Method for endocardial activation mapping using a multi-electrode catheter

A system and method is disclosed to improve the ability of physicians, researchers and others to digest large amounts of cardiac electrophysiologic data acquired during cardiac catheterization, improving their ability to visualize, interpret and act on its content. The technique addresses previous limitations imposed by the number of electrodes which can be simultaneously recorded and by difficulty in determining their locations in reference to intracardiac anatomy. It is based on the composition of multiple activation sequence mappings made in a single rhythm, effected by referencing the position of each of the catheter electrodes to a measurement grid which is stable with respect to the patient's heart. Using this approach, the number of endocardial sites which can be sampled in a stable rhythm is theoretically unlimited, resulting in realization of high resolution activation maps. Rather than imposing a geometry determined primarily by the measurement apparatus, sites of measurement are scattered over the endocardial surface in a semi-random manner, and the real geometry of that surface may be reconstructed by measurements made of electrode position.

RELATED APPLICATIONS 
The instant application is related to the copending provisional 
application, Ser. No. 60/006901, filed on Sep. 15, 1995, and abandoned as 
of Sep. 16, 1996 entitled "Rapid, High-Density Endocardial Activation 
Mapping by Composition of Multi-Electrode Catheter Recordings", and claims 
priority therefrom. 
1. Field of the Invention 
The current invention relates generally to methods for measuring electrical 
properties of anatomic organs, and more particularly to measuring and 
plotting the electrical properties of the endocardium using a 
multi-electrode catheter as probe and fluorography to identify position. 
2. Background of the Invention 
Catheter-based radio frequency (RF) ablative techniques have become the 
treatment of choice for curing many types of supraventricular tachycardia 
(SVT) in both children and adults. To perform a successful ablation, two 
issues must be addressed: the location of a vulnerable site must be 
identified by mapping the arrhythmia, and energy must be safely delivered 
to that site. 
The arrhythmias which have been most amenable to RF ablation have been 
those types of SVT in which the appropriate Site for energy delivery is 
known and anatomically constant (e.g., AV node reentrant tachycardia), and 
those in which the accessory fiber mediating the SVT is known to traverse 
one of the two atrioventricular grooves. For these rhythms, the 
potentially complicated process of mapping electrical activation patterns 
in the heart can be reduced to identifying either an anatomic location, or 
a single point known to be located on the AV groove. 
Unfortunately, a large group of patients, often with severe underlying 
cardiovascular disease, suffer from other, more intractable atrial or 
ventricular reentrant arrhythmias in which the vulnerable sites for 
ablation are not constrained in any simple manner by known cardiac 
anatomy. Although extensive experimental studies have demonstrated the 
reentrant pathways critical to such arrythmias, we are as yet unable to 
map these rhythms clinically in an easy, effective and reproducible 
manner. This is reflected in much the lower success rates for treatment of 
these rhythms using RF ablation. 
As mentioned, there exists considerable experience in the construction of 
high density activation sequence maps of the atria and the ventricle in 
experimental applications, dating back almost 100 years. This generally 
involves surgical exposure of the endocardial and/or epicardial surface of 
the chamber to be mapped, and placement of an electrode grid of known 
geometry to record electrical activation. Although the nature of the 
techniques used to construct these maps is inherently complex and 
invasive, it has been possible to apply them intraoperatively to assist in 
the successful surgical resection of foci of ventricular tachycardia. 
Structural constraints in catheter fabrication limit both the number of 
electrodes which can be simultaneously deployed in the heart and their 
geometry, and have precluded the development of similar useful techniques 
to assist RF ablation. Advances which have recently occurred in catheter 
design and signal processing technology have allowed a substantial 
increase in the number of electrical signals which can simultaneously be 
recorded in the catherization laboratory. Using the so-called "basket 
catheter" it is now feasible to record as many as 25 bipolar signals 
simultaneously from the atrium or ventricle. This number, however, is 
still far fewer than the optimal number for complete mapping of a cardiac 
chamber. 
It is thus desirable to discover a method for complete mapping of a 
cardiactic chamber using a limited number of electrodes. 
OBJECTS OF THE INVENTION 
Due to the foregoing and other disadvantages of prior methods of measuring 
electrical activity and performing activation mapping of the endocardium 
or other anatomical structure, an object of this invention is to provide a 
method that allows one to make a large number of measurements during a 
stable rhythm of limited duration. 
It is a further object of the invention to derive coordinates with respect 
to a defined coordinate system so as to describe the geometry of the 
endocardium or other anatomical structure. 
