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 . 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 . fig1 , 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 . fig1 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 . 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 . fig1 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 . 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 . fig2 a - c are schematics showing various cardiac monitoring and pacing sites according to some methods of the present disclosure . fig2 a - 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 ( fig1 ), 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 . fig3 is a plan view of a distal portion of lead 252 r , according to some embodiments of the present disclosure . fig3 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 ( fig1 ); 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 fig3 a - 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 fig3 a - 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 . with reference back to fig2 a - 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 fig2 c . it should be noted that , although fig2 a - 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 . with further reference to fig2 a - 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 ( fig2 a ) 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 ( fig2 b ) 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 ( fig1 ). 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 ( fig1 ), 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 . fig4 a 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 fig4 a , 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 fig4 b - c . 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 fig2 b 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 . 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 . 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 . fig4 b 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 fig4 b , in conjunction with fig2 a , 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 fig4 b 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 . 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 fig2 c , 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 fig2 c and from a plurality of receiver coils disposed along a length of lead 252 l positioned in the cardiac vein , as shown in fig2 c , 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 . fig4 c 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 fig4 c , 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 . with reference back to fig1 , 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 fig4 a - 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 . 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 .
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 .