Download Using Vectorcardiography In Cardiac Resynchronization Therapy

Using Vectorcardiography In
Cardiac Resynchronization
Therapy
By L.Lindeboom
BMTE 10.19
Report Internship
Catharina Hospital Eindhoven
Eindhoven University of Technology
Supervisors
Dr. Ir. M. van ‘t Veer
Dr. B.M. van Gelder
Dr. Ir. M.C.M. Rutten
Prof. Dr. N.H.J. Pijls
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Abstract
(English)
The conductive system of the heart may be affected by a heart disease due to direct damage
of the Purkinje bundle branches or by a changed geometry in a dilated heart. As a result the
electrical activation impulse will no longer travel across the preferred pathway and a loss of
ventricular synchrony, prolonged ventricular depolarization and a corresponding drop in the
cardiac output is observed. During cardiac resynchronization therapy (CRT) a biventricular
pacemaker is implanted, which is used to resynchronise the contraction between different
parts of the myocardium. Optimize pacing lead placement and CRT device programming, is
important to maximize the benefit for the selected patients. The use of vectorcardiography
(VCG) for CRT optimization is investigated.
In current clinical practice a 12-lead electrocardiogram (ECG) is used to measure the electric
cardiac activity of a patient. Each cell in the heart can be represented as an electrical dipole
with differing direction during a heartbeat. A collection of all cellular dipoles will result in a
single dipole, the cardiac electrical vector. Spatial visualization of the intrinsically threedimensional phenomena, using VCG, might allow for an improved interpretation of the
electric cardiac activity as compared to the one dimensional projections of a scalar ECG.
The VCG loops of one healthy subject and two subjects with a left bundle branch block (LBBB)
and two subjects with a right bundle branch block (RBBB) are qualitatively described. It is
shown that different electrical activation patterns will indeed result in different VCG’s and
that the VCG loops give an intuitive insight into the conductive pathways. Differences in VCG
loops after right ventricular and left ventricular pacing and the influence of lead placement
are analyzed.
(Dutch - Samenvatting)
Door directe of indirecte schade aan het elektrische geleidingssysteem van het hart, bereikt
de elektrische prikkel verschillende delen van het hart niet gelijktijdig, waardoor de
samentrekking van het hart niet synchroon zal plaatsvinden. Door het implanteren van een
biventriculaire pacemaker wordt getracht het hartspierweefsel op verschillende plaatsen,
met verschillende tijdsintervallen, elektrisch te stimuleren om het samentrekken van het
hart te resynchroniseren. Het gebruik van vectorcardiografie (VCG) bij het zoeken naar de
optimale plaatsing en de optimale tijdsinstellingen van de pacemaker, wordt onderzocht.
Elke cel in het hart kan worden gezien als een kleine elektrische dipool, verschillend van
grootte en richting gedurende een hartcyclus. Door het optellen van alle dipolen ontstaat de
elektrische hartvector. Met gebruik van het standaard 12-afleidingen electrocardiogram
(ECG), is het mogelijk om de elektrische hartvector uit te rekenen en weer te geven in een
vectorcardiogram, waarbij de twaalf figuren van het ECG worden gereduceerd tot één figuur
voor het VCG.
De VCG’s van één gezond testpersoon, van twee testpersonen met een linkerbundeltakblok
en van twee testpersonen met een rechterbundeltakblok worden kwalitatief beschreven en
vergeleken. De invloed van rechter- en linkerventrikel pacing wordt tevens beschreven. De
VCG’s lijken een goed inzicht te geven in de geleiding van de elektrische prikkel over het hart.
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Contents
1. Introduction
1.1 Background
1.2 Cardiac Resynchronization Therapy
1.3 Electrocardiography
1.4 Description of Vectorcardiogram
1.4.1 Healthy subject
1.4.2 LBBB
1.4.3 RBBB
1.4.4 Pacing
1.5 Project goal
2. Materials and Methods
2.1 Patient population and data acquisition
2.2 Signal analysis
2.2.1 Filtering of the ECG signal
2.2.2 Calculation of the VCG
2.2.3 Definition of an average heartbeat
2.2.4 Differentiation of the depolarization wave
2.3 Qualitative measures of the VCG
2.4 Quantitative measures of the VCG
2.4.1 Mean electrical axis
2.4.2 VCG loop area
2.4.3 Duration of depolarization
3. Results
3.1 Measurements
3.2 Healthy subject, LBBB subjects and RBBB subjects
3.2.1 Healthy subject
3.2.2 Bundle branch blocks
3.2.3 Overall observations
3.3 RV and LV pacing
3.3.1 RV pacing
3.3.2 LV pacing
3.3.3 Overall observations
4. Discussion and Conclusions
4.1 Healthy VCG
4.2 VCG of LBBB intrinsic and with pacing
4.3 VCG of RBBB intrinsic and with pacing
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4.4 Pacing lead position
4.5 Normalization of the VCG
4.6 VCG in CRT
4.7 Quantification of the VCG
4.8 Repolarization
4.9 Summary of conclusions
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5. Future Directions
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6. References
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Appendix A – ECG’s
Appendix B – 3D Representations of VCG loops
Appendix C – 2D Representations of VCG loops
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1. Introduction
1.1 Background
Heart failure, defined as the inability of the heart to supply sufficient blood flow to the
organs in the body, is an important cause of hospitalization in patients older than 65 years.
Due to the impairment of the cardiac output, patients suffer from breathlessness and fatigue.
Initial causes for heart failure include hypertension, valvular heart disease, cardiomyopathy,
and ischemic heart disease, often accompanied by dilatation of the ventricles of the heart
and alterations in the electrical activation patterns [1].
The heart is endowed with a system for the generation and conduction of electrical impulses
to cause rhythmical and synchronic contractions of the heart muscle (see figure 1). Initially
the rhythmical electrical impulses are
generated in the sinus node (or S-A node)
and travel via the atria towards the
atrioventricular node (A-V node).
Special Purkinje fibers lead the impulses from
the A-V node through the A-V bundle into the
ventricles. These fibers divide into a left and a
right bundle branch. Each branch spreads
downward towards the apex of the heart,
progressively dividing into smaller branches.
