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Development of simulation
system of cardiac arrhythmia
and artificial pacemaker
Akihiro Takeuchi, Atsushi Hamada, Nozomu
*
*
Yamanaka, Mototake Taguchi , Minoru Hirose ,
Noriaki Ikeda, Hideo Miyahara
Dept. of Medical Informatics, Kitasato University
*
Graduate School, Dept. of Clinical Engineering,
School of Allied Health Sciences, Kitasato University,
Sagamihara, Kanagawa, 228-8555, Japan
Abstract: A simulation system which consisted of the cardiac
module and the artificial pacemaker module was developed using
a Window-based software technology, ActiveX control. The
cardiac module was composed of six kinds of cells, the sinus,
atrium, A-V node, ventricle, and ectopic focuses. The
electrophysiological properties of the cell were modeled by the
phase response curve that defined automaticity, and the
excitability recovery curve that defined the conduction delay
depending on the previous interval. The ECG waves were drawn
in real time on a ladder diagram. The pacemaker module sensed
the cardiac impulses, and stimulates the cardiac cell according to
the user setting,e.g., ICHD modes, pacing rate, and output level.
This system was useful to understand about arrhythmias and the
interaction between the cardiac cell and the artificial pacemaker,
and to experience the electrophysiological experiment virtually.
URL http://info.ahs.kitasato-u.ac.jp/tkweb/tkp/tkpace.html
Introduction
Two mechanisms that produce arrhythmias are automaticity
and conductivity. These physiological properties of cardiac cells
are represented by a phase response curve and an excitability
recovery curve, respectively. Some simulation systems including
the curves were developed by the standard programming style
[Ikeda et al., 1997, Abramovich-Sivan et al., 1998]. The cells and
its mathematical functions can be considered as objects and its
properties from a point of view of object-oriented programming.
The aim of this study is to develop an integrated simulation
system of electrical activity of the heart using a Window-based
software technology, ActiveX control, and to reproduce
theoretically various arrhythmias.
Model of cardiac cells
Electrophysiological properties of the cardiac autorhythmic
cells are characterized by the automaticity and conductivity. The
autorhythmic cells can periodically generate spontaneous
electrical impulses. The oscillatory systems have a phase
dependent property; the response of the cell to the external
stimuli varies depending on the phase at which the stimuli were
applied.
The conductivity is represented by an excitability recovery
curve (ERC). The ERC of the AV-node, AV-nodal conduction
curve, defines the conduction delay and a timing to stimulate the
ventricle in this system.
The refractory period of the cell was assumed to be
proportional to the previous RR interval, so that it was assumed
to be equal to the QT interval.
a w
c
b
Y0
T0
Ymin
Xmin
Figure 1. Electrophysiological properties of the
cardiac cell. PRC(left) and ERC(right).
Cell model by ActiveX control
The electrophysiological functions that were defined by a
spontaneous rate, PRC and ERC, were implemented as a set of
the ActiveX control (tkpace.ocx 90 kB, Visual C++6.0) (Fig. 1).
The PRC that specified the cell activity was defined by four
parameters, slopes of three line (a,c,b), and intersection on the
time axis (w) by the line c.
The ERC was approximated by the decaying exponential
function
conduction time = (Y0-Ymin)*Exp(-(x-Xmin)/T0) +Ymin,
where x is the extrastimulus coupling interval (H1A2), Xmin
corresponds to the effective refractory period, Y0 is a maximum
connection delay at Xmin, Ymin means an intrinsic conduction
time, and T0 means a time constant of the curve. For simplicity,
these parameters are not dependent on the autorhythmic rate.
The antergrade conduction time was calculated by the ERC,
but the retrograde conduction time was assumed to be constant.
Sinus
Atrium
AV node
Ventricle
Electrophysiological stimulation
Atrial ectopic focus
User command
antegrade conduction
retrograde conduction
extra stimuli
Ventricular ectopic focus
Defibrillator
Figure 2. Cardiac model using six types of cells
Description of the system
The simulation system consisted of the cardiac module and the
artificial pacemaker module. The cardiac module (tkp3.exe 130
kB, Visual Basic 6.0) was composed of six types of autorhythmic
cardiac cells (Fig. 2). The cardiac cells run independently at
every timer event from the cardiac module. The antegrade and
retrograde conduction between these cells were controlled by the
user's operation on the check boxes in the window.
The artificial pacemaker module had two electrodes to sense
the cardiac impulses, and recognized the rhythm disturbances
automatically. This module stimulated the cardiac cell through a
connection lines between the ActiveX controls of two modules.
Result
The application runs under MS-Windows (98 or higher) with
a wide screen on a high performance computer (CPU: Pentium III,
500 MHz or higher).
Figure 3 shows the control panel and the ECG panel of the
cardiac module. Each electrical activity of the cells is drawn at
the predefined position on a ladder diagram. The predicted
conduction is drawn with a blue line at the activation of the cell.
The slanted yellow lines show the spread of activation
transmitted actually. The ECG waveforms are drawn in real time
as the integrated activity of all cells.
In addition, the dynamic state of contraction and relaxation of
the atrium and the ventricle are presented graphically.
Figure 3. Sample window: normal state of the
heart.
Figure 4 shows a Wenckebach sequence with 5:4 ratios
produced by the special condition of the ERC of the AV-node at a
sinus rate of 70/min. User could modify the parameters of PRC
and ERC in the dialog box.
Figure 5 shows a simulation of cardiac rhythm disturbances
induced by the conduction between the atrium and the ectopic
focus.
Figure 6 shows the ECG sequence of the ventricular flutter,
fibrillation, and the defibrillation procedure achieved by the user
operation. It shows a pause period caused by the defibrillation
that resumed all the activity of the heart.
Figure 7 shows the interaction between the heart and an
artificial pacemaker. User can experience the artificial
pacemaker operation virtually.
References
Ikeda et al., Arrhythmia curve interpretation using a dynamic system
model of the myocardial pacemaker. Meth Inform Med 1997;36:286-9.
Abramovich-Sivan et al., A single pacemaker cell model based on the
phase response curve. Biol. Cybern. 1998;79:67-76.
Figure 4. Wenckebach sequence
Figure 5. Reentry in the atrium
Figure 6. Defibrillation
Figure 7. Interaction between the heart and
an artificial pacemaker
Figure 8 shows a simulation
of the electrophysiological
experiment to study the
conductivity of the AV node.
AV nodal responses to the
premature coupling interval
S1S2 were recorded and the
corresponding V1V2 interval
and S2V2 intervals were
plotted against S1S2. S1S2
(sinus-stimulation) intervals
were decreased with a 20msec
decrement.
Figure 8. Simulated refractory curve