Three Distinct Directions of Intramural Activation Reveal
Nonuniform Side-to-Side Electrical Coupling of
Ventricular Myocytes
Bryan J. Caldwell, PhD; Mark L. Trew, PhD; Gregory B. Sands, PhD;
Darren A. Hooks, MBChB, PhD; Ian J. LeGrice, MBChB, PhD; Bruce H. Smaill, PhD
Background—The anisotropy of cardiac tissue is a key determinant of 3D electric propagation and the stability of
activation wave fronts in the heart. The electric properties of ventricular myocardium are widely assumed to be axially
anisotropic, with activation propagating most rapidly in the myofiber direction and at uniform velocity transverse to this.
We present new experimental evidence that contradicts this view.
Methods and Results—For the first time, high-density intramural electric mapping (325 electrodes at ⬇4⫻4⫻1-mm
spacing) from pig left ventricular tissue was used to reconstruct 3D paced activation surfaces projected directly onto 3D
tissue structure imaged throughout the same left ventricular volume. These data from 5 hearts demonstrate that
ventricular tissue is electrically orthotropic with 3 distinct propagation directions that coincide with local microstructural
axes defined by the laminar arrangement of ventricular myocytes. The maximum conduction velocity of 0.67⫾0.019
ms⫺1 was aligned with the myofiber axis. However, transverse to this, the maximum conduction velocity was
0.30⫾0.010 ms⫺1, parallel to the myocyte layers and 0.17⫾0.004 ms⫺1 normal to them. These orthotropic conduction
velocities give rise to preferential activation pathways across the left ventricular free wall that are not captured by
structurally detailed computer models, which incorporate axially anisotropic electric properties.
Conclusions—Our findings suggest that current views on uniform side-to-side electric coupling in the heart need to be
revised. In particular, nonuniform laminar myocardial architecture and associated electric orthotropy should be
included in future models of initiation and maintenance of ventricular arrhythmia. (Circ Arrhythmia Electrophysiol.
2009;2:433-440.)
Key Words: anisotropy 䡲 mapping 䡲 structure 䡲 computer modeling 䡲 intramural pacing
A
ccurate information about the electric properties of
cardiac tissue is central to understanding the biophysical
basis of normal and aberrant heart rhythm. Electric anisotropy
influences the spread of activation in the heart, plays a critical
role both in the initiation and maintenance of reentrant
arrhythmia, and is an important determinant of the effectiveness of cardioversion. Knowledge of the nature and extent of
electric anisotropy is required for computer models of heart
activation that provide a means of probing intramural electric
behavior that cannot be accessed from clinical and experimental measurements made on the surfaces of the heart.
Clinical Perspective on p 440
Normal ventricular myocardium is generally thought to
function as a syncytium in which side-to-side electric coupling between adjacent myocytes is uniform.1,2 The electric
properties of ventricular myocardium are assumed to be
axially anisotropic with respect to the local myofiber axis,1,2
with activation propagating most rapidly in the myofiber
direction and at uniform velocity in planes transverse to this.
This view is not consistent with the laminar model of
ventricular myocardium that has resulted from detailed morphometric investigations of 3D cardiac tissue architecture.3–5
Ventricular myocardium is described as having a laminar
organization in which myocytes are arranged in layers (myolaminae) approximately 4 cells thick. Adjacent layers branch
and interconnect but are separated by cleavage planes across
which there can be little direct cell-to-cell coupling. The
laminar model of myocardial architecture is supported by the
findings of numerous analyses of cardiac mechanical function6 – 8 and by the results of cardiac magnetic resonance
diffusion–tensor imaging studies.9,10 A structurally detailed
computer model predicted that the laminar architecture of
ventricular myocardium will give rise to orthotropic electric
Received October 19, 2008; accepted June 8, 2009.
From the Auckland Bioengineering Institute (B.J.C., M.L.T., G.B.S., D.A.H., I.J.L., B.H.S.) and the Department of Physiology (I.J.L., B.H.S.), Faculty
of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.
Correspondence to Bruce H. Smaill, PhD, Auckland Bioengineering Institute, University of Auckland, Private Bag 92019, Auckland 1001, New
Zealand. E-mail b.smaill@auckland.ac.nz
© 2009 American Heart Association, Inc.