The ends of the Purkinje fibers penetrate
about one third of the muscle mass. Because
the speed of transmission of the impulse in
these Purkinje fibers is about 5 times higher
than transmission through the heart muscle
Figure 1 The cardiac rhytmical excitation system
itself, the conduction system causes the
(Adapted from Textbook of Medical Physiology Eleventh
Edition, Guyton and Hall, Chapter 10)
electrical impulse to arrive at almost all
segments of the ventricles within a narrow
time span resulting in a synchronous contraction.
This is required for an effective pumping by the two ventricular chambers of the heart [2].
In a number of patients suffering from heart failure the conduction system might be affected
due to direct damage of the conductive system by myocardial ischemia or by the changed
geometry in the dilated heart. In these cases the bundle braches of the Purkinje system
become diseased or damaged and will stop to conduct the electrical impulses. Since the
electrical impulse can no longer travel across the preferred pathway, it will move through
the muscle fibers, which slows the electrical conduction and which changes the directional
propagation of the impulse. The impulse will arrive at different segments of the heart with
an increased time interval. A differentiation between a left of a right bundle branch block
(LBBB and RBBB respectively) is made. As a result of the BBB, there is a loss of ventricular
synchrony, ventricular depolarization is prolonged and a corresponding drop in the cardiac
output is observed.
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1.2 Cardiac Resynchronization Therapy
Cardiac resynchronization therapy (CRT) aims to reduce the symptoms of breathlessness and
fatigue in patients suffering from heart failure, with delayed and dyssynchronous left
ventricular contraction as a result, by
restoration of a more physiological sequence in
cardiac activation. Resynchronisation is
achieved by the implantation of a biventricular
pacemaker or biventricular ICD (implantable
cardioverter defibrillator) which enables
activation of the myocardium at different
locations and with different time delays (see
figure 2). The goal of CRT is to activate both
ventricles simultaneously to restore the
synchrony between the ventricles [1].
To predict the optimal lead placement
Figure 2 Graphical representation of the heart in a section
and the optimal time interval between
perpendicular to the septum of the heart. The red dots indicate the
the pacing electrodes currently
regions for possible lead positions.
echocardiography and measurements of
pressure build-up are used. Echocardiography is used to identify mechanical dyssynchrony
with the use of tissue Doppler imaging (TDI), which measures the systolic and diastolic
velocity in different segments of the myocardium. A notable regional variation in velocities is
a typical appearance in patients with dyssychronous ventricle contraction. This method
requires experience in echocardiography and is very time consuming and rather operant
dependent [3].
A more objective and reproducible method is to measure the amount of pressure increase
per unit of time (or dP/dt) in the left ventricle, which evaluates the pumping effectiveness of
the heart. An optimal lead placement and an optimal timing interval between the pacing
electrodes for the biventricular pacemaker are found with a maximal value in dP/dt [4].
Several randomized controlled trials and numerous observational studies have
demonstrated improvements in exercise capacity and quality of life after CRT procedures in
patients suffering from heart failure. Despite these advances approximately 25% of patients
who meet current criteria for implantation of a CRT device do not show objective evidence
of clinical benefit (the ‘non-responders’) [3]. Implantation of a CRT device is expensive, time
consuming and involves invasive medical surgery. It is important to optimize pacing lead
placement and device programming to maximize the benefit for the selected patients.
Alternative methods for optimization are investigated.
1.3 Electrocardiography
In current clinical practice a standard 12-lead electrocardiogram (ECG) is used to measure
the electric cardiac activity of a patient. The change in the electrical activation of the heart
caused by a LBBB and a RBBB are recognized as different waves in the different
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electrocardiographic leads. Besides these intrinsic conduction disorders, different activation
patterns as a result of different pacing lead positions in CRT
will also result in different ECG’s.
In a standard 12-lead ECG a projection of the electric
cardiac vector is drawn along standardized leads. The
connections for the standard ECG are shown in figure 3.
The electric cardiac vector is a superposition of the
numerous electrical dipoles that exist during the
depolarization of the cardiac muscle at each point in time.
This is further explained in paragraph 1.4.
The voltage which is recorded in a normal ECG depends on
the placement of the electrodes on the body surface of a
patient, the amount of excited cardiac muscle and the body
composition of the patient. Because of resistance of the
tissue between the heart and the skin, the measured signal
will decrease with an increasing distance from the heart muscle.
Figure 3 Arrangement of electrodes for a standard
Because the separate leads in the ECG give a projection
12-lead ECG
of the cardiac electrical vector along one single direction
(Adapted from Textbook of Medical Physiology
Eleventh Edition, Guyton and Hall, Chapter 11)
spatial information is difficult to interpret from 12
different images. Thereby, redundant information is
presented in a 12-lead ECG. Spatial visualization of the intrinsically three-dimensional
phenomenon in a single image might allow for an improved interpretation of the electric
cardiac activity as compared to the one-dimensional projections of a scalar ECG.
In 1956 Frank developed a method with which he was able to obtain
a 3D representation of the electrical activity of the heart, represented
by a right-left axis (X), a head-to-feet (cranial to caudal) axis (Y) and a
front-back (anterior to posterior) axis (Z) [5]. Frank used seven
electrodes to calculate the X, Y and Z potentials. The electrode
placement and the X, Y and Z directions are shown in figure 4. From
the scalar X, Y and Z coordinates it is possible to obtain the
instantaneous cardiac vector, whose path in space builds the
vectorcardiographic loop or a vectorcardiogram.
1.4 Description of the Vectorcardiogram
1.4.1 Healthy subject
Figure 5 shows the depolarization of the heart for a healthy
subject, together with the expected ECG and VCG. The figure
shows the result of the depolarization on the cardiac vector
in the frontal plane as indicated by the red axes in figure 5A.
Figure 4 Frank vectorcardiographic lead
system. Seven electrodes used are denoted
with A, C, E, I, M, H and F.
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Figure 5 Schematic drawing of different time steps of the cardiac electrical impulse pathway during
depolarization for a healthy subject in the frontal plane as indicated by the red arrows in panel A. The
expected VCG and ECG (shown under the heart respectively) are plotted for successive time steps in panel
B-F. While the impulse travels over the heart, the resultant vector changes in direction and magnitude.
Tracking the direction and magnitude of the resultant vector in time results in the VCG loop. The ECG only
gives the magnitude of the vector. The small loop indicated with “p” within the larger VCG loop represents
the electrical activity of the atria.
(Adapted from 'Presentation and Analysis of Vector Electrocardiograms', Anna Redz, March 1998)
As the impulse travels from the A-V node into the ventricles through the Purkinje system,
the impulse arrives in the left bundle branch slightly before the right bundle branch.
