Abstract:
Methods and systems for preparing electroanatomic maps of the heart operate using a probe that has been inserted into a heart chamber by emitting electrical calibration signals from external locations that are outside the subjects body, receiving the calibration signals in a plurality of intracardiac electrodes on the probe, and determining functional relationships between the emitted calibration signals and the received calibration signals. Thereafter, electrophysiological signals from respective origins in the heart are detected in the external locations, and the functional relationships are applied to the detected electrophysiological signals to calculate intracardiac potentials at the respective origins.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit of U.S. Provisional Application No. 61/903,484, filed Nov. 13, 2013, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to evaluation and treatment of cardiac arrhythmias. More particularly, this invention relates to improvements in electrical mapping of the heart for use in evaluation of cardiac arrhythmias and ablative therapy thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    Methods are known for noninvasive mapping of electrical potentials in the heart based on body surface electrocardiographic (ECG) techniques. These methods combine 3-dimensional imaging with the ECG data in order to generate 3-dimensional maps of the electrical potentials on the epicardial surface, and on the endocardial surface, as well. 
         [0006]    The document Modre et al., Atrial Noninvasive Activation Mapping of Paced Rhythm Data, J. Cardiovasc. Electrophysiology 14:712-719 (July 2003), describes a surface heart model activation time (AT) imaging approach, based on magnetic resonance imaging (MRI) and ECG mapping data. Both endocardial and epicardial surfaces might be mapped in this way. The AT pattern was compared to a CARTO™ map of atrial potentials. External anatomic markers were used to couple the CARTO data to the MRI coordinate system, by moving the catheter tip to marker locations at the body surface after internal mapping. It is proposed that AT imaging within the atria may be useful for noninvasive imaging of atrial activity in patients with focal arrhythmias. 
         [0007]    U.S. Pat. No. 7,983,743 to Rudy et al., which is herein incorporated by reference, proposes noninvasive systems and methods for determining electrical activity for a heart of a living being. A processor is configured to meshlessly compute data that represents heart electrical activity from a set of noninvasively measured body surface electrical potentials. This is accomplished using data that describes a geometric relationship between a plurality of locations corresponding to where the body surface electrical potentials were measured and the heart. 
         [0008]    Commonly assigned U.S. Patent Application Publication No. 2008/0058657 by Schwartz et al., which is herein incorporated by reference, describes construction of a matrix relationship between a small number of endocardial points and a large number of external receiving points using a multi-electrode chest panel. Inversion of the matrix yields information allowing the endocardial map to be constructed. 
       SUMMARY OF THE INVENTION 
       [0009]    According to disclosed embodiments of the invention, weak stimulation signals are applied to skin patches and measure the signals received at a catheter in different locations in the heart to establish calibrate a functional relationship between the emitted and the received electrical signals. The relationship is reversible, so that the same calibration relations apply in reverse, allowing activity originating in the heart at different locations to be detected at the skin surface and mapped back to their sources in the heart. The stimulation signals used for calibration have no effect on cardiac electrical activity, and constitute no risk to the subject. 
         [0010]    There is provided according to embodiments of the invention a method, which is carried out by inserting a probe into a chamber of a heart of a living subject, emitting electrical calibration signals from external locations that are outside a body of the subject, receiving the calibration signals in a plurality of intracardiac electrodes disposed in a distal portion of the probe the intracardiac electrodes, determining functional relationships between the emitted calibration signals and the received calibration signals. Thereafter, the method is further carried out by detecting electrophysiological signals at the external locations from respective origins in the heart, and applying the functional relationships to the detected electrophysiological signals to calculate intracardiac potentials at the respective origins. 
         [0011]    One aspect of the method includes removing the probe from the subject prior to detecting electrophysiological signals at the external locations. 
         [0012]    A further aspect of the method includes mapping the electrophysiological signals to the respective origins. 
         [0013]    According to yet another aspect of the method, the external locations comprise a plurality of skin electrodes disposed within a torso vest, the skin electrodes being in galvanic contact with a skin surface of the subject. 
         [0014]    According to still another aspect of the method, there are between 125 and 250 skin electrodes. 
         [0015]    According to an additional aspect of the method, receiving the calibration signals includes filtering the calibration signals to exclude potentials that are generated from electrical activity of the heart. 
         [0016]    According to another aspect of the method, determining functional relationships includes expressing the functional relationships as a matrix has elements, wherein values of the elements depend on respective distances and conductivities between the external locations and the intracardiac electrodes. 
         [0017]    According to one aspect of the method, applying the functional relationships to the detected electrophysiological signals includes inverting the matrix. 
