Page 129 - Cardiac Electrophysiology | A Modeling and Imaging Approach

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reconstruction, and the recorded epicardial potentials provide a “gold standard” for evaluation. It

represents accurately the geometrical relationship between the heart and torso surfaces in the
human anatomy (the proportion of size between the dog heart and the tank that was molded on
a 10 years old boy torso, are representative of an adult human). This relationship determines the
transfer matrix that is used in the ECGI computations. Recordings were conducted with 256 or

384 body-surface electrodes, 490 or 64 electrodes in a nylon sock placed over the heart, and 184
electrodes on rod tips pushed against the heart (Burnes, JACC 2001;38:2071 265 and Oster, Circulation
1997;96:1012 266 ). Advantages of this experimental setup include: 1. Consistency of the torso volume
conductor between measurements and computations (homogeneous in both, so the evaluation of

the ECGI computation method is not affected by a volume-conductor inconsistency); 2. Epicardial
electrodes cover the entire heart; 3. The heart can be manipulated during the experiment – for
example, necrosis was produced so that its effect on ECGI reconstruction could be evaluated by
comparing electrograms from the same locations before and after necrosis formation. In addition,

accessibility of the heart during the experiment provides tight control over electrodes contact,
placement, etc. Results from the torso-tank experiments are summarized below.

Sub-epicardial pacing from 4 left ventricular (LV) sites and 1 right atrial (RA) site near

the SA node (Oster, Circulation1997;96:1012) 266 . Single – site LV pacing: An anterior pacing site was
reconstructed 7 mm from the measured one; a posterolateral site within 4 mm. Two other sites,
between these two extreme ones, were reconstructed 0 mm and 10 mm from the measured
pacing sites. Average distance between reconstructed and measured sites was 5 mm (range 0 mm

to 10 mm). The reconstructed pacing sites were determined from the reconstructed epicardial
potential maps (not isochrones). In this method, the initiation site is determined at the center of
a quasi-elliptical negative potential minimum on the epicardium during early activation (this is
based on the properties of wave front propagation in the anisotropic myocardium). 2 simultan-

eous pacing sites 52 mm apart (anterior and posterolateral) were reconstructed 7 mm and 4 mm
from the actual locations. Two anterior simultaneous sites were reconstructed at their exact
positions. Two posterolateral sites only 17 mm apart were reconstructed with 5 mm and 4 mm
errors of position. Electrograms (EGMs): Correlation coefficients (CC) between reconstructed and

measured electrograms were greater than 0.9 for 72% of all epicardial electrodes (54% with CC
greater than 0.95). Some outliers (only 5% of electrodes) had poor CC, smaller than 0.5. Isochrones:
Regions of earliest and latest activation, regions of crowded and sparse isochrones were
reproduced correctly. Pacing site locations should be determined from the potential pattern,

not from isochrones (greater precision).

Intramural Pacing (Oster, Circulation 1998;97:1496) . Anterior pacing – errors in positions of
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reconstructed pacing sites relative to the corresponding sites in the measured maps, determined

from the potential pattern, were 4, 6, 2, and 2 mm (average 3.5 mm) for pacing depths increasing
from 0 to 9.6 mm, respectively. The reconstructed potential pattern was very similar to the
measured, capturing its counterclockwise (CCW) rotation in time as the wave front propagated
into deeper myocardium.