Circ Arrhythmia Electrophysiol is available at http://circep.ahajournals.org
DOI: 10.1161/CIRCEP.108.830133
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Circ Arrhythmia Electrophysiol
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Figure 1. Intramural activation mapping in pig heart. A, Extracellular potentials were recorded from the LV anterior region indicated using a 5⫻5 array of plunge needles, each ⬇4 mm apart
at the epicardium and containing 13 electrodes at 1-mm transmural spacing. Fiducial markers outside corners of array are
also shown. B, Activation isosurfaces from midwall unipolar
stimulus reconstructed at 6-ms intervals after first activation.
Electrode locations and PAA are indicated. Activation isochrones (2 ms) are superimposed on tissue structure rendered in a
plane (C) parallel to the epicardial surface containing the stimulus (indicated) and (D) transverse to the PAA and also containing
the stimulus. Data are from a typical experiment (experiment 1)
stimulated 6.5 mm below epicardium. LAD indicates left anterior
descending.
properties with slow propagation normal to laminae due to
the tortuous conduction paths involved.11 Experimental data
that would enable this hypothesis to be tested objectively
have not previously been available. The few studies in which
attempts have been made to reconstruct 3D intramural electric activation12,13 have not had the spatial resolution nor
provided sufficient structural data to address this issue.
This work, for the first time, provides experimental evidence that the spread of electric activation in ventricular
myocardium is not axially anisotropic. We demonstrate
preferential transmural activation directions that coincide
with the orientation of muscle layers across the left ventricular (LV) wall. Conduction is significantly slower perpendicular to muscle layers, suggesting that structural discontinuities affect side-to-side electric coupling.
Methods
The authors had full access to and take full responsibility for the
integrity of the data. All authors have read and agree to the
manuscript as written. All surgical procedures were approved by
the Animal Ethics Committee of The University of Auckland and
conform to the Guide for the Care and Use of Laboratory Animals
(NIH publication No. 85-23).
Key features of the study design are represented in Figure 1. Five
pigs (weight, 45 to 55 kg) were anesthetized and maintained with
halothane (2% to 5%) in oxygen. The heart was exposed, and an
array of 25 plunge needles (0.5-mm OD with 13 electrodes at 1-mm
spacing; see Rogers14 for details of fabrication) was introduced into
the LV free wall. Unipolar extracellular potentials (ECPs), referred
to the pulmonary artery root, were acquired (sampling rate, 4 KHz;
12-bit resolution; bandwidth, 0.01 to 2000 Hz) at up to 325 sites
using a purpose-developed mapping system (UnEmap, Auckland
UniServices Ltd, New Zealand).
Recordings were made when ST-segment elevation was fully
resolved, 40 to 60 minutes after insertion of all needles. ECPs were
recorded during intramural unipolar pacing (typically 80 to 100 bpm
for 15 to 30 seconds) from each of the 13 electrodes along the central
needle and subsequently in sinus rhythm. This procedure was
repeated with bipolar stimulation using adjacent electrodes along the
same needle. Constant-current stimulation was used at 1.5⫻ capture
threshold.
On completion of the experiment, the heart was arrested, excised
with needles in place, and immersed in chilled (4°C) physiological
saline solution. Hearts were perfusion-fixed (3% formalin in phosphate buffer), and a transmural segment containing the recording
array was removed. T2-weighted MR images of the excised LV
segment were acquired at 0.313-mm voxel size (1.5-T Siemens
Avantos, MAGNETOM Avanto, Siemans AG Medical Solutions)
shortly after fixation. Plunge needles were carefully removed from
the heart before MR imaging. Previously measured electrode positions were mapped onto best-fit lines superimposed on the segmented needle tracks to determine 3D electrode locations in vivo.
