Abstract:
A method for selecting a cardiac pacing site includes steps of: securing first and second electromagnetic receiver coils at first and second positions, respectively, along a heart wall; collecting a set of non-paced heart wall motion data from each of the coils secured at the corresponding positions; applying cardiac pacing stimulation at at least one first pacing site; collecting a first set of paced heart wall motion data from each of the secured coils; comparing the non-paced heart wall motion data to the first set of paced heart wall motion data; and determining, based on the comparing, whether to maintain pacing at the at least one first cardiac pacing site or to apply pacing stimulation at a second pacing site for collection of a second set of paced heart wall motion data. The at least one first pacing site may include a right ventricular site and a left ventricular site.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority and other benefits from U.S. Provisional Patent Application Ser. No. 60/977,098, which was filed on Oct. 3, 2007, and which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure pertains to cardiac pacing and more particularly to methods for selecting cardiac pacing sites. 
       BACKGROUND 
       [0003]    In recent years cardiac resynchronization therapy (CRT) for patients suffering from chronic heart failure has been shown to increase exercise capacity and a quality of life for these patients. CRT is typically administered via bi-ventricular pacing delivered via implanted medical electrodes, and the outcome of the therapy is often highly dependent upon selecting, and then successfully implanting the electrodes at appropriate pacing sites. In this context, as well as others, for example, physiological or dual chamber pacing, alternative pacing sites may be evaluated via measures of the electrical and/or mechanical response of the heart to the pacing. Many assert that pacing is most effective if mechanical synchrony between the right and left ventricle can be maintained or re-established, thus many physicians prefer to assess a mechanical, or hemodynamic, response of the heart to pacing at various implant sites before selecting one or more locations for chronic pacing. Tissue Doppler Imaging (TDI) is one of several methods currently employed to assess the mechanical response of a heart to pacing, but there is still a need for methods that can simplify intra-operative monitoring of the mechanical response of the heart to pacing at various sites, for example, to facilitate selection of effective bi-ventricular pacing sites. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0004]    The following drawings are illustrative of particular embodiments of the present disclosure and therefore do not limit the scope of the disclosure. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present disclosure will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. 
           [0005]      FIG. 1  is a diagram of an exemplary system for carrying out methods of the present disclosure. 
           [0006]      FIGS. 2A-C  are schematics showing various cardiac monitoring and pacing sites according to some methods of the present disclosure. 
           [0007]      FIG. 3  is a plan view of a distal portion of a lead employed by some methods of the present disclosure. 
           [0008]      FIGS. 4A-C  are exemplary analysis plots which may be generated with data collected by some methods of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION  
       [0009]    The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the following description provides practical illustrations for implementing exemplary embodiments of the present disclosure. Constructions, materials, dimensions, and manufacturing processes suitable for making embodiments of the present are known to those of skill in the field of the disclosure. 
         [0010]    In parallel with the development of CRT, techniques employing image-guided surgical navigation technology have been developed for the navigation of catheters, or leads, within the heart in order to assist in the placement of pacing electrodes. A particular image-guided navigation system, described in co-pending and commonly assigned U.S. patent application  2004 / 0097806  entitled NAVIGATION SYSTEM FOR CARDIAC THERAPIES, which is hereby incorporated by reference in its entirety, may be employed, by methods of the present disclosure, for the monitoring of cardiac wall motion in response to pacing at various sites.  FIG. 1 , which has been borrowed from the aforementioned patent application, is a diagram of the system  10 . It should be noted that the principles described herein may be applied in alternative contexts in which medical electrical leads are employed. 
