Identifying instances of cardiac defibrillation while building a location map

Abstract

To track probes during intracardiac procedures.SOLUTION: A method includes: computing a position of an intrabody probe, which includes one or more electrodes and an electromagnetic sensor, within a heart of a subject, based on an induced signal received from the electromagnetic sensor; ascertaining a set of properties of signals passed between the electrodes and multiple reference electrodes located at respective reference positions; based on the set of properties, deriving an estimated position of the probe from a position map that maps multiple sets of properties to respective estimated positions; in response to a distance between the computed position and the estimated position being greater than a predefined threshold, ascertaining whether an electrocardiographic signal from the subject is saturated; and, in response to the electrocardiographic signal not being saturated, updating the position map so as to map the set of properties to the computed position. Other embodiments are also described.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to tracking probes during intracardiac procedures. [Background technology]

[0002] U.S. Patent No. 7,536,218 (Govari et al.), the disclosure of which is incorporated herein by reference, describes a position sensing system including a probe adapted to be introduced into a body cavity of a subject. The probe has a magnetic field transducer and at least one probe electrode. A control unit is configured to measure position coordinates of the probe using the magnetic field transducer. The control unit also measures impedance between the at least one probe electrode and one or more points on the subject's body surface. Using the measured position coordinates, the control unit calibrates the measured impedance. Summary of the Invention [Means for solving the problem]

[0003] According to some embodiments of the present invention, a system is provided that includes a memory configured to store a location map that maps a plurality of sets of characteristics to respective estimated locations, and a processor. The processor is configured to calculate a location of an intracorporeal probe, including one or more electrodes and an electromagnetic sensor, within a subject's heart based on induced signals received from the electromagnetic sensor. The processor is further configured to ascertain a set of characteristics of signals passed between the electrodes and a plurality of reference electrodes located at respective reference locations. The processor is further configured to derive an estimated location of the probe from the location map based on the set of characteristics. The processor is further configured to ascertain whether an electrocardiogram signal from the subject is saturated in response to a distance between the calculated location and the estimated location being greater than a predetermined threshold. The processor is further configured to update the location map in memory to map the set of characteristics to the calculated location in response to the electrocardiogram signal not being saturated.

[0004] In some embodiments, the predetermined threshold is between 8 and 15 mm.

[0005] In some embodiments, the predetermined threshold is a first predetermined threshold, and the processor is configured to update the location map in response to the distance not exceeding a second predetermined threshold.

[0006] In some embodiments, the processor is configured to determine whether an electrocardiogram signal from the subject is saturated in response to the distance not exceeding a second predetermined threshold.

[0007] In some embodiments, the second predetermined threshold is greater than 15 mm.

[0008] In some embodiments, the second predetermined threshold is between 15 and 30 mm.

[0009] In some embodiments, the processor: Calculate other positions of the probe to check other sets of properties, The device is further configured to, in response to the electrocardiogram signal being saturated, discontinue updating the location map to map a different set of characteristics to a different calculated location.

[0010] In some embodiments, the processor is further configured to cease updating the location map for a predetermined duration in response to the electrocardiogram signal being saturated.

[0011] In some embodiments, the predetermined duration is 4 to 5 seconds.

[0012] Some embodiments of the present invention further provide a method including calculating a position of an intracorporeal probe including one or more electrodes and an electromagnetic sensor within a subject's heart based on induced signals received from the electromagnetic sensor. The method further includes ascertaining a characteristic set of signals passed between the electrode and a plurality of reference electrodes located at respective reference positions. The method further includes deriving an estimated position of the probe based on the characteristic set from a location map that maps the plurality of characteristic sets to respective estimated positions. The method further includes ascertaining whether an electrocardiogram signal from the subject is saturated in response to a distance between the calculated position and the estimated position being greater than a predetermined threshold. The method further includes updating the location map to map the characteristic set to the calculated position in response to the electrocardiogram signal not being saturated.