Activation times were estimated from dV/dTmin for the intrinsic
ECP deflection. Where dV/dT exhibited a prolonged plateau or
showed fractionation (17% of all pacing data), the principal deflection was identified by convolving the ECP with adjacent signals from
the same plunge needle using the high pass filter weights (⫺0.5 1.0
to 0.5). Activation contours were reconstructed in 3D using Hardy
interpolation,15 with R2⫽0.05. Activation data were interpolated
onto a 0.5-mm 3D grid for analysis of activation contours in 2D
slices and evolution of the wave front in 3D. The direction of most
rapid propagation, defined as the principal axis of activation (PAA),
was identified as the longest line through the stimulus site enclosed
by the 12-ms isosurface. The spread of activation transverse to the
PAA was characterized as follows. In the transverse plane containing
the origin of activation, the distance from origin to points on a
specified isochronal contour was measured and magnitudes and
orientations of local maxima and minima were recorded. Local 3D
conduction velocities (CVs) were estimated as follows. Isochronal
activation surfaces at 1-ms intervals were interpolated at 0.2-mm
grid resolution and vectors normal to these surfaces were determined. The orientation and magnitude of local velocity were given
by the direction of the normal vector and the distance along it to the
next isochronal surface. Maximum and minimum CVs were extracted from volumes immediately adjacent to the PAA and the
transverse plane containing the origin of activation. Activation
isosurfaces were constructed and analyzed using MATLAB (MathWorks Inc, Cambridge, Mass).
Tissue architecture was reconstructed in 3D throughout the volume from which intramural potentials were acquired as follows
(Figure 2). The excised LV segment was embedded in wax and the
entire volume was imaged using techniques described in detail
elsewhere. 16 In brief, specimens were mounted with the
circumferential-radial surface uppermost. This surface was trimmed
with an ultramiller to expose the tissue, stained with Toluidine blue
(0.12% in 1% borax) to a depth of ⬇2 m, and imaged at 8.33-m
pixel size using a digital camera. The process of milling, surface
staining, and imaging was repeated throughout the volume. The full
volume was imaged at 50-m steps, whereas 16.7-m steps were
used for a 5-mm-wide central subvolume.
MR volume images of the excised LV segment were resliced
parallel to the epicardial surface (XY plane). Plunge needle tracks
and corner fiducial markers were segmented and the coordinates of
epicardial entry and endocardial exit points were determined. Tissue
shrinkage and distortion caused by wax embedding were corrected
using 3D cubic mappings, constrained so that needle entry and exit
points in tissue volumes and corresponding MR images were best
matched. RMS distance errors for the corrections varied from 0.62 to
1.75 mm, with a measurement error of 0.62 mm.
To determine myofiber orientation, corrected volume images were
resliced on XY planes parallel to the epicardial surface, subdivided
with a 24⫻24 pixel grid, and dominant angles in each subregion
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Figure 2. Reconstruction of 3D morphology in tissue volume containing recording array. The volume was imaged initially with MRI and
subsequently using extended volume microscopy. A, 3D reconstruction of extended tissue volume (8.33-m voxels). Processing artifact
is corrected by transforming the volume to best match key features with corresponding structures in equivalent MR volume. Sections
are resliced parallel to the epicardial tangent plane (XY) and transverse to the mean myofiber direction in that plane. B, XY slice with
myofiber orientations overlaid. Locations of needle tracks and corner fiducial markers are highlighted. C, Transverse section with muscle layer orientations overlaid. D, Model of 3D geometry and fiber field in the tissue volume used for computation. Epicardial and endocardial surfaces are rendered and local myofiber orientation are represented by ribbons.
were extracted (Figure 2B) using a Sobel gradient operator.17
Myolaminar orientations were measured similarly in resliced transmural planes (Figure 2C).
A computer activation model was constructed using surface
geometry, myofiber orientations, and electrode locations measured in
1 heart (Figure 2D). Electric properties were assumed to be axially
anisotropic, with conductivities of 0.4 and 0.1 mS/mm, respectively,
in the fiber direction and transverse to the fiber axis. A monodomain
activation model with a cubic ionic current was discretized using
finite elements at 0.16-mm resolution and solved on the domain
containing the recording array (25⫻31⫻18 mm), using a highperformance computer (IBM p595).
Data are expressed as mean⫾SEM. Comparison of grouped data
were made using multiway, repeated-measures analysis of variance.
Results
The ECPs acquired in this study were stable and noise-free
over the length of the experiment (typically ⬍120 minutes),
with no differences distinguishable between consecutive
trains of unipolar and bipolar stimulation. ECPs in sinus
rhythm were highly reproducible throughout each experiment
and exhibited a narrow activation complex. During pacing,
activation complexes were broadened and less smooth. STsegment elevation was negligible in all cases.