         [0011]      FIG. 1  illustrates system  10  including a fluoroscopic C-arm imaging device  12 , an electromagnetic navigation or tracking device  44 , a gating device or electrocardiograph  62 , and a controller or work station  34 , which receives input from each of the aforementioned devices. Tracking device  44  includes a transmitter coil array  46 , which is controlled, or driven, by a coil array controller  48 . Coil array controller  48  may drive each coil, in transmitter coil array  46 , in a time division multiplex or a frequency division multiplex manner. In this regard, each coil may be driven separately, at a distinct time, or all of the coils may be driven simultaneously, wherein each is driven at a different frequency. Thus, coil array controller  48  drives coils in array  46  in order to generate electromagnetic fields, within a patient  14 , in the area where the medical procedure is being performed, which is sometimes referred to as the patient space. The electromagnetic fields, generated within the patient space, induce currents in at least one localization sensor  58 , for example, an electromagnetic receiver coil, which is coupled to a lead or catheter  52 , as is further discussed herein. These induced currents, or signals, are delivered from catheter  52  to a navigation probe interface  50 , which provides the necessary electrical isolation for navigation system  10 . Probe interface  50  further includes amplifiers, filters and buffers required to directly interface with sensor(s)  58  of catheter  52 . Catheter  52  may employ a wireless communications channel, as opposed to being directly coupled to probe interface  50 . 
         [0012]    Tracking device  44  functions to transfer the signals to coil array controller  48 , which then processes the signals in order to generate, and superimpose, an icon, which represents the location of the catheter, onto images generated by imaging device  12 , which are displayed on a display  36  of workstation  34 . Electrocardiograph  62  provides for a time-gated acquisition of the signals from coil  58  and/or the images from imaging device  12 , for example, by triggering acquisition off of a measured R-wave, or ventricular depolarization, which may be sensed by skin electrodes  64 , which are coupled to electrocardiograph  62 .  FIG. 1  further illustrates tracking device  44  including a dynamic reference frame  54 , which is fixed to patient  14  to track movement of patient  14  for registration correlation in order to maintain accurate information concerning the catheter location. Patient registration may be accomplished by selecting and storing particular points or landmarks  60  in memory, from pre-acquired images and then by touching the corresponding points on a patient&#39;s anatomy with a pointer probe  66 . A landmark is an anatomical feature that is generally common to all patients. A complete and detailed description of system  10  can be found in the aforementioned &#39;806 application, which has been incorporated by reference. 
         [0013]    According to embodiments of the present disclosure, a system, similar to system  10 , includes at least one pair of electromagnetic receiver coils utilized not only in a navigational capacity, as described in the &#39;806 application, but also in a monitoring capacity for the purpose of selecting one or more cardiac pacing sites intra-operatively, that is, at a time of pacing electrode implant.  FIGS. 2A-C  are schematics showing various cardiac monitoring and pacing sites according to some methods of the present disclosure.  FIGS. 2A-C  illustrate a first elongate lead  252 R extending into a right ventricle (RV) and a second elongate lead  252 L extending into a coronary vein over a surface of a left ventricle (LV); each of leads  252 R and  252 L include an electromagnetic receiver coil  258 R,  258 L, respectively, which has been positioned to monitor cardiac wall motion. Voltage signals from coils  258 L,  258 R, which are generated by a current induced therein by an external magnetic field, for example, created by coil array controller  48  driving coils in array  46  ( FIG. 1 ), facilitate creation of a virtual representation of leads  252 R,  252 L, respectively, in proximity to the RV and LV walls, and thereby provide RV and LV heart wall motion data. (The term ‘lead’ is employed in a generic sense to denote a body carrying at least one receiver coil and an associated lead wire; as such, either or both of leads  252 R and  252 L may further be adapted to carry out addition functions, for example, in facilitating delivery of a pacing electrode to a target site, and can, thus, in various embodiments, take the form of a guidewire or catheter.) It should be noted that the voltage signals from each of coils  258 R,  258 L may be used for image guided navigation of leads  252 R and  252 L, respectively, to the illustrated positions, for example, according to methods described in the aforementioned &#39;806 application. Furthermore, it should be noted, that each of leads  252 R,  252 L may include a plurality of receiver coils spaced apart from one another along a length thereof, in order to provide more enhanced wall motion data. 