Depolarization starts from the left side of the septum. Initially the cardiac vector is pointing
from left to right, which is visible in a vector pointing to the right in the VCG (figure 5B). Then
the vector changes direction as the depolarization wave expands towards the apex and the
LV and RV (figure 5C). Because the left ventricle consists of more muscle mass, it will take
slightly longer to completely depolarize this ventricle. The vector in the VCG is a resultant
vector from multiple existing dipoles over the heart, with the larger potential differences
appearing in the left ventricle. Hence, the resultant vector in the VCG will therefore mainly
be directed to the left (figure 5D and 5E). Furthermore the loop will mainly be directed
caudal as can be seen by the vector pointing towards the apex (figure 5C and 5D). When the
heart is completely depolarized, no dipoles will exist and the loop is closed.
In the horizontal plane (defined by the x- and z-axis, see figure 4) the left ventricle is
positioned behind the right ventricle. Because larger potential differences appear in the left
ventricle the main direction of the loop in 3D will be posterior.
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1.4.2 LBBB
Figure 6 Schematic overview of depolarization with LBBB in the frontal plane, as indicated by
the red arrows in A. The right ventricle will depolarize before the left ventricle. The impulse will
travel via the apex of the heart towards the left ventricle resulting in a mainly cranial oriented
VCG (C and D)
Figure 6 gives a schematic overview of the depolarization for a subject with a LBBB in the
frontal plane, as indicated with the red arrows in figure 6A . The depolarization of the right
ventricle will be normal and the impulse will travel from the right ventricle to the left
ventricle via the apex. The vector starts pointing caudal (see figure 6B) changing in a vector
with the main direction being cranial ( figure 6C and 6D). The last part of the VCG loop will
be directed more to the left because the depolarization follows the left ventricular free wall
(figure 6D).
Looking in the horizontal plane, the VCG will mainly be pointing posterior towards the left
ventricle, because the right ventricle depolarization is ahead of the left ventricular
depolarization.
1.4.3 RBBB
Figure 7 Schematic overview of depolarization with RBBB in the frontal plane, as indicated by the red
arrows in A. Depolarization of the left ventricle will be ahead of depolarization of the left ventricle. The
impulse will travels via the apex of the heart towards the right ventricle resulting in a mainly cranial
direction (C en D).
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In figure 7 a schematic overview of the depolarization in the frontal plane for a subject with
RBBB is given. The red arrows in figure 7A indicate the frontal plane. The impulse will travel
from the left ventricle to the right ventricle via the apex. One would expect again a mainly
cranial direction of the loop (figure 7C and 7D). The vector will point to the right during the
beginning of depolarization (figure 7B) and will swing to the left as depolarization in the left
ventricle takes place. When the depolarization in the left ventricle is completed, the vector
will turn to the right side again (figure 7D)
The horizontal plane will indicate that the cardiac vector points anterior in this case, because
the left ventricle will be depolarized before of the right ventricle.
1.4.4 Pacing
Figure 8 Pacing in the horizontal plane (B and C) and in the frontal plane (E). The
green arrows in A indicate the horizontal plane. The red arrows in D indicate the
frontal plane. Lead placement will influence the direction of the cardiac electrical
vector as is shown with the red arrows in B,C and E.
A change in electrical activation will not only occur in case of bundle branch blocks, but the
use of a pacemaker will also influence the electrical impulse propagation over the heart. The
changes in the impulse conduction will be visible in the VCG as well.
Generally speaking it is possible to pace the left or the right ventricle (LV or RV). The main
difference in the VCG between LV and RV will be observed in the horizontal plane, as is
shown in figure 8B. When pacing the right ventricle, the vector will be directed posterior.
Pacing the left ventricle will lead to a vector directed anterior.
To indicate vector directions on the right-left and the cranial-caudal axes, the exact
placement of the lead will influence this direction. The position of the leads with respect to
the X-axis will influence the right-left direction. A lead positioned to the right, will generally
cause the vector to be directed to the left and vice versa (figure 8C).
When positioning the lead towards the apex of the heart this will lead to a vector in cranial
direction, while a pace position towards the basis of the heart will give a more caudal
directed vector (figure 8E).
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It is important to notice that RV pacing for a LBBB subject will most likely not change the
directions of the vector drastically. The same characteristics will be visible because the
pacing will cause the right ventricle to depolarize ahead of the left ventricle, as is the case
with a LBBB. The same holds for LV pacing of a RBBB subject.
1.5 Project goal
The goal of this project is to qualitatively describe vectorcardiograms for patients suffering
from heart failure accompanied with conduction disorders that require CRT. The pathologic
VCG’s will be compared to a normal VCG. Influence of lead placement will be investigated by
analysis of vectorcardiograms after right and left ventricular pacing.
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2. Materials and Methods
2.1 Patient population and data acquisition
Standard 12-lead ECGs were recorded for patients with a LBBB, a RBBB, and for healthy
individuals. The differentiation between patients with a LBBB and a RBBB was made
according to the standard criteria on the 12-lead ECG. A bundle branch block can be
diagnosed when the duration of the QRS complex on the ECG exceeds 120 milliseconds. A
right bundle branch block typically causes prolongation of the last part of the QRS complex,
and may shift the heart's electrical axis slightly to the right. Left bundle branch block widens
the entire QRS complex, and in most cases shifts the heart's electrical axis to the left [2].
All patients with a LBBB and patients with a RBBB required CRT. For these latter groups the
intrinsic rhythm was measured, before the pacemaker was activated. The pacemaker was
then used for RV and LV pacing and the ECG was recorded again. For the healthy individuals
only the intrinsic rhythm was recorded.
During the implantation of the CRT device the 12-lead ECG was recorded for a period of 5
seconds for each setting (intrinsic, RV pacing, and LV pacing). Data was exported and
analyzed offline. The data sampling rate was 977 points per second and the measured
voltages were in mV. All procedures were performed in the Catharina Hospital in Eindhoven.