Typical activation isosurfaces generated by intramural
point stimulation are shown in Figure 1. The spread of
activation was initially ellipsoid with the PAA—the direction
of most rapid early propagation—typically lying in a plane
adjacent to the stimulus site and parallel to the epicardial
surface. Activation isochrones are superimposed on tissue
structure rendered in this plane (Figure 1C), and it is evident
that the PAA aligns with local myofiber direction. However,
the spread of activation transverse to the PAA (Figure 1D) is
also markedly nonuniform with most rapid propagation at
around ⫺20° to the transmural z-axis. This coincides with the
orientation of myolaminae in these transmural planes.
Figure 3 shows activation contours generated by stimulation at other intramural sites in the same heart. Panels B and
C are XY and transverse planes that contain the stimulus site
as outlined above. Again, the spread of activation is elliptical
in both planes. The orientation of the PAA varies through
⬇130° in the XY plane as the stimulus is moved progressively across the LV wall (Figure 3B). Transverse to the
PAA, activation consistently spreads most rapidly oblique to
the transmural z-axis. The correspondence between transverse
activation spread in the vicinity of the stimulus and local
laminar architecture is shown in Figure 3C. Although the 3D
spread of activation was relatively uniform, local inhomogeneity was common. Activation was nonsequential on occasion, and the point of first activation was displaced
1.1⫾1.3 mm from the stimulus site.
The correspondence between the 3D activation spread and
local laminar structure was quantified across all hearts and is
represented in Figure 4. The orientation of the PAA is
compared with myofiber direction at stimulus sites through
the LV wall, for a typical experiment. The correspondence
between PAA and myofiber direction is remarkable and this
was reproduced across all studies: average difference between pairs 8⫾7°. In Figure 4B, we present distributions of
myolaminar orientation adjacent to the stimulus site in the
plane transverse to the PAA. Despite local variability, myolamina orientation distributions were surprisingly reproducible both within and across experiments. In all cases, the
orientation of the dominant population of myolaminae was
tightly distributed around a mean angle of ⫺25° relative to
transmural z-axis. A more scattered secondary population
with a mean orientation ⬇15° was also observed. To quantify
the spread of activation in this plane, maximum and minimum
distances from the point of first activation to the 10-ms
activation contour are plotted in polar coordinates across all
experiments (Figure 4C). The transverse spread of activation
is greatest in the upper right and lower left quadrants of the
plane (3.8⫾1.3 mm and ⫺42⫾36°) and least in the quadrants
at right angles (2.1⫾0.5 mm and 51⫾39°). Features to be
noted here are the significant differences between maxima
and minima and the fact that they cluster in orthogonal
orientations. The directions in which activation spreads most
rapidly in the transverse plane coincide with the dominant
layer orientation.
Local CV was consistently greatest in the fiber direction. In
the plane transverse to the PAA, maximum and minimum
local velocities clustered as in Figure 4C. For a given
stimulus site, maximum velocity was relatively consistent
during the interval 5 to 12 ms after first activation. There
were too few points for accurate local velocity estimation
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location on mean conduction velocity in the 3 orthogonal
directions.
To test further the hypotheses that the electric properties of
ventricular myocardium are transversely isotropic and that
the intramural spread of activation is determined by principally by myofiber rotation, we implemented an experimentspecific activation model. This incorporates detailed measurements of 3D surface geometry and myofiber orientations
throughout the region containing the recording array, and
electric properties are assumed to be transversely isotropic
with respect to the fiber axis. Predicted and experimental 3D
spread of activation are compared directly in Figure 5 for
midwall and subendocardial unipolar stimulation. It is evident
that the model does not capture key features of corresponding
experimental data. The predicted spread of activation in the
transverse plane containing the stimulus is relatively circular
in the model but is elliptical in the experiment. Although the
direction of principal activation spread in transverse planes
on either side of the stimulus (A and C) is in opposite
directions in the model, this is not seen experimentally. The
difference between predicted and observed results is marked
for stimuli in subendocardial regions where variation of fiber
orientation is relatively small (see Figure 5).
Discussion
Figure 3. Three-dimensional spread of activation from intramural stimulus at a series of sites from subepicardium to subendocardium in experiment 1. A, Broken lines indicate PAA and red
arrows the viewing direction of transverse plane (shaded).