It is a yet further object of the invention to derive coordinates with 
respect to a defined coordinate system so as to graphically map the 
geometry of the endocardium or other anatomical structure in three 
dimensions. 
It is a still further object of the invention to animate on a graphical 
display electrical activation sequences associated with observed 
endocardial activity. 
Other general and more specific objects of the invention will in part be 
obvious and will in part appear from the drawings and description which 
follow. 
SUMMARY OF THE INVENTION 
The present invention provides systems and methods for obtaining coordinate 
data representing the surface geometry of an anatomical structure. 
Furthermore, the methods and systems described herein disclose a system 
and method of obtaining data on electrical activity of the anatomic 
structure during a stable period which is suitable to be displayed using 
three dimensional graphical techniques. 
This invention attains the foregoing and other objects with a novel method 
of which is summarized as follows. 
For each electrical systole to be mapped, a multi-electrode catheter 
recording is made in an arbitrarily defined position in the heart of an 
arbitrary number of electrical channels appropriately amplified and 
filtered, and AP and lateral fluorogram frames are obtained to establish 
catheter position and orientation. Temporal and spatial reference points 
are established. A temporal reference point consists of a recurrent 
electrical event, used to reference activation times, which is associated 
with the rhythm of interest and easily identified on the surface ECG, for 
example, the onset of the P-wave in sinus rhythm. Spatial reference 
points, used to reference AP and lateral fluoroscopic locations of the 
basket electrodes, consist of any intrathoracic radiopaque objects 
occurring in the fluorographic field. Examples of these reference points 
might be surgical clips, sternal wires, or pacemaker leads. 
For each recording, a four column numeric matrix is generated. The first 
three columns consist of spatial location in X, Y, Z ! Cartesian 
coordinates, taken from the AP and lateral fluoroscopic views of the 
catheter at the time the recording was made. The fourth column consists of 
activation times obtained during that recording, measured against the 
temporal reference point. These spatial and temporal measurements may be 
generated manually, from digital images using interactive, 
computer-assisted techniques, or completely automatically using image- and 
signal-processing software currently available. 
An arbitrarily large number of such mappings can be made as long as the 
rhythm of interest is sustained and the fixed spatial and temporal 
reference points remain discernible. Between each recording, the catheter 
is moved by advancing or retracting it from the body, and/or rotating it, 
in order to sample another set of endocardial locations. 
Because the spatial and temporal reference points are chosen to be as 
nearly identical as possible between individual recordings, the four 
column space-time array can be concatenated to form a much larger array 
which can be interpreted as a mapping of a single beat. However, due to 
cardiac movement, respiratory movement and inaccuracies in measurement, it 
is to be expected that a certain amount of error will be present in this 
data set, especially in the spatial measurement component. For this 
reason, it is desirable to employ algorithms to smooth the cloud of points 
generated in space, and/or time. 
Display of activation sequence mapping can be effected by animating the 
three-dimensional spatial framework of the mapped cardiac chamber using 
activation time to represent the wave of electrical activity traversing 
the endocardial surface of the heart. After determining the total duration 
of the activation sequence to be animated, and specifying the isochronal 
window to be used for time steps, corresponding to individual frames of 
the animated sequence, and other details affecting the visual display. 
Using commercial software, the animated sequence may be replayed at any 
specified rate, thus allowing examination of the rhythm's activation 
sequence, and correlation of the anatomic pattern with the known 
fluoroscopic anatomy of the patient. Additionally, commercial software can 
be used to create the true illusion of depth using stereoptic projections 
with an appropriately chosen parallax angle. The three-dimensional data 
can be easily transformed into two-dimensional polar coordinates for 
graphing of activation times on paper. This is useful in the preparation 
of more standard forms of isochronal activation maps.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
The method and system described herein can be utilized advantageously to 
map the geometry of generic anatomic structures that allow for the 
placement of electrodes on the structure to sample electrical activity. 
This method is described in particular with regard to performing 
endocardial activation mapping using a multi-electrode catheter. 