         [0018]    Yet another aspect of the method includes gating the electrical calibration signals only during phases of a cardiac cycle of the subject, wherein expressing the functional relationships as a matrix includes constructing a plurality of gated matrices for respective ones of the phases of the cardiac cycle. 
         [0019]    According to a further aspect of the method, gating the electrical calibration signals includes generating the electrical calibration signals only during phases of a respiratory cycle of the subject. 
         [0020]    There is further provided according to embodiments of the invention an apparatus for carrying out the method. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0021]    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: 
           [0022]      FIG. 1  is a pictorial illustration of a system, which is constructed and operative in accordance with an embodiment of the invention; and 
           [0023]      FIG. 2  is a flow-chart of a procedure for calibration and operation of the instrumentation used in reverse ECG mapping, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    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. 
         [0025]    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 USB memory, 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. 
         [0026]    A conventional method for mapping electropotentials of the heart, i.e., measuring intra-cardiac ECG signals, involves inserting a catheter with electrodes into the heart, and measuring potentials as the electrodes are moved to different locations within the heart. 
         [0027]    Reverse ECG mapping, an example of which is described in U.S. Pat. No. 7,983,743, cited above, attempts to generate an intra-cardiac ECG map by measuring body surface potentials at an array of positions on the skin of a patient. The method assumes that intra-cardiac ECG potentials {right arrow over (E)} generate body surface potentials {right arrow over (S)} S and that the two sets of potentials are related by an equation of the form: 
         [0000]        {right arrow over (S)}=M·{right arrow over (E)}   (1),
 
         [0000]    where M is a matrix, having elements m ij . 
         [0028]    The values of elements of the matrix M depend, inter alia, on the distance between the positions on the heart surface and the positions on the patient&#39;s skin, and on the conductivity of the material between these positions. The &#39;743 Patent uses a non-invasive approach to estimate the matrix M (using systems such as MRI or CT to image the heart and thus find heart surface−skin distances). According to the &#39;743 Patent vector {right arrow over (E)} is then estimated from measured values of the vector {right arrow over (S)}. 
         [0029]    In contrast, embodiments of the present invention take an invasive approach to determine the matrix M. In an initial phase of a mapping procedure for a patient an electrode array, with electrodes in known positions, is attached to the patient&#39;s skin. Assuming the procedure is being performed, a system having location tracking capabilities is used. For example, the positions of the electrodes in the array and the electrodes of a cardiac catheter can be determined by the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. Any other method for finding electrode position may be used. 
         [0030]    Reference is now made to  FIG. 1 , which is a pictorial illustration of a system  10 , which is constructed and operative in accordance with an embodiment of the invention. A subject  12  is clothed in a torso vest  14 . A plurality of electrodes  16 , typically between about 125 and 250 electrodes, are disposed within the torso vest  14  in galvanic contact with the skin of the subject  12 , and can transmit and receive electrical potentials over the anterior, posterior and lateral aspects of the torso of the subject  12 . The electrodes  16  are connected via leads  18  and cable  20  to a control processor  22 , which is typically disposed in a console  24 . 
         [0031]    The console  24  may include a signal generator  26 , an EKG processor  28  and an image processor  30 . 
         [0032]    A catheter  32  has been introduced into a heart  34  by an operator  36 . Information relating to the data obtained from the catheter  32 , the status of the electrodes  16  of the torso vest  14  and the signal generator  26 , EKG processor  28  and image processor  30  may be displayed on a monitor  38 . 
         [0033]    U.S. Pat. No. 7,869,865 to Govari, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference discloses an embodiment of the control processor  22  that contains electrical circuitry for which can be used for impedance detection, as described in U.S. Patent Application Publication No. 2007/0106147 and commonly assigned copending application Ser. No. 14/086,265, both of which are herein incorporated by reference. The system generates, based on impedance measurements between a small number of endocardial points and the electrodes  16 , a multidimensional matrix of coefficients, referred to herein as a lead field matrix. The inverse of the matrix is then estimated, for example, as described in U.S. Patent Application Publication No. 2003/0120163 by Yoram Rudy et al., whose disclosure is herein incorporated by reference. In the Rudy et al. disclosure, the inverse matrix corresponds to epicardial electrical potentials. In the system  10 , however, the inverse of the matrix corresponds to a map of endocardial conductances. Practical techniques and optimizations for the inversion of the lead field matrix are known from the above-referenced U.S. Patent Application Publication No. 2008/0058657. 
         [0034]    As will be seen from the discussion below, according to embodiments of the invention, in a calibration operation for the system  10 , the electrodes  16  of the torso vest  14  may inject known source signals into the body of the subject  12 . The signals are detected by electrodes of the catheter  32 , which is located within the heart. The control processor  22  is programmed to cooperate with the signal generator  26  for selection of the electrodes  16 , for configuration and transmission of the injected signals, and for receiving and processing data from receiving elements within the subject  12  to establish the elements of the matrix M. 