B, Planes containing the stimulus: transverse to PAA and parallel to the epicardial surface (1-ms isochrones). Red lines indicate
orientation of the transverse plane above. C, Isochrones (2 ms)
superimposed on 8⫻8 mm region of rendered laminar structure.
before this and maximum velocity in this plane increased
subsequently, presumably as a result of fiber rotation. Principal local CVs in the fiber direction and transverse to it were
averaged across this time interval and are presented across all
experiments in Figure 4D; mean values are 0.67⫾0.019,
0.30⫾0.010, and 0.17⫾0.004 ms⫺1. Each of these is significantly different (P⬍0.009, n⫽195), and they separate in the
ratio 4:1.8:1. There was no significant effect of intramural
In this study, high-density 3D electric mapping has been used
for the first time to reconstruct activation sequences and
project these directly onto tissue structure imaged at high
resolution throughout the same LV volumes. The following
important findings flow from this approach. Electric activation spreads in 3 distinct directions from an intramural LV
point stimulus, and these directions correspond with local
microstructural axes. Active electric properties are therefore
anisotropic transverse to the myofiber direction, and this
gives rise to preferential transmural activation pathways.
That electric activation propagates most rapidly in the fiber
direction is wholly expected and has been shown in numerous
studies.2,12,18 Our estimates of both axial and maximum
transverse CV are very similar to results reported for ventricular epicardium.2,12 However, anisotropic spread of activation
transverse to the fiber axis has not previously been demonstrated. This is because the few studies in which the spread of
electric activation has been reconstructed in 3D12,13,19 had
insufficient spatial resolution to detect such directional variation. We inserted 325 electrodes at 1-mm transmural spacing
into a segment approximately 16⫻16⫻12 mm of the anterior
free wall in the pig heart. This enabled us to map 3D electric
activation with sufficient spatial resolution to reconstruct
accurate isochronal contours transverse to the PAA in planes
containing the site of first excitation. Transverse activation
follows a consistent transmural pattern across all experiments, with maximum and minimum transverse local velocities separating in a ratio ⬇2:1. This is not consistent with the
widely held view that the electric properties of ventricular
myocardium are isotropic transverse to the fiber axis.
The strong association between the direction of preferential
transverse activation and the orientation of laminae in the
same transverse plane does not prove that there is a mecha-
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437
Figure 4. Comparison of myocardial
structure with spread of activation.
A, Myofiber and PAA orientations with
respect to circumferential y-axis as a
function of transmural depth in experiment 1. B, Distributions of transverse
myolaminar orientations (binned at 5°) for
each experiment. C, Maximum (circles)
and minimum (squares) transverse spread
10 ms after first activation across all
experiments. Polar plots for C and D
oriented with respect to the transmural
z-axis. D, Principal local conduction
velocities 5 to 12 ms after first activation.
Mean CV in fiber direction is 0.67⫾0.019
ms⫺1, whereas maximum and minimum
transverse CVs are 0.30⫾0.010 ms⫺1 and
0.17⫾0.004 ms⫺1, respectively.
nistic link between the two. However, it provides additional
support for the view that the laminar architecture of LV
myocardium gives rise to orthotropic electric properties. In
our initial article reporting laminar arrangement of myocytes
in ventricular myocardium,3 we argued that, in the absence of
electric coupling across the clefts that separate adjacent
layers, the spread of activation perpendicular to laminae
would be forced to a follow a relatively tortuous path and
therefore occur more slowly than the transverse spread of
activation within layers. Hooks et al11 tested this hypothesis
in a computer model that incorporated detailed information
about laminar structure through the rat LV free wall.5 The
model predicted a transverse velocity ratio of 1.68 in the
directions parallel with and normal to laminae, respectively.
Most recently, Hooks et al20 used a combination of highresolution intramural electric mapping, detailed structural
Figure 5. Predictions of structurally
detailed, experiment-specific activation
model, incorporating axially anisotropic
electric properties, compared with observed
data (experiment 1). The stimulus depth
(indicated) is 6.5 mm in the upper panel and
10.5 mm (in anterior papillary muscle) in the
lower panel. The predicted 3D spread of
electric activation is represented by isosurfaces at 6-ms intervals after first activation.