2.2 Signal analysis
2.2.1 Filtering of the ECG signal
Pacemaker spikes and baseline wander in the ECG signal were removed by a Butterworth
filter. Frequencies above 30 Hz and frequency below 0.25 Hz were eliminated. The lowpass
filter had less than 3 dB of ripple in the passband, defined from 0 to 30 Hz, and at least 50 dB
of attenuation in the stopband, which was defined from 60 Hz to 489 Hz (the Nyquist
frequency). The response of the highpass filter was set to less than 3 dB of ripple in the
passband, defined from 0.95 Hz to the Nyquist frequency at 489 Hz, and at least 50 dB of
attenuation in the stopband from 0 Hz to 0.25 Hz.
2.2.2 Calculation of the VCG
As described in paragraph 1.3, Frank used a special electrocardiographic lead system,
consisting of seven electrodes, to derive the X, Y and Z leads [5]. To minimize clinical
interference the standard recorded 12-lead ECG was used to calculate the Frank X, Y and Z
leads, instead of changing to a Frank lead system.
To calculate the X, Y and Z leads from a standard 12-lead ECG the inverse Dower matrix was
used [6]. This method operates by calculating the VCG as a fixed linear combination of ECG
signals as depicted below.
ܺ = ሺ−0.172ܸଵ − 0.074ܸଶ + 0.122ܸଷ + 0.231ܸସ + 0.239ܸହ + 0.194ܸ଺ + 0.156‫ ܫܮ‬− 0.010‫ܫܫܮ‬ሻሺ1ሻ
ܻ = ሺ0.057ܸଵ − 0.019ܸଶ − 0.106ܸଷ − 0.022ܸସ + 0.041ܸହ + 0.048ܸ଺ − 0.227‫ ܫܮ‬+ 0.887‫( )ܫܫܮ‬2)
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ܼ = ሺ−0.229ܸଵ − 0.310ܸଶ − 0.246ܸଷ − 0.063ܸସ + 0.055ܸହ + 0.108ܸ଺ + 0.022‫ ܫܮ‬+ 0.102‫ܫܫܮ‬ሻ (3)
Herein V1 to V6 indicate the precordial leads and LI and LII the standard bipolar leads I and II.
One of the main problems with this method is that the measurement is very dependent of
the placement of the electrodes and is not adapted for patients with different body
compositions and geometries. Compared to other proposed methods for the synthesis of a
VCG this method gives the best results with a minimum of interference with clinical practice
[7]
.
2.2.3 Definition of an average heartbeat
The ECG signal was recorded for approximately 5 seconds, to find a periodic electrical
rhythm consisting of a number of 5 to 10 heartbeats without any premature ventricular
contractions. For every separate heartbeat the electrical activation will be similar. To
enhance the signal to noise ratio one average heartbeat was calculated, which contains the
same characteristics of the separate heartbeats. Plotting several heartbeats apart will not
give any additional characteristics.
The first step to differentiate between separate heartbeats was to calculate the magnitude
of the cardiac vector (Vc) at each instant using equation 4:
||ܸ௖ ሺ݅ሻ|| = ඥܺሺ݅ሻଶ + ܻሺ݅ሻଶ + ܼሺ݅ሻଶ
(4)
With X (i), Y (i) and Z (i) the calculated Frank leads at the ith sample point.
Local maxima were then detected which corresponded with the depolarization peak, or the
R-wave, during the heart cycle. The part of the signal between two peaks therefore
corresponds with one single heartbeat. To normalize the separate heartbeats, each
heartbeat was resampled to 1000 sample points. An average heartbeat was calculated by
taking the mean of all differentiated heartbeats in the signal.
2.2.4 Differentiation of the depolarization wave
Because CRT aims to resynchronize the contraction of both ventricles, the focus of the
description of the VCG lies in the depolarization loop, since depolarization precedes the
ventricular contraction. To be able to differentiate between depolarization and
repolarization waves in the averaged heartbeat, the beginning and the end of depolarization
wave need to be defined.
Figure 9 shows the steps to differentiate the depolarization wave. First a global
representation of the magnitude of the cardiac vector was obtained by smoothing of the
signal with a lowpass filter, which has less than 3 dB of ripple in the passband, defined from
0 to 8 Hz, and at least 50 dB of attenuation in the stopband, which is defined from 30 Hz to
the Nyquist frequency.
Starting from sample point 1 in the smoothed signal (represented by the red signal in figure
9A), the first minimum of the time derivative was found (indicated by w1 in figure 9A).
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Subsequently, the next maximum of the time derivative was found (indicated by w2 in figure
9A). These two points set the window for finding the minimum in the unfiltered signal (the
green signal in figure 9A and 9B), which represents the end of depolarization.
To find the beginning of depolarization, first the minimum between sample points 700 and
1000 of the filtered signal was used as a reference point (indicated by point w3). Then the
maximum value of the time derivative of the filtered signal following the minimum, is used
as a second reference point (indicated by point w4 in figure 9A). A window was created
again between the two reference points and the minimum value in the unfiltered signal was
set as the beginning of depolarization.
Figure 9B shows the same representation of the magnitude of the cardiac vector (the green
signal from signal 9A), with the marked points for the depolarization wave. Following the
global maximum in the filtered signal after the end of depolarization (point p1 in figure 9A),
the first point in the time derivative of the filtered signal with a positive value was found
(point p2 in figure 9B). The signal is shifted to set point p2 as the start of the heartbeat.
Figure 9. Example of detection of the depolarization wave. The unfiltered signal is plotted in green (panel A and
B), the filtered signal in red (panel A). In panel A, the point indicated with w1 represents the first minimum in the
time derivative of the red signal and w2 indicates the following maximum in the time derivative of this signal.
Point w3 is found to be the minimum of the filtered signal for the depolarization peak and point w4 indicated
the maximum in the time derivative of the filtered signal. The black dots indicate the beginning and end of the
depolarization wave. Points p1 and p2 (in panel A) are used to shift the signal as shown in panel B.
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2.3 Qualitative measures of the VCG
To qualitatively describe the VCG, the depolarization loop was plotted in a 3D representation
and in three 2D representations (the frontal plane, the sagittal plane and the horizontal
plane). As mentioned in paragraph 1.3, axes defined for the Frank leads are represented by a
right-left axis (X), a head-to-feet (cranial to caudal) axis (Y) and a front-back (anteriorposterior) axis (Z). In figure 10 the frontal plane (indicated by the blue arrows), the sagittal
plane (indicated by the red arrows) and the horizontal plane (indicated by the green arrows)
are shown, in correspondence with the planes defined in paragraph 1.4. With the use of the
directional terms defined in each plane, it is possible to qualitatively describe the movement
of the cardiac vector in time.
