Abstract:
Conduction volumetry is used to determine the hemo-dynamic performance of the heart under various pacing protocols to optimize cardiac output as a function of the pacing protocol.

Description:
CROSS REFERENCE TO RELATED CASES 
   The present application is a continuation in part of U.S. patent application Ser. Nos. 09/107,371 filed Jun. 30, 1998; Ser. No. 09/589,387 filed Jun. 7, 2000; Ser No. 09/589,322 filed Jun. 7, 2000, now abandoned and 09/588,930 now U.S. Pat. No. 6,603,996 filed Jun. 7, 2000 each of which is incorporated by reference in its entirety herein. Application Ser. No. 09/588,930 is a divisional of U.S. application Ser. No. 08/387,832, filed May 26, 1995, now U.S. Pat. No. 6,240,307, which is a National Phase of PCT/US93/09015, filed Sep. 23, 1993, which is a Continuation-in-Part of U.S. application Ser. No. 07/950,448, filed Sep. 23, 1992, now U.S. Pat. No. 5,297,549, and which is a Continuation-in-Part of U.S. application Ser. No. 07/949,690, filed Sep. 23, 1992, now U.S. Pat. No. 5,311,866. 

   FIELD OF THE INVENTION 
   The present invention relates generally to cardiac pacing therapy and more particularly to biventricular pacing for the treatment of congestive heart failure. 
   BACKGROUND OF THE INVENTION 
   Congestive heart failure (CHF) is a disease state characterized by an enlargement of the heart with a concomitant reduction in pumping efficiency. Treatment regimes for CHF have included drugs, specifically diuretics, as well surgical interventions to remodel the heart. More recently it has been shown that pacing both ventricular chambers of the heart is close temporal sequence can improve the cardiac performance for CHF patients. It is believed that conduction disturbances contribute to CHF and replicating a “normal” activation sequence will improve heart function reducing or relieving symptoms. 
   The primary variables in biventricular pacing are the A-V delay and the V—V right and left ventricular pacing delay. In general the pacer synchronizes with the atrium and paces both ventricular chambers in sequence (V—V) after an appropriate A-V delay. 
   SUMMARY OF THE INVENTION 
   The purpose of the applicant&#39;s invention is to provide the physician with a tool to allow him to optimize the biventricular pacing therapy. The applicant proposes pacing the heart at a variety of sites in the cardiac chambers using a conventional pacing lead to survey potential sites for permanent implantation of pacing leads. During the survey the physician would have access to electrophysiological (EP) data taken on a beat-by-beat basis along with a calculated index of hemodynamic performance. In general the physician will try to maximize the hemodynamic performance based on the index of performance and then confirm that the pacing stimulus is creating an appropriate pattern of conduction with reference to the observed EP data. 
   The method of the invention begins with pacing the heart. This is done for several sites selected in the ventricles. This process is carried out with a pacing catheter that can be easily moved between the sites. At each site or candidate location, electrophysiologic data is collected. This data may be displayed to the physician as an activation map to show the interaction of the heart tissue with the pacing stimulus. The most typical display of data will be false color activation maps showing the propagation of the depolarization wave front over the heart as a function of time. 
   At each candidate pacing site, conduction volumetry is carried out with an indwelling multiple electrode array catheter such as the commercially available “ENSITE catheter” to compute volumetric changes associated with the pacing stimuli. Typically, the best cardiac performance is correlated with the most homogenous activation of the basal region of the heart chamber. 
   This coherence of action can be seen from the single beat activation map created with the ENSITE system. A hemodynamically based indication of coherence can be computed and expressed as a figure of merit corresponding to the homogeneity in the volume change in the chamber as the heart contracts as well. It is proposed to define and use this hemodynamic index of performance alone or together with electrical conduction measures to allow pacing optimization. 
   The index is based in part of the “homogeneity” or coherence of the contraction which is believed to correlate with the “vigor” of the contraction. It is preferred to compute the index on a beat-by-beat basis and to display the index of performance along with the electrophysiology data taken during the same beat. Thus the displayed data sets are from the same pacing event. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Throughout the figures identical reference numerals refer to identical structure wherein: 
       FIG. 1  is a schematic diagram of the EP system; 
       FIG. 2  is a schematic diagram of a portion of the system; 
       FIG. 3  is an equivalent circuit of a measurement made by the system; 
       FIG. 4  is a diagram of an output display from the EP system; 
       FIG. 5  is a diagram of a measurement made by the system; 
       FIG. 6  is a display representing the “coherence” measure and the index of hemodynamic performance. 
   

   DETAILED DESCRIPTION 
   Theory 
   The disclosure is based on the collection of new data, and a new use of data presently collected within the ENSITE system as sold by Endocardial Solutions (ESI) of St Paul Minn. In this specification reference is made to patents held by ESI, each patent is incorporated by reference herein. The use of the trade marked term ENSITE is intended to refer to commercially available structures. 