         [0014]      FIG. 3  is a plan view of a distal portion of lead  252 R, according to some embodiments of the present disclosure.  FIG. 3  illustrates a fixation element  259  terminating a distal segment  303  of lead  252 R, coil  258 R extending proximally from segment  303 , and a body  302  of lead  252 R extending proximally from coil  258 R; element  259  serves to secure coil  258 R at a position along a heart wall. According to preferred embodiments of the present disclosure, segment  303  is relatively rigid, for example, being formed from a 75D durometer polyurethane, so that coil  258 R will move in sync with that portion of the heart wall to which element  259  is fixed, while body  302  is relatively supple, or flexible, for example, being formed predominately from silicone rubber, so as not to influence the response of coil  258 R to the wall motion. Those skilled in the art will appreciate that lead wires for coil  258 R extend proximally therefrom, within body  302  to couple, for example, with probe interface  50  ( FIG. 1 ); an exemplary assembly for coil  258 R (as well as for coil  258 L), which may be incorporated by embodiments of the present disclosure, is described in conjunction with  FIGS. 3A-C  of a commonly assigned and co-pending patent application entitled THERAPY DELIVERY SYSTEM INCLUDING A NAVIGATION ELEMENT and having the Ser. No. 11/322,393 (Atty. Docket no. P-20898.00), and the  FIGS. 3A-C , along with the associated description, of this application are hereby incorporated by reference. It should be noted that, in the context of the present disclosure, fixation of a receiver coil, for example, coil  258 L, to a heart wall can encompass fixation to a coronary vein. Furthermore, it should be noted that methods of the present disclosure may alternately be carried out by leadless, or wireless, electromagnetic receiver coils, an example of which is described in co-pending and commonly-assigned patent application Ser. No. 11/565,283 (Atty. Docket no. P-22326.00), which is hereby incorporated by reference in its entirety. 
         [0015]    With reference back to  FIGS. 2A-B , according to some methods of the present disclosure, coil  258 R is fixed, or secured, at a position along the RV septal wall by fixation element  259  of lead  252 R, and coil  258 L has been secured along the LV wall by lodging a distal tip of lead  252 L deep within the coronary vein. It should be noted that lead  252 L may also include a fixation element to secure coil  258 R at a position along the LV wall, so that the secured position is not dependent upon an anatomy of the coronary vasculature. An alternate position for the fixation of coil  258 R, which is in closer proximity to the RV apex, is shown in  FIG. 2C . It should be noted that, although  FIGS. 2A-C  illustrate transvenous approaches for positioning coils  258 R,  258 L, within the venous system, the disclosure is not so limited, and one or both of coils  258 R,  258 L may be fixed, or secured to an epicardial surface of the heart, for example, via a trans-thoracic or sub-xiphoid approach known to those skilled in the art. 
         [0016]    With further reference to  FIGS. 2A-C , non-paced heart wall motion data may be collected, or sampled, using conventional techniques, from coils  258 R,  258 L for comparison with sets of paced heart wall motion data that result from pacing at an RV site RV 1  ( FIG. 2A ) in combination with pacing at different LV sites LV 1 , LV 2 , LV 3 . Alternately, or additionally, sets of paced heart wall motion data that result from pacing at another RV site RV 2  ( FIG. 2B ) in combination with pacing at the LV sites LV 1 , LV 2 , LV 3  may be compared to the non-paced heart wall motion data. According to one method, heart wall motion data sets, for example, averaged over five heart beats, for the non-paced condition and each of the paced conditions that correspond to each pair of selected pacing sites, may be collected and stored for projection onto a pre-acquired image of the patient&#39;s heart, for example, a fluoroscopic image generated by imaging device  12  ( FIG. 1 ). Each of these wall motion data sets, which are presented by the motion of the virtual representation of receiver coil  258 R on the pre-acquired image, may then be viewed, for example, on display  36  of workstation  34  ( FIG. 1 ), when a user ‘clicks on’, or selects via an interface of workstation  34 , landmarks in the pre-acquired image that have been associated with each of the selected pacing sites. 