Figure 10 3D representation and definition of the 2D planes. The frontal plane is indicated by the red
arrows, the sagittal plane is indicated by the blue arrows and the horizontal plane is visualized with the
green arrows. Directional terms are defined in each plane to allow for a qualitative description of the
movement of the cardiac vector in time.
2.4 Quantitative measures of the VCG
2.4.1 Mean electrical axis
To help analyzing the VCG loops qualitatively,
some parameters are calculated to quantify
changes in direction and magnitude of the
cardiac electrical vector. The maximal
depolarization vectors in 3D and in the 2D
planes are found by calculating the
magnitude of the vectors at the top of the
depolarization wave (as defined in figure 9).
Using geometry formulas (the inverse
tangent) in the 2D planes, the angle of the
maximal vector was calculated. The
calculated angles were defined as in figure 11.
Figure 11 Angle definitions for the 2D planes
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To diagnose bundle branch blocks from the 12-lead ECG, not only the duration of the QRS
complex but normally also the mean electrical axis is calculated. It is defined as the
preponderant direction of the potential during depolarization. Changes in the conduction
pathway of the impulse are thus visible by looking at this mean electrical axis. For the 2D
projections the mean electrical axis is calculated and plotted in the figure. The axis can be
calculated by summation of the maximal value and the minimum value for the X, Y and Z
leads [2]. A mean electrical axis was calculated in the 2D planes according to the definitions in
figure 11.
2.4.2 VCG loop area
The area covered by the VCG loop in the different
quadrants in the 2D planes is calculated as well.
As shown in figure 12, a grid of pixels is set up on an
area with known dimensions. The number of pixels
inside the loop is counted and divided by the total
number of pixels on the grid. Because the dimensions
are known, the area covered by the loop could be
calculated now. The four quadrants in every plane are
defined as in figure 10. The quadrant with the
maximum area will indicate the preponderant
direction of the VCG loop.
2.4.3 Duration of depolarization
From a standard 12-lead ECG it is possible to find the
width of the depolarization wave. With the use of
begin- and endpoint of the depolarization wave (as
found in figure 9), the duration of the depolarization
loop was calculated.
Figure 12 Schematic drawing of an example
for area calculation in quadrant three. The
total number of pixels is 100 on a total area of
2
1mV . 23 pixels inside the loop are colored
2
black, indicating a loop area of 0.23 mV
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3. Results
3.1 Measurements
Based on available subjects and the availability of the heart catheterization room, the ECG’s
of one healthy subject and two subjects with a RBBB (RBBB1 and RBBB2 respectively) and
two subjects with a LBBB (LBBB1 and LBBB2 respectively) were recorded. All four BBB
subjects were paced from the right and the left ventricle, but the exact pacing location was
unknown. VCG loops of all measurements were calculated and analysed.
3.2 Healthy subject, LBBB subjects and RBBB subjects
3.2.1 Healthy subject
Figure 13 shows the 3D representation of the VCG loop for the healthy subject. To visualize a
timescale in the VCG loop, all depolarization loops are plotted in red and green, switching
colour every 10 milliseconds. The projection of the loop in the frontal, sagittal and horizontal
plane is also visible in grey, with arrows indicating the followed direction in all figures. An
increased loop length for the separate red and green parts corresponds with a higher speed
of conduction of the electrical impulse.
Figure 13 3D plot of VCG loop for the healthy subject. Depolarization loop is colored red and
green every 10 ms. Projections on the 2D planes are shown in grey. The axes are in mV. The
preponderant direction of the loop is caudial, posterior and to the left.
Figure 14 gives the projections of the same loop of the healthy subject in the frontal and the
horizontal plane. The quantitative measures are printed below the loops. In the frontal plane
the vector starts with a cranial and right deflection and moves towards a left and caudial
direction. The loop is closed with a vector pointing in the cranial and right direction again.
The maximal depolarization vector, the mean electrical axis and the maximal area in the
quadrant are all found in the caudal and left quadrant.
17
Figure 14 2D Representation of the VCG loop in the frontal and horizontal plane for a healthy
subject. Coloring of the depolarization loop is in correspondence with the 3D plot. The bold black
vector with the closed tail is the maximal depolarization vector and the thin speckled vector that
is plotted corresponds with the mean electrical axis. Axes are in mV. Figures and measures
indicate a mainly caudal, posterior and left direction in correspondence with figure 13.
In the horizontal plane the depolarization starts in the anterior and right direction. The
vector moves towards the posterior and left quadrant. Quantitative measures all indicate a
preponderant direction in this quadrant. The loop ends with a small posterior and right
vector.
3.2.2 Bundle branch blocks
In figure 15 the 3D representations of the VCG loop for the subject LBBB1 and for subject
RBBB1 are shown. As compared to the healthy subject the depolarization time is longer,
indicating a lower speed of conduction of the electrical impulse. For the RBBB1 subject, as
compared to the LBBB1 subject, the speed of conduction is higher, as indicated by the
shorter depolarization time and the increased length for the separate red and green loop
parts.
Figure 15 3D plots of the VCG loop for the LBBB1 and the RBBB1 subject. Projections
of the 2D planes are visible in grey, with arrows indicating the followed path. Axes are
in mV. The preponderant direction of the loop for the LBBB1 is posterior, while the
loop of the RBBB1 subject is mainly pointing anterior.
18
From both loops in figure 15, it is clear that for the LBBB1 the vector is pointing posterior,
while for the RBBB1 subject this is mainly in the anterior direction.
To describe the similarities and differences between the two loops, figure 16 shows the 2D
representations of the VCG loop of both subjects in the frontal and the horizontal plane.
Figure 16 Frontal and horizontal plane representation of the VCG loop of the LBBB1 and RBBB1 subjects. The
maximal vector and mean electrical axes are plotted in the figures as described with figure 14. Axes are in mV.
The main difference between the loops can be found in the horizontal plane, while in the frontal plane the
loops look very similar.
For both subjects the loop in the frontal plane is very similar. A clearly cranial direction of
the vector is seen, moving from the left to the right. For both subjects the preponderant
direction is to the right, as the calculated areas indicate. The main difference between both
loops can be found in the horizontal plane. The maximal vector, the mean electrical axes and
the calculated area indicate a posterior direction for the LBBB1 subject, while a anterior
direction is seen for the RBBB1 subject.