   It has been widely known that one may pace the heart through an EP catheter or through a separate pacing catheter to explore the electrical behavior of the heart during a diagnostic or ablation procedure. More recently it has been determined that pacing in both the left and right ventricle or bi-ventricular pacing is a useful therapy for the treatment of congestive heart failure. By closely coordinating the contraction of both ventricular chambers, an improved cardiac output can be achieved which tends over time to reduce the overt symptoms of congestive heart failure. It is recently, but not widely, recognized that the timing intervals and pacing sites of biventricular pacing must be carefully selected to generate the benefits of biventricular pacing. 
   It is becoming well understood that the precise placement of ventricular pacing leads in the heart is critical to achieving success with biventricular pacing or other pacing therapies directed to patients with CHF. It is believed that if the lead system is located in tissue that is refractory, ischemic or scarred, the propagation of activation is delayed and the resulting contraction is disorganized and less effective than normal. 
   The coherence of electrical activation is a non standard but useful way of expressing the requirement that the electrical activation of the heart be propagated over the diseased tissue in a way to result in an effective contraction. From a hemodynamic viewpoint a coherent contraction arises from a homogenous volumetric contraction, in which all portion of the observable heart chamber contract progressively and in “unison”. 
   The coherence of electrical activation can be directly observed by the ENSITE system in the EP data while the homogeneity or hydralic coherence measure is a hemodynamic index computed beat to beat by a modified ENSITE system. 
   Users of the ENSITE system become skilled at interrupting the propagation of such waveforms and can readily determine the location of infarcted regions in the myocardium based upon their electrical behavior. It is generally wise to avoid attempting to pace these regions of the heart. 
   Implementation 
     FIG. 1  shows a commercially available ENSITE electrophysiology mapping system sold by Endocardial Solutions of St. Paul, Minn. Although the ENSITE system in its current commercial embodiments presents electrophysiologic data on a static geometry of the heart, it should be recognized that certain heart information (EP activation) is available on a single beat basis this attribute is important in understanding the use of the system in this application. 
   In this system a patient  10  is undergoing a diagnostic procedure through a minimally invasive procedure involving the introduction of an ENSITE catheter coupled to the breakout box  12 . A conventional electrophysiology catheter  16  is also introduced into the patient while a variety of surface electrodes  11  are used to monitor cardiac activity during the procedure. The breakout box  12  permits the ECG cables and EP system to be coupled to additional hardware, which is not shown in this figure. The patient interface unit  18  couples the ENSITE catheter to the workstation computer and its related peripherals.  20 . The workstation operates under the control of a software program, which provides a substantial amount of information to the attending physician. 
   In use the physician will see an activation map image similar to that shown in  FIG. 3  on the monitor  23 . The computed index  51  will also been shown to the physician as indicated by index value “0.93” seen on the monitor  23 . In general, the physician is able to visualize the intracardiac cavity  32  containing the ENSITE catheter  14  as seen in  FIG. 3  on a color monitor  23 . Color is used to reduce the clutter in the image. Expressed or displayed on this wire frame geometry image  50  are activation maps and other electrophysiology information derived from the ENSITE catheter in conjunction with the EP catheter. In this particular instance, the patient is also provided with one or more pacing catheters  24  which are coupled to a temporary pacer  26  through the breakout box  12 .The temporary pacer  26  allows the physician to make measurements while varying the A-V delay and the V—V delay time. Pacing rate may be varied to ensure capture. 
   Turning to  FIG. 2  the heart  30  is shown schematically with a right ventricle  32  containing the ENSITE catheter  14  and a conventional EP catheter  16  as well as the pacemaker lead  24 . In brief, software running on the workstation  20  in  FIG. 2  can create an electrophysiological map of the heart during a single heartbeat as follows. In operation current sourced from a pair of electrodes (electrode  40  and  42 ) and injected into the heart chamber  32 , chamber. A roving catheter, shown as EP catheter  16 , is located on the endocardial surface  31  toward the exterior of the heart this catheter may be moved widely and may be placed on the interior heart surface along the septum is shown by reference numeral  33 . The injected current is detected through the electrode  44  on the EP catheter  16 . This location is determined and as the catheter is moved about the chamber, complete chamber geometry can be built up by noting the sequential positions of the electrode  44 . Incorporated references describe this process in more detail but for purposes of this disclosure a convex hull modeling technique is used to build a statically displayed interior geometry of the heart chamber by selecting certain locations developed from the electrode motion. The convex hull model of the interior chamber of the heart can be smoothed and a representative wire grid displayed to the physician. Such a wire grid is shown in  FIG. 3  as element  50 . 