[0013] According to some embodiments of the present invention, there is further provided a computer software product including a tangible, non-transitory computer-readable medium having program instructions stored thereon, the instructions, when read by a processor, causing the processor to calculate a position of an intracorporeal probe including one or more electrodes and an electromagnetic sensor within a subject's heart based on induced signals received from the electromagnetic sensor. The instructions further cause the processor to ascertain characteristic sets of signals passed between the electrodes and a plurality of reference electrodes located at respective reference positions. The instructions further cause the processor to derive an estimated position of the probe based on the characteristic sets from a location map that maps the plurality of characteristic sets to respective estimated positions. The instructions further cause the processor to ascertain whether an electrocardiogram signal from the subject is saturated in response to a distance between the calculated position and the estimated position being greater than a predetermined threshold. The instructions further cause the processor to update the location map to map the characteristic set to the calculated position in response to the electrocardiogram signal not being saturated.

[0014] The invention is more fully understood when considered in conjunction with the following detailed description of the invention, in which: [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram of a system for calculating a position map according to some embodiments of the present invention; [Figure 2A] 1 is a schematic diagram of an intracellular probe, according to some embodiments of the present invention. [Figure 2B] 1 is a schematic diagram of an intracellular probe, according to some embodiments of the present invention. [Figure 3] FIG. 1 is a flow diagram of an algorithm for identifying instances of cardiac defibrillation while building a location map, according to some embodiments of the present invention. [Figure 4] 1A-1D illustrate experimental data obtained from treatments on human subjects performed in accordance with some embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] overview U.S. Patent No. 7,536,218 to Govari et al., cited in the Background section above, describes a hybrid tracking system for tracking the position of an internal probe. In this system, the probe is equipped with an electromagnetic sensor and one or more position tracking electrodes. As the probe moves within a subject's body cavity, an external magnetic field induces a signal in the electromagnetic sensor, and a processor calculates the position of the probe based on the induced signal. In addition, the position tracking signal, whose characteristics vary with the probe's position, is passed between the position tracking electrode and one or more reference electrodes on the subject's body surface. The processor constructs a position map that maps the characteristics of the position tracking signal to the probe's position calculated from the induced signal. Subsequently, another probe that does not include an electromagnetic sensor but does include position tracking electrodes can be tracked using the position tracking electrodes and the position map.

[0017] A challenge with constructing a position map as described above is that any cardioversion treatment administered to the subject may alter the characteristics of the position tracking signal, causing the position tracking signal to not accurately indicate the location of the probe. Thus, if construction of the position map continues during the cardioversion treatment, the accuracy of the map is compromised. Given that the cardioversion device used for cardioversion is typically not connected to a processor, the processor does not know when cardioversion has occurred.

[0018] To address this issue, embodiments of the present invention use a position map to calculate an estimated probe position based on the position tracking signals. If the estimated position deviates from the "true" position (calculated from the induced signals) by more than a threshold distance, a processor checks whether the electrocardiogram (ECG) signal from the subject is saturated. If the signal is saturated, indicating a high likelihood of cardiac defibrillation in progress, the processor typically ceases updating the position map until a predetermined time has elapsed.

[0019] Typically, a second, higher threshold distance is also defined, and if the estimated position deviates from the true position by more than the second threshold distance, the processor can stop updating the position map without checking the ECG signal.

[0020] System Description Reference is first made to Figure 1, which is a schematic illustration of a system 20 for calculating a position map, according to some embodiments of the present invention. Reference is also made to Figures 2A-2B, which are schematic illustrations of an intracorporeal probe 40, according to some embodiments of the present invention. Probe 40 comprises a shaft 22, which may be coupled at its distal end to a plurality of deflectable arms 54 (Figure 2A), an inflatable balloon 45 (Figure 2B), or any other suitable structure.