Planes transverse to the PAA (A, behind,
B, containing, and C, in front of the stimulus
site) are superimposed on the 3D maps
(viewing direction indicated by red arrows).
Predicted and observed activation contours
in these planes, and the differences
between them are presented.
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Circ Arrhythmia Electrophysiol
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measurement, and experiment-specific modeling to analyze
the passive electric response of LV myocardium to injection
of a current pulse. It was found that bulk conductivity
transverse to the fiber axis (affected by extracellular and
intracellular conductivities) divided in a ratio of approximately 2:1, with maximum bulk conductivity in the direction of myolaminae and minimum conductivity perpendicular to this.
Although most laminae are oriented between ⫺20 to ⫺30°
to the transmural z-axis, there are regions with interspersed
myolaminae approximately at right angles to this (see Figure
4B), as has been reported elsewhere.3,9,10,20 Although the
direction of the principal laminar orientation is surprisingly
consistent across all studies, the relative frequency of the
second orthogonal direction varies between and within hearts
and appears to be greatest in the midwall. This would be
expected to introduce a stochastic component into the spread
of activation.
An important further question is whether it is necessary to
incorporate orthotropic electric properties in computer activation models to capture key features of intramural propagation. The results of computational studies that have sought to
address this issue21,22 have been equivocal. Colli-Franzone et
al22 have argued that the effects of intramural fiber rotation
would mask the orthotropic nature of the medium, particularly for epicardial activation maps. They state that, from a
qualitative point of view, both axially anisotropic and orthotropic models are compatible with the experimental findings
“. . . and only a quantitative comparison with the experimental data could provide a means for validating one of the two
assumptions.” We have used experiment-specific computer
modeling to test the hypothesis that it is not necessary to use
locally orthotropic electric properties in activation models,
because the intramural spread of activation is determined
principally by myofiber rotation.
The 3D spread of electric activation in a typical heart has
been interpreted using a computer model that assumes transverse isotropy and incorporates detailed information on myofiber orientation and surface geometry in that heart (see
Figure 5). The transverse isotropic model predicts a relatively
uniform spread of activation in the plane transverse to the
fiber direction at the point of activation. In parallel planes in
front of and behind the stimulus, however, the predicted
spread is elliptical with the major axis of the ellipse in
opposite directions due to transmural fiber rotation. These
predictions are consistent with the results of Colli-Franzone
et al22 for an axially anisotropic medium but fail to reproduce
even the qualitative features of experimental data obtained
under exactly comparable conditions. The spread of activation in transverse planes in front of and behind the stimulus is
in the same direction, whereas differences between predicted
and observed results are most marked in subendocardial
regions where variation of fiber orientation is relatively small.
Whole-heart computer models of cardiac electric activation23–25 are playing an increasingly important role in aiding
understanding of the mechanisms that give rise to reentrant
arrhythmia and ventricular fibrillation (VF). Electric anisotropy and structural heterogeneity are clearly implicated in the
initiation of ventricular tachycardia, wave break, and the
induction of VF.26,27 Moreover, the spatio-temporal characteristics of reentrant wave front dynamics during VF26 –28 are
all expected to depend on the electric orthotropy of ventricular tissue. Whereas the laminar arrangement of myocytes
probably increases conduction safety29 transmurally by minimizing current-load mismatch, intrinsically slow electric
propagation perpendicular to myolaminae may contribute to
the initiation and maintenance of reentrant arrhythmia, particularly in ischemia. The present study strongly reinforces
the view that the nonuniform laminar architecture of myocardium and associated electric orthotropy should be included in
future computer models of cardiac arrhythmia.