         [0035]    Reference is now made to  FIG. 2 , which is a flow-chart of a procedure for calibration and operation of the instrumentation used in reverse ECG mapping, in accordance with an embodiment of the invention. The process steps are shown in a particular linear sequence in  FIG. 2  and other drawing figures herein for clarity of presentation. However, it will be evident that many of them can be performed in parallel, asynchronously, or in different orders. Those skilled in the art will also appreciate that a process could alternatively be represented as a number of interrelated states or events, e.g., in a state diagram. Moreover, not all illustrated process steps may be required to implement the method. 
         [0036]    At initial step  40  A catheter having an electrode is inserted into the patient&#39;s heart, and the position of the electrode is tracked by the processor. While the catheter is in the heart, at step  42  the processor injects respective known source signals that do not affect the patient&#39;s heart or interfere with the generated ECG potentials, into the electrodes  16  of the torso vest  14 . For example, an injected source signal may have an amplitude of 2 mV and a frequency of 300 Hz. The injection is repeated at multiple known electrode positions within the heart. Using filtering or multiplexing, multiple signals at different frequencies can be injected into more than one of the electrodes  16 . 
         [0037]    During the signal injection, at step  44  the system processor records the signals picked up by electrodes in the distal portion of the catheter  32 . At step  46  the raw signals at the catheter electrodes include are filtered to exclude potentials that are generated from the electrical activity of the heart using the known characteristics of the injected signal. Appropriate filters are provided in the above-referenced CARTO system. At the conclusion of the calibration phase, at step  48  the processor uses the recorded signals, together with the known positions of the electrodes injecting and receiving the signals to evaluate elements of M, {m ij }. Step  48  comprises the following steps. 
         [0038]    The elements m ij  of the elements of the matrix M vary with the cardiac and respiratory cycles, as the distances between the electrodes  16  and the catheter electrodes vary. While these could be ignored by providing average values for the elements m ij , it is preferable to provide reliable calibration information at all phases of the physiologic cycles at a desired resolution by collecting the signals over time. Step  50  provides one way of accomplishing this: the phase of the respiratory cycle is detected by known methods, e.g., measurement of intrathoracic pressures or pressure-volume relationships. In step  52  these relationships, together with information obtained from the EKG processor  28  in step  54  may be used by the control processor  22  to gate the signal generator  26 . An array of gated matrices M k  is generated, and then evaluated to determine their respective elements m ij . For each of the matrices, at the conclusion of the calibration phase, the control processor  22  uses the recorded signals from the catheter electrodes, together with the known positions of the electrodes  16  that injected the signals (and thus the distances to the catheter electrodes) to generate the elements m ij  of each the matrices M k  in step  56  and evaluate the matrices M k  in step  58 . 
         [0039]    Optionally, gated arrays of matrices Mk may be generated at different locations within the heart, either by employing catheters having multiple mapping electrodes, or by navigating the catheters to positions relative to known landmarks. 
         [0040]    Subsequently, in an operational phase of the procedure, (during which phase the catheter may or may not be present) in step  60  the processor monitors signals {right arrow over (S)} registered by the skin electrodes. Then in step  62  The processor then uses selected matrices M k  as the matrix M by rearranging equation (1) to calculate intra-cardiac potentials {right arrow over (E)} at different phases of the cardiorespiratory cycles: 
         [0000]        {right arrow over (E)}=M   −1   ·{right arrow over (S)}   (2).
 
         [0041]    Typically, as shown in the figure, in final step  64  the processor displays potentials {right arrow over (E)} on a map of the heart, although other types of display, such as potential-time graphs, may also be used. 
         [0042]    Equation (1) assumes a generally linear relationship between a given element of {right arrow over (S)} and the elements of {right arrow over (E)}, with matrix elements m ij  as coefficients of {right arrow over (E)}. There is a similar relationship for equation (2). For example, patient breathing and the heart beating change the distances involved, which change the values of the elements m ij  as noted above. 
         [0043]    Conventional invasive mapping procedures, using one or more catheters with multiple electrodes, can only provide electropotential mapping while the catheters are in the heart, and the mapping is only valid at the positions of the electrodes. In contrast, once the matrix M is obtained by reverse ECG mapping, such as is described herein, an accurate electroanatomic mapping is obtained. Indeed, the procedure can provide real-time patient-specific electropotential mapping over the whole heart surface, without the necessity of a catheter being in the heart. Additionally or alternatively, the mapping procedure described herein may be performed together with another invasive procedure, such as ablation. 
         [0044]    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.