[0021] The probe 40 includes one or more position tracking electrodes 52. For example, as shown in FIGS. 2A-2B, the probe may include a proximal position tracking electrode 52a and a distal position tracking electrode 52b. The probe 40 further includes an electromagnetic sensor 50, which may be coupled to the shaft 22 between the two tracking electrodes (FIG. 2A), proximal to the proximal position tracking electrode 52a (FIG. 2B), or at any other suitable location. Optionally, the probe may further include additional electrodes 55 for ablating cardiac tissue and / or sensing electrogram signals from the cardiac tissue. The sensors and electrodes are connected to an interface circuit 44 within the console 24 via wires extending through the shaft 22. The interface circuit 44 may include an analog-to-digital (A / D) converter and / or any other suitable components.

[0022] 1, a physician 30 inserts a probe 40 into the vasculature of a subject 28 and then navigates the probe to a target location within the heart 26 of the subject 28, typically using a control handle 32 to manipulate the shaft 22. Typically, the probe is navigated through a sheath 23 that constrains the distal end of the probe. Following reaching the target location, the sheath 23 is retracted, expanding the distal end of the probe.

[0023] 1, subject 28 is positioned within a magnetic field generated by field generator coil 42. Specifically, first signal generator (SIG GEN) 43 drives a signal through coil 42, causing the coil to generate a magnetic field. The magnetic field induces a signal in electromagnetic sensor 50, which varies with the position of the sensor. The induced signal from sensor 50 is received by interface circuitry 44.

[0024] Additionally, as the probe moves within heart 26, second signal generator 47 passes position tracking signals between position tracking electrodes 52 and reference electrodes 49. Reference electrodes 49 are located at respective reference positions that do not move with the probe. For example, the reference electrodes may be coupled to the subject's body surface, such as the subject's chest and / or back. Specifically, three reference electrodes may be coupled to the subject's chest (as shown in FIG. 1 ) and three reference electrodes may be coupled to the subject's back. (The reference electrodes are typically connected to interface circuit 44 via cable 39.) As the probe moves, the impedance between position tracking electrodes 52 and reference electrode 49 changes, causing the characteristics of the position tracking signal to vary with the probe's position.

[0025] System 20 further includes a processor (PROC) 41, typically included in console 24. Processor 41 is configured to control various other components of system 20, such as first signal generator 43 and second signal generator 47. Processor 41 is further configured to receive an induced signal from electromagnetic sensor 50 via interface circuit 44. Based on the induced signal, the processor calculates the position of probe 40. (The position of the probe may be defined as the position of the sensor or as the position of another portion of probe 40 at a fixed displacement from the sensor.) In making this calculation, the processor may use any suitable technique, such as those described in U.S. Pat. Nos. 5,391,199, 5,443,489, and 6,788,967 (Ben-Haim), U.S. Pat. No. 6,690,963 (Ben-Haim et al.), U.S. Pat. No. 5,558,091 (Acker et al.), and U.S. Pat. No. 6,177,792 (Govari).

[0026] Processor 41 is further configured to receive the position tracking signals via the interface circuit and to ascertain a characteristic set for each position tracking signal. Each characteristic set may include, for example, voltage and / or current between each pair of electrodes, e.g., between proximal position tracking electrode 52 a and each of the reference electrodes, and between distal position tracking electrode 52 b and each of the reference electrodes. (Specifically, in embodiments in which second signal generator 47 acts as a voltage source, each characteristic set may include current, while in embodiments in which second signal generator 47 acts as a current source, each characteristic set may include voltage.) Alternatively or additionally, each characteristic set may include a calculated impedance between each pair of electrodes.

[0027] (Note that each of the aforementioned voltages, currents, and impedances may be expressed as absolute numbers or as relative numbers. As an example of the latter, the current between the proximal position tracking electrode 52a and one of the reference electrodes may be expressed as a percentage of the total current between the proximal position tracking electrode 52a and that reference electrode.)

[0028] Processor 41 is further configured to receive electrocardiogram potentials from electrocardiogram (ECG) electrodes (not shown) coupled to the subject's body via the interface circuit. (The ECG electrodes may be connected to the interface circuit via cable 39 or a separate cable.) Using techniques known in the art, the processor combines the potentials into a single electrocardiogram signal.