19
3.2.3 Overall observations
In the frontal plane the caudal direction for a healthy subject is opposite to the observed
cranial direction for the BBB subjects. Another difference can be found in the mainly right
direction for the BBB subject, as compared to the left direction for the healthy subject. All is
summarized in table 1.
INTRINSIC RHYTHM
Depolarization time in ms
Angle max. vector in
degrees
(frontal/sagittal/horizontal)
Angle mean electrical axis
in degrees
(frontal/sagittal/horizontal)
Quadrant with max. area
(frontal)
Quadrant with max. area
(sagittal)
Quadrant with max. area
(horizontal)
Healthy
112
50/43/
-54
LBBB1
196
-88/-55/
-103
LBBB2
143
-121/-9/
-93
RBBB1
141
-83/-141/
105
RBBB2
208
-114/-107/
137
59/47/
-58
-100/-39/
-98
-126/-18/
-103
-100/-133/
101
-102/-112/
117
CaudalLeft
PosteriorCaudal
PosteriorLeft
CranialRight
PosteriorCranial
PosteriorRight
CranialRight
PosteriorCranial
PosteriorRight
CranialRight
AnteriorCranial
AnteriorRight
CranialRight
AnteriorCranial
AnteriorRight
Table 1 Summary of quantitative measurements for the healthy and the BBB subjects. The difference
between a healthy subject and the BBB subject is found in the caudal and left direction. The main difference
between the LBBB and RBBB subjects is found in the posterior and anterior directions.
3.3 RV and LV pacing
3.3.1 RV pacing
In figure 17 the 2D representations of the VCG loop after RV pacing for the LBBB1 and RBBB1
subject are shown.
Figure 17 2D Representation of the VCG loop of the LBBB1 and RBBB1 subject in the sagittal plane after RV
pacing. The maximal vector and mean electrical axes are plotted in the figures as described with figure 14.
Axes are in mV. Main difference between the loops can be found in the cranial direction for LBBB1 and the
caudal direction for RBBB1. Both loops show a posterior direction.
20
A mainly posterior direction, which is found for all four BBB subjects, is observed. A
difference is seen in the cranial direction for LBBB1 and a caudal direction for RBBB1. All
quantitative measures are summarized in table 2 for the RV pacing. The table shows a mainly
right and posterior direction for all subjects. As was visible in figure 17, a difference is found
in the cranial direction for the LBBB subjects and a caudal direction for the RBBB subjects.
RV PACING
Depolarization time in ms
Angle max. vector in
degrees
(frontal/sagittal/horizontal)
Angle mean electrical axis
in degrees
(frontal/sagittal/horizontal)
Quadrant with max. area
(frontal)
Quadrant with max. area
(sagittal)
Quadrant with max. area
(horizontal)
LBBB1
203
-110/-71/
-138
LBBB2
170
-111/-55/
-119
RBBB1
147
127/18/
-103
RBBB2
192
136/17/
-109
-115/-75/
-150
-111/-59/
-123
136/11/
-102
139/14/
-106
CranialRight
PosteriorCranial
PosteriorRight
CranialRight
PosteriorCranial
PosteriorRight
CaudalRight
PosteriorCaudal
PosteriorRight
CaudalRight
PosteriorCaudal
PosteriorRight
Table 2 Summary of quantitative measures after RV pacing for the four BBB subjects. Similarities are found in
the posterior direction for all subjects. Main difference is found between LBBB subject and RBBB subjects in
the cranial and caudal directions.
3.3.2 LV pacing
For the LV pacing the 2D representations of the VCG loop of LBBB1 and RBBB1 in the frontal
plane are shown in figure 18. A mainly left direction for the LBBB1 subject is observed, while
for the RBBB1 subject this main direction is to the right.
Figure 18 2D Representations of the VCG loops in the frontal plane for subjects LBBB1 and RBBB1 after LV
pacing. The maximal vector and mean electrical axes are plotted in the figures as described with figure
14. Axes are in mV. The plots in the frontal plane indicate a difference in the right and left direction. For
LBBB1 the loop is mainly in the cranial/left quadrant, while for the RBBB1 subject the loop is mainly in
the cranial/right quadrant.
21
For both 2D projections of the VCG loop from figure 18, a cranial direction is observed and
calculated from the quantitative measures. All quantitative measures are summarized in
table 3 for the LV pacing. The table gives a preponderant anterior and cranial direction. A
difference is found between the left direction for both LBBB subject and a right direction for
both RBBB subjects.
LV PACING
Depolarization time in ms
Angle max. vector in
degrees
(frontal/sagittal/horizontal)
Angle mean electrical axis
in degrees
(frontal/sagittal/horizontal)
Quadrant with max. area
(frontal)
Quadrant with max. area
(sagittal)
Quadrant with max. area
(horizontal)
LBBB1
159
-41/-159/
63
LBBB2
178
-7/-175/
63
RBBB1
214
-131/-160/
79
RBBB2
213
-1/-171/
80
-39/-165/
71
-46/-165/
75
-102/-157/
96
-32/-172/
78
CranialLeft
AnteriorCranial
AnteriorLeft
CranialLeft
AnteriorCranial
AnteriorLeft
CranialRight
AnteriorCranial
AnteriorRight
CranialRight
AnteriorCranial
AnteriorRight
Table 3 Summary of quantitative measures after LV pacing for the four BBB subjects. Similarities are found
in the anterior direction for all subjects. Main difference is found between LBBB subject and RBBB subjects in
the left and right directions.
3.3.3 Overall observations
RV pacing results in a cardiac vector directed posterior for all BBB subjects, while for LV
pacing an anterior vector is observed for all BBB subjects.
When the LBBB subjects are paced from the right ventricle, the direction of the VCG loop
does not change with respect to the intrinsic rhythm. RV pacing does influence the VCG loop
direction for the RBBB subjects, as can be observed in table 1 and table 2. The VCG loop is
directed posterior and caudal after pacing, while the loop was intrinisically directed anterior
and cranial.
For the RBBB subjects pacing from the left ventricle does not influence the main direction of
th VCG loop. For the LBBB subjects, when comparing table 1 and table 3, the VCG loop is
directed anterior and left after LV pacing, with an intrinsic posterior and right directed loop.