Catheterization is performed under general anesthesia with femoral venous 
access. After estimation of the right atrial dimension is made by 
angiography, an appropriate Mullins sheath is used to guide a 
multi-electrode bipolar catheter 10 onto the right atrium so as to cover 
the organ, as shown in FIG. 1. In the preferred embodiment, a 50-electrode 
(25 bipolar pairs) catheter, the Webster-Jenkins Basket Catheter from 
Cordis-Webster, of Baldwin Park, Calif. is employed. One of ordinary skill 
in the art will recognize that a multi-electrode catheter of any geometry 
can be used to practice the invention. Endocardial contact is improved by 
expanding the basket with a coaxial puller mechanism attached to the 
catheter tip. In FIG. 1 filled circles 20 indicate electrode pairs 
registering atrial electrograms and open circles 30 indicate electrode 
pairs with no discernible electron activity due to absence of close 
proximity to the endocardial wall. 
Multiple recordings of atrial activation times are obtained of a stable 
rhythm consisting of a plurality of periods, with the basket rotated, 
advanced and retracted in the atrium to sample as much of the endocardial 
surface as possible. FIG. 2 depicts the universe of sampled points of the 
endocardium after the completion of the sampling. Atrial activation times 
were determined for each electrode pair bearing a signal using electronic 
calipers after selection of a constant fiducial point. Examples of this 
temporal reference might be onset of P-wave, pacing artifact, or 
esophageal clectrogram. Total right atrial activation times are calculated 
as the duration in milliseconds from the earliest to the latest atrial 
electrograms recorded from all electrode pairs during a given atrial 
rhythm. Rotation and repositioning of the multi-electrode catheters allow 
acquisition of a high density mapping of endocardial points. 
For studies in which several discrete recordings of a stable rhythm of 
interest are available, approximation of the mapped endocardial surface in 
3 dimensions is achieved by identifying the spatial locations of recording 
electrode pairs with biplane fluorography, using a modified model of the 
technique previously described by Hauer et al., Hauer R N W, Heethaar R M, 
dezwart M T, van Dijk R N, van der Tweel I, Borst C, Robles de Medina E O: 
Endocardial catheter mapping: validation of a radiographic method for 
accurate localization of left ventricular sites. Circulation 1986;74:862 
which is incorporated by reference herein. Coordinates of individual 
electrode pairs and fixed intrathoracic reference points (e.g., surgical 
clips and wires) are obtained in orthogonal views and used to generate an 
X Y Z !-Cartesian coordinate vector for each measured point, using both 
manual and computer-assisted (Adobe Photoshop v3.1, Optimas v5.1a) 
techniques. More particularly, each recording is documented with 
fluorograms, utilizing conventional fluoroscopy. The fluorograms are taken 
in AP and lateral views to establish catheter electrode locations. These 
fluorograms are used to establish the two planes defining for example the 
XY, and YZ planes for spatial locationing. The third plane, XZ, is derived 
from the other two by taking the appropriate coordinates from the other 
two planes. The planes define the rectangular Cartesian coordinates of the 
electrode locations and thus describe the geometry of the anatomical 
structure. Two reference markers such as staples, clips or the like are 
positioned so as to define absolute spatial reference points for the 
anatomic structure, in this case the endocardium. Most conveniently, one 
of the markers is initially designated as the origin of the coordinate 
system, and the other marker is used to define the coordinate system 
relative to that origin. Using the two fluorograms and the markers 
establishing reference points, the geometry of the structure is mapped by 
measuring distances of the structure to be mapped from the two 
fluoroscopic views, relative to the established coordinate system. In the 
preferred embodiment the two fluorograms are orthogonal views of the 
organ, in order to reduce the mathematical complexity of the derivations. 
However, one of ordinary skill in the art will recognize that the planes 
are not required to be orthogonal, only that they be non coplanar with a 
known angle of rotation about their common axis. In cases where electrode 
location is not well-delineated, symmetry of catheter design is used to 
interpolate electrode pair location when possible. Alignment of spatial 
locations of pairs from multiple recordings was achieved by subtraction of 
the X Y Z !-coordinates of the reference markers. 
Scaling to accommodate differences in magnification between the orthogonal 
view was accomplished using the slope of the linear regression equation 
relating the points identified along the craniocaudal axis of each view. 
FIG. 4 depicts a flow chart of the procedure. 
Processing of raw fluorographic and electrophysiologic data for display is 
outlined in FIG. 3. Employing the assumption that spatial and temporal 
reference points are "constant" (based on selection criteria) between 
recordings, data from individual records was combined to increase mapping 
density. To reduce the error in spatial measurement due to 
cardiorespiratory motion and other causes, spatial locations are smoothed 
using an algorithm developed for this purpose, based on a spherical 
approximation of true endocardial geometry. The matrix of spatial X Y Z ! 
data representing the mapped endocardial points for a single beat is 
converted to a zero-mean matrix by subtracting the mean spatial position 
of all points: 
EQU X Y Z !.sub.0 =X Y Z !-x y z !mean 1) 
Data points are then converted to spherical coordinates .rho..theta..phi.! 