[0029] As described further below with reference to FIG. 3 , the processor is configured to build a position map 36 that maps various sets of characteristics (PROPs) to respective positions (POS) of the probe. While the map 36 is being built, the map may be stored in memory 34, such as random access memory (RAM). The process of building the position map 36 may be referred to as “calibrating” the electrode-based tracking system, which includes the position tracking electrodes 52 and the reference electrode 49, in that the processor learns how the characteristics of the position tracking signal indicate the position of the probe. Following calibration of the electrode-based tracking system, the electrode-based tracking system can be used to track another probe, which includes the position tracking electrodes 52 but does not include the sensor 50, during a subsequent procedure.

[0030] While calibration is being performed, probe 40 can be used to ablate intracardiac tissue, construct an electrophysiological map of the target location, and / or perform any other suitable procedure. Alternatively, calibration can be performed without simultaneously performing any other procedure. In such embodiments, the probe need not necessarily include an electrode distal to shaft 22.

[0031] Typically, system 20 further includes a display 27. Based on the calculated position of the probe, processor 41 may display on display 27 an icon representing the probe superimposed over an image of the target location.

[0032] In some embodiments, electrodes 55 are used for position tracking, i.e., position tracking signals are passed between electrodes 55 and reference electrode 49, and a processor checks the characteristics of these signals and builds position map 36 accordingly. In such embodiments, probe 40 does not necessarily need to include position tracking electrodes 52.

[0033] While the calibration is taking place, the subject may need to undergo defibrillation treatment using cardioverter-defibrillator 51. As described in more detail below with reference to FIG. 3, processor 41 is configured to identify any instances of defibrillation and suspend the calibration accordingly.

[0034] In general, the processor 41 may be embodied as a single processor or as a set of cooperatively networked or clustered processors. Some functions of the processor 41 may be implemented solely in hardware, e.g., using one or more fixed-function or general-purpose integrated circuits, application-specific integrated circuits (ASICs), and / or field-programmable gate arrays (FPGAs). Alternatively, the functions may be implemented at least partially in software. For example, the processor 41 may be embodied as a programmed processor, e.g., including a central processing unit (CPU) and / or a graphics processing unit (GPU). Program code and / or data, including software programs, may be uploaded to RAM for execution and processing by the CPU and / or GPU. The program code and / or data may be downloadable to the processor in electronic form, e.g., over a network. Alternatively or additionally, the program code and / or data may be provided and / or stored on a non-transitory tangible medium, such as magnetic, optical, or electronic memory. Such program code and / or data, when provided to the processor, causes a machine or special-purpose computer configured to perform the tasks described herein.

[0035] Identifying instances of cardiac defibrillation Reference is now made to Figure 3, which is a flow diagram of an algorithm 56 for identifying instances of cardiac defibrillation while building location map 36, according to some embodiments of the present invention. Algorithm 56 is executed by processor 41, typically in real time, while the probe is inside the subject's heart.

[0036] Algorithm 56 causes the processor to iteratively calculate the position of the probe within the subject's heart based on the induced signals received from the electromagnetic (EM) sensors, at a position calculation step 58. Each time the processor calculates the probe position, it also checks the property set of the position tracking signals, at a property set check step 60. Based on the property set, the processor derives an estimated position of the probe from position map 36 (FIG. 1) at an estimated position derivation step 62.

[0037] (Note that during each iteration of algorithm 56, characteristic set validation step 60 and optionally estimated position derivation step 62 may occur before position calculation step 58.)

[0038] Following calculation of the probe position and deriving the estimated position, the processor calculates the distance between the calculated position and the estimated position in a distance calculation step 64. Next, in a first comparison step 66, the processor compares the distance to a first predetermined threshold, typically between 8 and 15 mm.