22
4. Discussion and Conclusions
4.1 Healthy VCG
In correspondence with the theory described in paragraph 1.4, the results show a VCG loop
for the healthy subject which is directed mainly to the left, caudal and posterior. However, at
the end of depolarization, a cranial and right pointing vector is observed, which is somewhat
unexpected. This deviation might lie within the normal limits and could be explained by a
slightly different orientation of the heart in the thorax. A rotation to the front around the
right-left axis could explain the unexpected projection in the frontal plane in this case.
Increasing the number of normal VCG’s will give more insight in this issue.
4.2 VCG of LBBB intrinsic and with pacing
The VCG loops of the subjects with an intrinsic LBBB rhythm are directed mainly right,
cranial and posterior, as was expected. RV pacing does not influence the direction of the
VCG loop, because it resembles the electrical impulse pathway for the intrinsic LBBB rhythm.
In both cases the right ventricle depolarizes ahead of the left ventricle. LV pacing influences
the direction of the VCG loop by changing the vector to a mainly anterior and left direction.
4.3 VCG of RBBB intrinsic and with pacing
For the intrinsic rhythm of the RBBB subjects an expected, mainly right, cranial and anterior
directed VCG loop, is observed. In this case LV pacing resembles the impulse pathway for the
intrinsic RBBB rhythm, while RV pacing changes the direction of the VCG loop. After RV
pacing the vector is directed mainly caudal and posterior.
4.4 Pacing lead position
As mentioned in paragraph 3.1, the
exact lead positions, for both RV and
LV pacing, were unknown. It seems
reasonable to conclude that this exact
lead position determines the main
direction of the VCG loop, as
differences were found between RV
and LV pacing measurements mutually.
In figure 19 two different lead
positions are schematically shown for
LV pacing (in red and green).
The direction to the left for the RBBB
subjects is explained by a position of
the pacemaker towards the septum
(shown in red in figure 19). The heart
muscle mass at the left side of the
Figure 19 Schematic view on the lead positions for LV pacing in a
section perpendicular to the septum of the heart. The red dot
indicated the pacing position for the RBBB subjects, with the red
line indicating an excess of muscle mass at the left side of the lead,
resulting in a left oriented vector. The green dot indicates the
pacing position for the LBBB subjects, with the green line
indicating an excess of muscle mass at the right side of the lead,
resulting in a right oriented vector.
23
pacemakers exceeds the muscle mass at the right side of the pacemaker. Therefore the
vector will be directed to the left first. For the LBBB subjects the lead was positioned more to
the left (shown in green in figure 19), resulting in a mainly right directed vector.
This principle holds for all planes or directions, but also for RV lead placement. This means
that it is possible to extract the exact lead position from the VCG. It is necessary to monitor
the exact lead positions in future, to get more data on lead placement and differences in
VCG loop directions.
4.5 Normalization of the VCG
In chapter 2 it was mentioned that the method chosen to calculate the X, Y and Z leads, with
the use of the inverse Dower matrix, is very dependent of the placement of the electrodes
and is not adapted for subjects with different body compositions and geometries. In
literature this method is described as the preferred method for VCG synthesis. For now, this
method is appropriate to focus on patterns visible in the VCG and the minimal interference
with clinical practice is a great benefit.
To compensate for different geometries, a change of basis of the coordinate system can help
to normalize the VCG loops. MRI images could provide information about the actual
anatomy and orientation of the heart. When the anatomy and orientation of the heart are
known, a coordinate system with one axis along with the heart axis (from basis to apex) can
be used for all subjects. To change the basis of the coordinate system a 3D rotation matrix
(R), using Euler angles, can be used:
‫ݏ݋ܥ‬ሺߙሻ‫ݏ݋ܥ‬ሺߛሻ − ‫ݏ݋ܥ‬ሺߚሻܵ݅݊ሺߙሻܵ݅݊ሺߛሻ
‫ݏ݋ܥ‬ሺߛሻܵ݅݊ሺߙሻ + ‫ݏ݋ܥ‬ሺߙሻ‫ݏ݋ܥ‬ሺߚሻܵ݅݊ሺߛሻ
ܵ݅݊ሺߚ_ܵ݅݊ሺߛሻ
R= ቌ‫ݏ݋ܥ‬ሺߚሻ‫ݏ݋ܥ‬ሺߛሻܵ݅݊ሺߙሻ − ‫ݏ݋ܥ‬ሺߙሻܵ݅݊ሺߛሻ ‫ݏ݋ܥ‬ሺߙሻ‫ݏ݋ܥ‬ሺߚሻ‫ݏ݋ܥ‬ሺߛሻ − ܵ݅݊ሺߙሻܵ݅݊ሺߛሻ ‫ݏ݋ܥ‬ሺߛሻܵ݅݊ሺߚሻቍ
ܵ݅݊ሺߙሻܵ݅݊ሺߚሻ
−‫ݏ݋ܥ‬ሺߙሻܵ݅݊ሺߚሻ
‫ݏ݋ܥ‬ሺߚሻ
With α, β and γ corresponding to the Euler angles. The
rotation by angle α represents the rotation around the axis ‘z’
of the Cartesian coordinate system or the angle between the
‘x’ axis and the line of nodes (N), which is shown in figure 20. N
is defined by the intersection between the ‘xy’ and ‘DE’
coordinate planes. β gives the rotation between the original ‘z’
axis and the new ‘F’ axis. Finally the γ rotation corresponds
with the rotation around the ‘F’ axis of the new reference
frame or, as figure 20 shows, the rotation between the line of
nodes and the new ‘D’ axis.
4.6 VCG in CRT
In CRT, a biventricular pacemaker is used, which
enables activation of the myocardium at different
locations and with different time delays. Compared
to the RV and LV pacing only, this will give a more
Figure 20 Change of basis using Euler angles (α, β en γ).
The line of nodes (N) is defined at the intersection
between the 'xy' and 'DE' coordinate planes. The blue
coordinate system is the initial xyz coordinate system. The
red coordinate system (DEF) gives the result after rotation.
24
complicate interpretation of the VCG loop. More research is needed to investigate the
influence of such a biventricular device.
A database of VCG characteristics with different lead positions and different timing
sequences should be set up, by conducting measurements during CRT procedures.
Correlation with optimal dP/dt measurements could eventually help to find the optimal lead
position and timing sequence, based on VCG loops.
4.7 Quantification of the VCG
Besides the qualitative description of the VCG loops, quantification of VCG loops can help to
find objective measures for optimal electrical conduction pathways. At this moment, the
number of measurements is too low to obtain any conclusions about quantitative measures.