   The ENSITE catheter also carries an array of passive electrode sites typified by electrode site  46 . These electrodes are arrayed around the geometric access of the ENSITE balloon  47 . At any given instant some of these electrode sites are pointed toward the exterior surface wall  31  and the septal wall  33 . By computing the inverse solution, the electrophysiologic potentials passing along these surfaces can be measured within one beat. Reference may be had to U.S. Pat. Nos. 5,297,549; 5,311,866; 6,240,307 and 5,553,611 for further discussion of the inverse solution and the creation of the electrophysiologic map. Each of these references is incorporated in its entirety in the present application. 
   In the commercially available ENSITE system the depolarization wavefront is displayed on a representative geometric surface such as the grid surface  50  of  FIG. 3 . The workstation  20  animates this electrophysiology data and the propagation of the electrical way front along the interior surfaces of the heart can be monitored. Wavefronts  80   82  and  83  are sequence movements of the stimulus from pacing site  84  seen in  FIG. 3 . 
     FIG. 4  shows an equivalent circuit implementation to facilitate a description of conduction volumetry measurements made from an ENSITE catheter. Returning to the geometry of the array on the ENSITE catheter  14  the interior of the balloon  47  is non-conductive which provides a limited field of view for each of the electrode sites on the surface of the balloon. In essence each electrode responds only to electrical activity bounded by the heart wall, which is directly opposite the electrode site. For example, an electrode such as electrode  46  sees electrical activity and conductance data bounded by the wall  31  and is blind to electrical activity on wall  33 . In a similar fashion, an electrode such as electrode  50  sees only electrical activity occurring on wall surface  33 . By monitoring the voltages on the array electrodes during the pulse, or more particularly measuring the resistance between adjacent columnar pairs of electrodes as indicated by exemplary difference amplifier  86  it is possible to compute the volume of a partial slice  88  of the chamber volume best seen in  FIG. 5 . It is important to note that the volume measurement is segmented into several local volumes typified by volume  88 . 
     FIG. 5  shows a slice of chamber volume computed by measuring the difference in resistance between electrodes adjacent along the axis  21 . This view shows that the volume segments are non-overlapping and extend along the axis  21 . The conductance term R is the resistance measured at electrodes in the passive array. This value is directly available to the software in the program, and Rho is the conductance of the blood in ohms-centimeters. D represents the distance between adjacent electrode sites in the passive array along the axis  21 . This value is known from the geometry of the ENSITE catheter. The preferred conduction volumetry algorithms can be computed very fast and the volume changes throughout a single beat of the heart may be tracked. The measurement of chamber volume is most accurate at the mid volume level indicated in  FIG. 4  at reference  90 . It is preferred but not required to sum or stack the independent volume measurements to create “columnar values” centered on the axis  21 . This is achieved by adding volumes  92  through  96  to create a column volume  90  located near the septum. A similar process is repeated to create a column volume near the wall  31  as shown as a slice  88  in  FIG. 5  as well as elsewhere around the chamber. 
   It is believed that the most effective heartbeat will involve the simultaneous and progressive activation of all of the muscle tissue, which should result in a self similar reduction in the measured volume among all of the volume segments measured. 
     FIG. 6  is a display of four representative volume segments of the heart chamber displayed as a function of time. It is expected that eight volumes will be used most effectively. Segment  88  may correspond to the antero-lateral volume while the other traces represent other volumes such as the Septal; antero-septal; anterior; antero-lateral; lateral or other volumes defined around axis  21 . The preferred way to compare the self-similarity of the volume waveforms is to cross correlate them statistically. By cross correlation of the values of the segment volumes over time one can compute a number that represents the similarity relationship of the various waveforms to each other. That is if the all the volumes contract identically then they should share the same waveform morphology and be completely self similar. In this instance the index value is unity. Real measurements taken have shown that a CHF patient in normal sinus rhythm has an index value of about 0.8. and that by manipulating the A-V delay time, location of stimulus and V—V delay interval this index can by increased to about 0.9 this is a very significant improvement in the heart contraction. In  FIG. 6  a computed value of 0.93 is delayed showing improved contraction behavior based on the selected pacing parameters. It is important to note that it is not intended to make a display like  FIG. 6  available to the physician because it is difficult to “compute” self-similarity qualitatively. The figure is designed to show how the performance index is calculated. index 
   In operation the physician will have the index saved for each pacing location and set of pacing variables. The physician will look for an improved contraction that is reflected by a high index value and a “normal” activation sequence. 
   For example a relatively invariant collection of volumes on one side of the heart or the other is some indication that wall is not contracting vigorously and that a better pacing site should be selected. This coherence of contraction index can be displayed as a simple number of percent of a total (unity). It is expected that simple figures of merit will be displayed for the physician to allow him to optimize the location of the pacing lead. It is expected that a measure of hemodynamic performance based upon conduction volumetry will be given independently of a coherence of contraction index. 
   It must be recognized that such measures are largely arbitrary and they may be combined in a variety of ways to improve the relationship between the hemodynamic performance index and the clinical outcome for the patient based upon pacing site.