: 
EQU .rho.=(x.sup.2 +y.sup.2 +z.sup.2).sup.1/2 2) 
EQU 0.theta.=cos.sup.-1 x/(x.sup.2 +y.sup.2).sup.1/2 !,y.gtoreq..theta.03) 
EQU 0.theta.=2.rho.-cos.sup.-1 x/(x.sup.2 +y.sup.2).sup.1/2 !, y&lt;.theta.04) 
EQU .phi.=.rho./2-cos.sup.-1 (x.sup.2 +y.sup.2).sup.1/2 
/.rho.!,z.gtoreq..theta.0 5) 
EQU .phi.=.rho./2+cos.sup.-1 (x.sup.2 +y.sup.2).sup.1/2 /.rho.!,z&lt;.theta.0.6) 
Smoothing was then performed by averaging the values of .rho., representing 
the vector length of a point from the approximate center of the atrium, 
with the .rho.-values of its near neighbors determined using the angular 
coordinates .theta. and .phi.. The size of the "neighborhood" and thus the 
degree of smoothing is determined by the user as a fraction of the surface 
area (FSA) of the unit sphere, which is converted into an arc length using 
the formula: 
EQU arc length=cos.sup.-1 (1-2.times.FSA). 7) 
Thus, specifying an FSA of 0 results in no smoothing, while a FSA of 1.0 
results in all values of .rho. being averaged over the entire sphere. In 
the preferred embodiment, low FSA values of 0.03-0.05 are chosen 
empirically, to preserve as much anatomic detail as possible in the maps. 
In summary in order to reduce the artifactual effect of mechanical motion 
on the reconstructed image of the atrial endocardial surface, the 
following algorithm for spherical smoothing is implemented: 
1. A zero-mean spatial array is created by subtracting the mean X Y Z 
!-coordinate for the entire array from each point of the data array X Y Z 
!; 
2. The zero-mean array is converted from X Y Z !-coordinates to a 
spherical coordinate system (.rho..theta..phi.!-coordinates); 
3. A user-specified smoothing factor indicating the fractional surface area 
of a sphere over which smoothing should occur (0.04-0.08) is converted 
into a reference arc measured in steradians; 
4. For each point, .rho. is reset to its mean value for all rays 
intersecting the arc specified by the smoothing factor and centered on the 
ray defined by .theta. and .phi.; 
5. The smoothed .rho..theta..phi.!-coordinate array is reconverted to X Y 
Z !-coordinates to facilitate display. 
This algorithm smoothes the spatial data using a spherical template. A 
smoothing factor of 1.0 results in all points being mapped to a perfect 
sphere with a radius equal to the mean radius of all points, while a 
smoothing factor of 0 results in no smoothing. 
Having derived the set of points that define the surface geometry of the 
anatomical structure, in this case the endocardium, commercially available 
software may be advantageously utilized to draw a three dimensional view 
of the structure. MATLAB from MathSoft of Natick, Mass. is one such 
software package that will allow one to display a three dimensional image, 
defined by coordinates. Associated with each coordinate point is a time of 
electrical activation. Thus, again using commercially available software 
such as MATLAB maps of electrical activation in the structure can be 
derived and displayed. Producing maps of activation sequences, over time, 
allows one to display an animation of electrical activity in the 
anatomical structure. 
Activation sequence maps are animated by superimposing activation times on 
the three-dimensional spatial framework, to represent the wave of 
electrical activity traversing the endocardial surface of the heart. 
Displays are projected in 3-dimensions from a point of view which can be 
specified by the user. Isochronal windows for time increments 
corresponding to individual frames of the animated sequence are also 
user-specified. The presentation software is also used to create the 
illusion of depth by using appropriate stereoptic projections. This 
technique further enhances the ability of the viewer to appreciate the 
third dimensional component of the activation sequence. 
It will thus be seen that the invention efficiently attains the objects set 
forth above, among those made apparent from the preceding description. 
Since certain changes may be made in the above constructions without 
departing from the scope of the invention, it is intended that all matter 
contained in the above description or shown in the accompanying drawings 
be interpreted as illustrative and not in a limiting sense. 
It is also to be understood that the following claims are to cover all 
generic and specific features of the invention described herein, and all 
statements of the scope of the invention which, as a matter of language, 
might be said to fall therebetween.