The loop area for different quadrants was calculated to help with the qualitative description
of the VCG loops. The total depolarization loop area can be used as a quantitative measure.
An increased are under the depolarization loop will correspond with an increased
depolarized muscle mass and an increased R-wave in the 12-lead ECG.
Besides the loop area, the loop length can be calculated by summing the magnitude of the
vectors between two consecutive sample points. An increased loop length, with a short
depolarization time will give an indication about the average speed of conduction of the
electrical impulse. An increased speed of conduction will result in a more synchronous
contraction.
The ratio between the loop area and the loop length will give information about the
morphology of the loop. It should be investigated which quantitative measures can help with
the interpretation of the VCG loops.
4.8 Repolarization
For now, the focus was on the description of the depolarization loop. Differentiation and
description of the repolarization loop can also give useful information. For the BBB subjects
the normal pathway for the impulse conduction is disturbed and the repolarization will start
in a part of the heart before depolarization is completed. Therefore, the direction of the
repolarization and the depolarization loops will be contrary for the BBB subjects, while for
the healthy subject the repolarization and depolarization loop are directed more parallel.
More research is needed to describe and analyze the repolarization loops.
4.9 Summary of conclusions
Because the direction of the VCG loop is a reflection of the electrical impulse pathway, the
presented VCG loops give an intuitive insight into the conduction of the electrical impulse
over the heart. Differences in the impulse conduction for subjects with a BBB as compared
to a healthy subject and as compared for LBBB and RBBB subjects apart, are found. Although
the number of measurements is low, it should be noticed that the same patterns are visible
25
for homologous subjects. Differences between RV pacing and LV pacing were found and
information about the lead position can be extracted from the VCG loops.
26
5. Future Directions
As mentioned already in paragraph 4.5, MRI images should be produced preceding a CRT
procedure, to indicate the orientation of the heart in the thorax. The MRI images can be
used to rotate the coordinate system, to a system with one axis along with the septum. The
plotted VCG loops will be indifferent with respect to heart orientation, which leads to a
normalization of the VCG loops.
Measurements were recorded now during CRT procedures and analyzed offline afterwards.
In future, real time visualization of the VCG loops during the procedure will make it easier to
make use of the interpretation of the VCG for CRT. Some first steps are already made in this
direction.
Furthermore, it should be investigated whether a finite element model of the cardiac electro
mechanics could help with the prediction of the VCG loops in more detail. In 2008 A. Lenssen
developed a model which could help to produce the 12-lead ECG signals [8]. The model can
help to simulate the influence of the lead positions and conduction blocks at different
regions of the heart, but also the influence of body composition and different geometries of
the heart. The model should be improved and can be used to develop VCG loops. Theoretical
insight into the influence on the VCG loop can help to improve insight in practice.
Increasing the number of measurement, for healthy subjects and for BBB subjects, will help
to get insight into the quantitative measures of the VCG loops and it will help to develop a
database for the qualitative description of the VCG loop. For the RV and LV pacing
measurements the lead position should be monitored and varied over the ventricles. This
will give more data concerning the position of the pacemaker and the change in direction of
the VCG loop. Also pressure data should be assembled to find correlation between dP/dt
measurements and VCG loops.
27
6. References
1. ESC Guidelines for the diagnosis and treatment of acute and chronic heart failure
2008, The Task Force for the Diagnosis and Treatment of Acute and Chronic Heart
Failure 2008 of the European Society of Cardiology. Developed in collaboration with
the Heart Failure Association of the ESC (HFA) and endorsed by the European Society
of Intensive Care Medicine (ESICM), Eur Heart J (2008) doi 10.1093/eurheartj/ehn309,
September 17th 2008
2. Textbook of Medical Physiology Eleventh Edition, Arthur C. Guyton and John E. Hall,
2006
3. Optimization of cardiac resynchronization therapy: addressing the problem of “nonresponders”, by D.J. Fox, A.P. Fitzpatrick and N.C. Davidson, Heart, 2005;91;10001002
4. Application of LVdP/dtMAX in the optimization of cardiac resynchronization therapy,
Berry M. van Gelder, Ph.D. , Department of Electrophysiology , Catharina Hospital
Eindhoven, April 2009
5. An Accurate, Clinically Practical System For Spatial Vectorcardiography, Ernest Frank,
Circulation 1956;13;737-749
6. On deriving the electrocardiogram from vectorcardiographic leads, by G.E. Dower
and H.B. Machado, Clin. Cardiology, 1980;3:87
7. Vectorcardiogram Synthesized From a 12-lead ECG: Superiority of the Inverse Dower
Matrix, by Lars Edenbrandt and Olle Pahlm, Journal of Electrocardiology, 1988;21(4);
361-367
8. From cardiac electrical activity to the ECG (a finite element model), A.M.J. Lenssen,
Master Thesis, Eindhoven University of Technology, 2008
28
Appendix A – ECG’s
Healthy Subject
LBBB1 Intrinsic
29
LBBB2 Intrinsic
RBBB1 Intrinsic
30
RBBB2 Intrinsic
LBBB1 RV Pacing
31
LBBB2 RV Pacing
RBBB1 RV Pacing
32
RBBB2 RV Pacing
LBBB1 LV Pacing
33
LBBB2 LV Pacing
RBBB1 LV Pacing
34
RBBB2 LV Pacing
35
Appendix B – 3D Representations of VCG loops
Healthy Subject
LBBB1 Intrinsic
36
LBBB2 Intrinsic
RBBB1 Intrinsic
37
RBBB2 Intrinsic
LBBB1 RV Pacing
38
LBBB2 RV Pacing
RBBB1 RV Pacing
39
RBBB2 RV Pacing
LBBB1 LV Pacing
40
LBBB2 LV Pacing
RBBB1 LV Pacing
41
RBBB2 LV Pacing
42
Appendix C – 2D Representations of VCG loops
Healthy Subject
LBBB1 Intrinsic
43
LBBB2 Intrinsic
RBBB1 Intrinsic
44
RBBB2 Intrinsic
LBBB1 RV Pacing
45
LBBB2 RV Pacing
RBBB1 RV Pacing
46
RBBB2 RV Pacing
LBBB1 LV Pacing
47
LBBB2 LV Pacing
RBBB1 LV Pacing
48
RBBB2 LV Pacing
49