an initial mean position step  210 . 
         [0078]    In initial mean position step  210  processor  38  calculates a mean position of reference distal end  30 , and uses this position as a mean position of the heart. The processor uses the reference distal end mean position to evaluate relative positions of distal ends  31  with respect to reference distal end  30 . In addition the processor may display a notice on screen  47  indicating to professional  28  that system  20  is tracking distal ends  30  and  31 . From step  210 , the flowchart returns to step  206 , so that steps  206 ,  208 , and  210  repeat iteratively; however, step  209  may not need to be performed after a first iteration has occurred. The reiteration of the steps corresponds to the valid system tracking state illustrated by overlapping lines  106  ( FIGS. 3A and 5A ). 
         [0079]    At a time after at least one of the iterations described above, a situation is assumed where decision step  208  returns a negative value, so that the positions determined by position sensor  36  are no longer constant, do not correspond to the mean position found in step  210 , and so that there has been a non-zero change in the mean position of the reference distal end. Such a non-zero change may be indicative of a change in the mean position of the heart, and/or a change in the body coordinate frame of reference. 
         [0080]    In this case there is no valid tracking state. Such a situation corresponds to broken arrows  110  and  150  ( FIGS. 3A and 5A ). Typically in this case, in a loss of valid state tracking step  212  a warning is displayed to professional  28  that tracking has been lost, and the flowchart continues to a second recording step  214 . 
         [0081]    Second recording step  214  and a second decision step  216  (following step  214 ) are substantially the same as first recording step  206  and first decision step  208 . 
         [0082]    If in second decision step  216  the positions do repeat, then the flow chart continues to a subsequent mean position step  218 , wherein processor  38  calculates a new mean position of reference distal end  30 . 
         [0083]    In calculating the new mean position the processor considers the electrical signals from electrodes  32 . If the parameter defined in setup step  202  (assumed therein to be the LAT of the signals) has not changed, corresponding to a situation illustrated by  FIGS. 3A-3C , then distal end  30  has not moved from the selected site in the coronary sinus. In this case the positions recorded in step  214  are used to calculate a new mean position for reference distal end  30 , and the processor assumes the new reference distal end mean position corresponds to the reference site selected in step  204 . The processor uses the new reference distal end mean position to evaluate relative positions of distal ends  31  with respect to reference distal end  30 . 
         [0084]    If in step  218  the LAT of the signals has changed, this corresponds to a situation illustrated by  FIGS. 5A-5C . In this case distal end  30  has moved from the selected site in the coronary sinus to a new reference site therein. Substantially as described above with respect to  FIGS. 4A-4D , and using the mapping generated in step  209 , processor  38  analyzes the signals from electrodes  32  to determine a new reference position of the new reference site within the coronary sinus. 
         [0085]    The positions recorded from position sensor  36  in step  214  are used to calculate new mean position coordinates for reference distal end  30 , and the processor assumes the new reference distal end mean position corresponds to the new reference site determined from the mapping. Using this correspondence, the processor uses the new reference distal end mean position to evaluate relative positions of distal ends  31  with respect to the new reference site. 
         [0086]    If in decision step  216  the positions do not repeat, then the flowchart continues to an invalid tracking state step  220 , wherein typically a warning is issued that tracking is not valid. From step  220  the flowchart returns to step  214 . Typically, steps  214 ,  216  and  218  reiterate until the procedure implemented by professional  28  is completed. 
         [0087]    It will be appreciated that the embodiments described above are cited by way of example, and 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 which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.