[0039] If the distance is less than or equal to the first threshold, the processor updates the location map to map the identified set of characteristics to a location in a map update step 74. If the distance is not less than or equal to the first threshold, the processor checks whether the electrocardiogram signal from the subject is saturated, i.e., whether the amplitude of the electrocardiogram signal exceeds a predetermined threshold, in a saturation check step 70. If the electrocardiogram signal is not saturated, the processor updates the location map; if the electrocardiogram signal is saturated, the processor cancels updating the location map, assuming that saturation of the electrocardiogram signal indicates cardiac defibrillation and therefore reduces the reliability of the location tracking signal.

[0040] Following updating the position map, the processor returns to the position calculation step 58.

[0041] Typically, if the distance is greater than a first threshold, updating the location map requires that two conditions be met: first, the ECG signal is not saturated (as described above), and second, the distance does not exceed a second predetermined threshold, typically greater than 15 mm, e.g., 15-30 mm.

[0042] For example, before performing saturation check step 70, the processor may compare the distance to a second predetermined threshold in a second comparison step 68. If the distance does not exceed the second predetermined threshold, the processor performs saturation check step 70. If the distance exceeds the second predetermined threshold, the processor discontinues updating the position map.

[0043] In some embodiments, the processor ceases updating the location map for a predetermined duration in response to the electrocardiogram signal being saturated (or the distance being greater than a second threshold). In other words, following determining that the electrocardiogram signal is saturated (or the distance is greater than a second threshold), the processor waits for a predetermined duration in wait step 72. Typically, the predetermined duration is 4-5 seconds, which is generally sufficient time for an electrode-based tracking system to recover from a cardiac defibrillation event. Following the wait, the processor returns to position calculation step 58.

[0044] In other embodiments, the processor may update the location map at any subsequent time, so long as the conditions for updating the location map are met. In such embodiments, following determining that the electrocardiogram signal is saturated (or the distance exceeds the second threshold), the processor returns to the location calculation step 58 without first waiting for the predetermined duration.

[0045] In some embodiments, the algorithm is executed offline based on recordings of the lead signals, ECG signals, and position tracking signals. In such embodiments, the processor may refrain from using any recorded data acquired within a predetermined duration when ECG saturation occurs (or when the distance begins to exceed the second threshold) to update the position map.

[0046] Experimental data Reference is now made to Figure 4, which illustrates experimental data obtained from a procedure on a human subject performed in accordance with some embodiments of the present invention. The data includes a first plot 76 of the distance between the position of the probe calculated based on induced signals received from the EM sensor and the estimated position of the probe derived from the position map. The data also includes a second plot 78 of ECG signals obtained from the subject during the procedure. The two plots are aligned with respect to the time axis, i.e., the plots are synchronized.

[0047] During the procedure, a cardioverting event occurred between approximately 4.525 seconds and 4.528 seconds. As a result of the cardioverting event, the distance increased substantially from its previous range, which was centered at approximately 2 mm. Additionally, instead of the ECG signal saturating and exhibiting a PQRST wave 80, the ECG signal was flat at zero, indicating that the electrocardiographic potentials seen at each of the ECG electrodes exceeded the maximum value measurable by system 20 (100 mV) and the difference between the potentials was zero.

[0048] Thus, the experimental data demonstrates that saturation of the ECG signal indicates a cardioversion event. The data further demonstrates that the time for an electrode-based tracking system to recover from a cardioversion event (and thus distance back to its pre-cardioversion range) can be substantially longer (e.g., 4-5 seconds longer) than the time required for the ECG signal to recover.

[0049] Those skilled in the art will understand that the present invention is not limited to what is specifically shown and described above in this specification. Rather, the scope of the embodiments of the present invention includes both combinations and subcombinations of the various features described above in this specification, as well as variations and modifications of features that are not present in the prior art and that would occur to one of ordinary skill in the art upon reading the above description. Documents incorporated by reference into this patent application are considered to be an integral part of this application, except that if any term is defined in these incorporated documents in a way that contradicts the definition given herein, either expressly or impliedly, then only the definition given herein shall be considered.