         [0017]      FIG. 4A  is an exemplary display including a three dimensional plot  420  of wall motion data, for example, averaged over six cycles, which is superimposed on an image of a patient&#39;s heart, and a two dimensional plot  430 , of distances mapped between coils  258 R,  258 L, at particular points in time for each of the six cycles. The plotted wall motion data is not actual data, but is representative of data that could be collected from coils  258 R,  258 L. Plot  420  shows a first condition represented by a pair of simultaneous motion loops L 1  and R 1  created, for example, from averaged wall motion data collected from coils  258 L and  258 R, respectively, either when the heart is not paced, or when the heart is paced at at least one of pacing sites LV 1 , LV 2 , or LV 3 . For comparison, plot  420  also shows a second condition, represented by a pair of simultaneous motion loops L 2  and R 2  created, for example, from averaged wall motion data collected from coils  258 L and  258 R, for pacing that has been adjusted, either being applied (vs. no pacing), or being applied at a different site, from that which resulted in loops L 1  and R 1 . Point S 1  on each of loops L 1  and R 1  corresponds to an approximate position of the respective heart wall portion at systole for the first condition, and point S 2  on each of loops L 2 , R 2  to an approximate position of the respective heart wall portion at systole for the second condition. With reference to points S 1 , S 2 , it may be appreciated that motion loops L 2 , R 2  show a greater contraction between the heart wall portions and a greater relative rotation therebetween, which is indicative of a twisting, or torsion, from apex to base, that will be described in greater detail below. Plot  430  presents the first and second conditions in a different manner wherein a distance between corresponding points of each of the motion loops that have been averaged to create loops L 1  and R 1 , are plotted over time for the six cycles for comparison with a distance between corresponding points of each of the motion loops that have been averaged to create loops L 2  and R 2 . The six cycles may be identified by the six peak magnitudes for each curve. Distances between points of loop L 1  and points of loop R 1  make up curve LR 1 , and distances between points of loop L 2  and points of loop R 2  make up curve LR 2 . With reference to plot  430  it may be appreciated that the repeatability of magnitudes of the distances making up curve LR 2  is greater than that for curve LR 1  over the six cycles, which may be an indication of better synchrony between left and right heart wall motion. Thus, with reference to the display of  FIG. 4A , one may determine that the pacing resulting in the second condition, represented by loops L 2 , R 2  and curve LR 2 , provides a better hemodynamic response than the lack of pacing or pacing at another site resulting in the first condition, represented by loops L 1 , R 1  and curve LR 1 . Other methods for comparing heart wall motion data will be discussed below, in conjunction with  FIGS. 4B-C . 
         [0018]    Pacing may be applied at the sites, either endocardial or epicardial, by pacing lead electrodes which have been delivered to the sites by a transvenous or a trans-thoracic or a sub-xiphoid approach, according to a variety of methods well known to those skilled in the art. According to some embodiments of the present disclosure, one or both of leads  252 R,  252 L further include an electrode for delivering the pacing stimulation; for example, in  FIG. 2B  fixation element  259  may double as a pacing electrode to deliver pacing stimulation at site RV 2 . According to methods of the present disclosure, wall motion data for any group of pacing sites may be iteratively collected for comparison with non-paced wall motion data, in order to select one or more preferred pacing sites. 
         [0019]    The pacing sites shown are in areas generally corresponding to effective bi-ventricular pacing sites, but, it should be noted that methods of the present disclosure are not limited to these particular pacing sites. In the context of bi-ventricular pacing for CRT, a difference between paced and non-paced heart wall motion is typically sought, since non-paced wall motion will be asynchronous and the objective is to achieve synchrony; however in a different context, for example, in selecting one or more pacing sites for bradycardia or tachyarrhythmia therapy, a similarity between paced and non-paced heart wall motion is sought, since the objective is to maintain the already synchronous heart wall motion. 