Possible artifacts introduced by the techniques used in this
study warrant careful consideration. First, there is no evidence that introduction of the electrode array has affected the
results. During data collection, ST-segment elevation was
negligible and results were highly repeatable both within and
between experiments. Second, reliable interpretation of the
results presented here depends on accurate spatial registration
of electric and structural data. Although the 3D locations of
individual electrodes were identified with considerable precision using MR imaging, structural data were acquired from
wax-embedded specimens that undergo substantial shrinkage
and distortion during processing. This deformation was corrected using a mapping procedure that matches features in the
wax-embedded specimens with corresponding structures in
MR volumes. The effectiveness of this correction is independently corroborated by the near-perfect alignment of the
principal axis of propagation with reconstructed myofiber
orientation at the stimulus site in all experiments. Third,
interpolation of 3D isochrones from activation times at
electrodes that are nonuniformly distributed (1-mm spacing
along each needle, but around 4 mm spacing between
needles) can introduce bias, particularly in the initial phases
of activation. We have elected to use a conservative interpolation scheme15 that tends initially to default to isotropic
interpolation and have taken pains to identify a value of the
R2 fitting parameter that is robust across the experimental
data sets used (see Methods). Finally, it could be argued that
virtual electrodes generated by intramural stimulation30 may
play some role in the 3D activation patterns reported here. We
have used near-threshold stimulation in all cases to minimize
this possibility and can demonstrate no difference between
unipolar and bipolar stimulation. In addition, we are confident that our results are not biased by intramural activation of
the specialized conduction system, which has much greater
penetration across the LV wall in pigs than in dogs or human
beings.31 The transmural spread of activation was relatively
uniform, with little evidence of the rapid local spread expected with activation of the conduction system. The spread
of excitation was also consistent in magnitude and direction
(relative to tissue microstructure) for each of the 13 sites
across the LV wall at which intramural stimulation was
applied. Moreover, the mean value of the principal velocities
estimated transverse to the fiber direction in this study lies
within the range of transverse velocities measured on the
epicardial surface in the dog.2,12
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Caldwell et al
Limitations
For more detailed analysis of the electric properties of
ventricular myocardium, it would be desirable to characterize
intramural activation at even higher spatial resolution. Ideally, the data obtained from such a study would be interpreted
with a fully orthotropic computer model that incorporates
detailed information not only on myofiber orientation and
boundary geometry but also the laminar arrangement of
myocytes and the distribution of other structures such as
coronary blood vessels. This is beyond the scope of the
current study, but work is proceeding in each of these areas in
our laboratory.
Conclusions
This study provides experimental evidence that the spread of
electric activation in LV myocardium is not uniform transverse to the local myofiber axis. We have demonstrated
distinct directions of slow and faster propagation, transmurally. Transverse to the fiber direction, maximum CV coincides with the orientation of muscle layers, and minimum CV
is approximately normal to the myolaminae. The effects of
myocardial tissue architecture on the spread of electric
activation have also been probed by comparing the predictions of an experiment-specific computer model with measured electric activation. Despite incorporation of detailed
information on surface geometry and fiber orientation, models with axially anisotropic electric properties do not capture
key features of the observed 3D spread of electric activation
in the heart. This work therefore provides a consistent body of
evidence that conflicts with the widespread assumption that
electric coupling in ventricular myocardium is uniform transverse to the local myocyte axis and suggests that current
views on side-to-side electric coupling in the heart should be
revised.
Acknowledgments
We thank Dr Denis Loiselle and Professor Andrew Pullan for their
contributions to this research.
Sources of Funding
This work was funded by a programme grant from the Health
Research Council of New Zealand.
Disclosures
None.
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CLINICAL PERSPECTIVE
The anisotropy of cardiac tissue is a key determinant of 3D electric propagation, affecting the spread of activation from
an intramural focus, the spatio-temporal dynamics of ventricular arrhythmia, and the response of the heart to defibrillation.
The traditional view of anisotropy is that electric propagation is fastest in the direction of the myofiber and uniformly
slower in all directions transverse to the myofiber. In this study, high-resolution activation maps combined with tissue
microstructural analysis reveal that in fact, 3 distinct velocities of propagation can be defined at any point within ventricular
myocardium. Most rapid propagation is observed in the direction of myofibers, and intermediate and slowest propagation
is seen within and across microscopic laminar groupings of myocytes. This new view of electric anisotropy, when
incorporated into models of the electric behavior of the heart, will enhance understanding of all anisotropy-dependent
phenomena.
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Three Distinct Directions of Intramural Activation Reveal Nonuniform Side-to-Side
Electrical Coupling of Ventricular Myocytes
Bryan J. Caldwell, Mark L. Trew, Gregory B. Sands, Darren A. Hooks, Ian J. LeGrice and
Bruce H. Smaill
Circ Arrhythm Electrophysiol. 2009;2:433-440; originally published online June 18, 2009;
doi: 10.1161/CIRCEP.108.830133
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