[0050] [Embodiment] (1) a memory configured to store a location map that maps a plurality of sets of characteristics to respective estimated locations; 1. A processor, comprising: calculating a position of an intracorporeal probe including one or more electrodes and an electromagnetic sensor within the subject's heart based on induced signals received from the electromagnetic sensor; determining a set of characteristics of signals passed between the electrode and a plurality of reference electrodes located at respective reference positions; deriving an estimated position of the probe from the position map based on the set of characteristics; determining whether an electrocardiogram signal from the subject is saturated in response to the distance between the calculated location and the estimated location being greater than a predetermined threshold; updating the location map in the memory to map the set of characteristics to the calculated location in response to the electrocardiogram signal not being saturated. a processor configured to: A system comprising: (2) The system of embodiment 1, wherein the predetermined threshold is 8 to 15 mm. (3) The system of embodiment 1, wherein the predetermined threshold is a first predetermined threshold, and the processor is configured to update the location map in response to the distance not exceeding a second predetermined threshold. (4) The system of embodiment 3, wherein the processor is configured to determine whether the electrocardiogram signal from the subject is saturated in response to the distance not exceeding the second predetermined threshold. (5) The system of embodiment 3, wherein the second predetermined threshold is greater than 15 mm.

[0051] (6) The system of embodiment 5, wherein the second predetermined threshold is 15 to 30 mm. (7) The processor: calculating another position of said probe to identify another set of properties; and ceasing to update the location map to map the different set of characteristics to the different calculated location in response to the electrocardiogram signal being saturated. The system of embodiment 1, further configured as follows: (8) The system of embodiment 7, wherein the processor is further configured to cease updating the location map for a predetermined duration in response to the electrocardiogram signal being saturated. (9) The system of embodiment 8, wherein the predetermined duration is 4 to 5 seconds. (10) calculating a position of an intracorporeal probe including one or more electrodes and an electromagnetic sensor within the subject's heart based on induced signals received from the electromagnetic sensor; ascertaining a set of characteristics of signals passed between the electrode and a plurality of reference electrodes located at respective reference positions; deriving an estimated location of the probe based on the characteristic sets from a location map that maps a plurality of characteristic sets to respective estimated locations; determining whether an electrocardiogram signal from the subject is saturated in response to the distance between the calculated location and the estimated location being greater than a predetermined threshold; updating the location map to map the set of characteristics to the calculated location in response to the electrocardiogram signal not being saturated; A method comprising:

[0052] (11) The method of claim 10, wherein the predetermined threshold is 8 to 15 mm. (12) The method of embodiment 10, wherein the predetermined threshold is a first predetermined threshold and updating the location map includes updating the location map in response to the distance not exceeding a second predetermined threshold. (13) The method of embodiment 12, wherein determining whether the electrocardiogram signal from the subject is saturated includes determining whether the electrocardiogram signal from the subject is saturated in response to the distance not exceeding the second predetermined threshold. (14) The method of claim 12, wherein the second predetermined threshold is greater than 15 mm. (15) The method of embodiment 14, wherein the second predetermined threshold is 15 to 30 mm.

[0053] (16) calculating another position of the probe to identify another set of properties; responsive to the electrocardiogram signal being saturated, ceasing to update the location map to map the different set of characteristics to the different calculated location; 11. The method of embodiment 10, further comprising: (17) The method of embodiment 16, further comprising: in response to the electrocardiogram signal being saturated, ceasing to update the location map for a predetermined duration. (18) The method of embodiment 17, wherein the predetermined duration is 4 to 5 seconds. (19) A computer software product including a tangible, non-transitory computer-readable medium having stored thereon program instructions, said instructions, when read by a processor, causing said processor to: calculating a position of an intracellular probe including one or more electrodes and an electromagnetic sensor within the subject's heart based on induced signals received from the electromagnetic sensor; ascertaining a set of characteristics of signals passed between the electrode and a plurality of reference electrodes located at respective reference positions; deriving an estimated location of the probe based on the property sets from a location map that maps a plurality of property sets to respective estimated locations; determining whether an electrocardiogram signal from the subject is saturated in response to the distance between the calculated location and the estimated location being greater than a predetermined threshold; updating the location map to map the set of characteristics to the calculated location in response to the electrocardiogram signal not being saturated; Computer software products. (20) The instructions further include: calculating another position of the probe to identify another set of properties; and canceling updating the location map to map the different set of characteristics to the different calculated location in response to the electrocardiogram signal being saturated. 20. A computer software product as described in embodiment 19.