         [0020]    According to some methods, the wall motion data corresponding to various pacing sites from secured RV and LV coils, for example, coils  258 R and  258 L, respectively, is processed and plotted to provide a picture of RV and LV wall motion with respect to one another, in the time domain.  FIG. 4B  is an exemplary plot of a net motion of three-dimensional wall motion data. The plotted wall motion data is not actual data, but is representative of data that could be collected from coils  258 R,  258 L. With reference to  FIG. 4B , in conjunction with  FIG. 2A , a first curve  48 R is generated from non-paced wall motion data collected from coil  258 R, a second curve  48 L 0  is generated from non-paced wall motion data collected from coil  258 L, a third curve  48 L 1  is generated from paced wall motion data collected from coil  258 L, wherein pacing is applied at a first pair of sites, RV 1  and LV 1 , and a fourth curve  48 L 2  is generated from paced wall motion data collected from coil  258 L, wherein pacing is applied at a second pair of sites, RV 1  and LV 2 . The plot of  FIG. 4B  indicates that pacing at sites RV 1  and LV 2 , which results in the wall motion depicted by curve  48 L 2 , brings LV heart wall motion closer into phase, or synchrony with RV heart wall motion, which is represented by first curve  48 R. 
         [0021]    According to some other methods, preferred pacing sites may be selected according to maximum cardiac wall motion, either RV, LV or both. According to an exemplary method of this type, the wall motion data from secured coils  258 R,  258 L, positioned as shown in  FIG. 2C , is processed to generate a plot describing a differential rotation between an apex and a base of the heart. Alternately, wall motion data from a plurality of receiver coils disposed along a length of lead  252 R positioned in the RV as shown in  FIG. 2C  and from a plurality of receiver coils disposed along a length of lead  252 L positioned in the cardiac vein, as shown in  FIG. 2C , can provide more detailed information concerning the differential rotation. This differential rotation is indicative of the characteristic twisting or torsion, from apex to base, of cardiac contraction; the twisting is commonly described as a wringing-out motion that ‘squeezes’ the blood out from the RV and LV during systole. The effectiveness of the motion is often measured in terms of an ejection fraction, that is, a ratio of the blood that is ejected from the LV to that which is contained in the LV at the peak of filling, or diastole.  FIG. 4C  is a plot of relative rotation (ordinate) between apex and base, in terms of degrees, versus time (abscissa), in terms of percent of systole, which may be generated from a torsion analysis of the wall motion data for a paced and an un-paced condition. Dashed line  400  corresponds to a closing of the aortic valve at 100% systole. A first curve  445  of the plot is indicative of a relatively low ejection fraction, and may correspond to an un-paced condition, while a second curve  446  is indicative of a more normal ejection fraction, wherein the relative rotation between apex and base has been increased, for example, via pacing. One or more additional pacing sites may be tested, and the corresponding sets of wall motion data collected and plotted, per  FIG. 4C , to find out if an even greater relative rotation can be induced. According to another exemplary method, wall motion indicative of ejection fraction may be observed in terms of short and/or long axis contraction and expansion for the LV. 
         [0022]    With reference back to  FIG. 1 , pre-programmed algorithms of workstation  34  may process wall motion data collected from coils  258 R,  258 L to generate plots, for example, like those described above in conjunction with  FIGS. 4A-C . Such plots, for example, displayed on display  36  of workstation  34 , can help a physician to select one or more effective pacing sites by facilitating a methodical comparison between baseline non-paced mechanical function of the heart and the mechanical function thereof in response to pacing at various sites. 
         [0023]    In the foregoing detailed description, the disclosure has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the disclosure as set forth in the appended claims.