Claims

1. a memory configured to store a location map that maps a plurality of sets of characteristics to respective estimated locations; 1. A processor, comprising: calculating a position of an intracellular probe including one or more electrodes and an electromagnetic sensor within the subject's heart based on induced signals received from the electromagnetic sensor; determining a set of characteristics of signals passed between the electrode and a plurality of reference electrodes located at respective reference positions; deriving an estimated position of the probe from the position map based on the set of characteristics; determining whether an electrocardiogram signal from the subject is saturated in response to the distance between the calculated location and the estimated location being greater than a predetermined threshold; updating the location map in the memory to map the set of characteristics to the calculated location in response to the electrocardiogram signal not being saturated. a processor configured to: A system comprising:

2. The system of claim 1 , wherein the predetermined threshold is between 8 and 15 mm.

3. 2. The system of claim 1, wherein the predetermined threshold is a first predetermined threshold, and the processor is configured to update the location map in response to the distance not exceeding a second predetermined threshold.

4. 4. The system of claim 3, wherein the processor is configured to determine whether the electrocardiogram signal from the subject is saturated in response to the distance not exceeding the second predetermined threshold.

5. The system of claim 3 , wherein the second predetermined threshold is greater than 15 mm.

6. The system of claim 5, wherein the second predetermined threshold is between 15 and 30 mm.

7. the processor: calculating another position of said probe to identify another set of properties; and ceasing to map the different set of characteristics to the calculated different location in response to the electrocardiogram signal being saturated. The system of claim 1 further configured to:

8. The system of claim 7 , wherein the processor is further configured to cease updating the location map for a predetermined duration in response to the electrocardiogram signal being saturated.

9. The system of claim 8, wherein the predetermined duration is between 4 and 5 seconds.

10. A program, wherein when the program is read by a processor, the processor: calculating a position of an intracellular probe including one or more electrodes and an electromagnetic sensor within the subject's heart based on induced signals received from the electromagnetic sensor; ascertaining a set of characteristics of signals passed between the electrode and a plurality of reference electrodes located at respective reference positions; deriving an estimated location of the probe based on the characteristic sets from a location map that maps a plurality of characteristic sets to respective estimated locations; determining whether an electrocardiogram signal from the subject is saturated in response to the distance between the calculated location and the estimated location being greater than a predetermined threshold; updating the location map to map the set of characteristics to the calculated location in response to the electrocardiogram signal not being saturated; Including, the program.

11. The program according to claim 10, wherein the predetermined threshold is 8 to 15 mm.

12. 11. The program of claim 10, wherein the predetermined threshold is a first predetermined threshold, and updating the position map includes updating the position map in response to the distance not exceeding a second predetermined threshold.

13. 13. The program of claim 12, wherein determining whether the electrocardiogram signal from the subject is saturated comprises determining whether the electrocardiogram signal from the subject is saturated in response to the distance not exceeding the second predetermined threshold.

14. The computer-readable medium of claim 12 , wherein the second predetermined threshold is greater than 15 mm.

15. The program according to claim 14, wherein the second predetermined threshold is 15 to 30 mm.

16. calculating another position of said probe to identify another set of properties; and ceasing to map the different set of characteristics to the calculated different location in response to the electrocardiogram signal being saturated. The program of claim 10, further comprising:

17. 17. The computer program product of claim 16, further comprising: in response to the electrocardiogram signal being saturated, ceasing to update the location map for a predetermined duration.

18. 18. The program of claim 17, wherein the predetermined duration is 4 to 5 seconds.

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