Biomedical Engineering Reference
In-Depth Information
2
Basic Principles of Cardiac
Pacemaker Technology
2.1
Cardiac Pacemaker Classi fi cations
2.2
Electric Cardiac Pacing
Nowadays, cardiac pacemaker technology includes a wide
range of implantable medical devices, the use of which has
increased worldwide. According to valid national and
European legislation, cardiac pacemaker technology falls
into a group of active implantable medical devices (AIMD).
This group is subject to the most severe requirements with
regard to safety and reliability.
An up-to-date cardiac pacing system is a medical device
that always consists of the main unit itself and between one
and three leads, the number of which depends on the type of
heart blockage. As a matter of principle, implantable systems
might be used within several situations. Treatment of a
patient's slow heart rhythm requires implantation of a lead in
either the atrium or the ventricle. In the case of absence of the
sensed intrinsic heart beat, the pacemaker (PM) sends a stim-
ulus based on defined parameters. With heart blockages of all
degrees, one lead senses the contractions of the atria and the
second lead initiates the contraction of the ventricle after a
delay. The most recent possibility is a solution to ventricular
dyssynchrony, created because of structural changes (cardiac
failure), by implantation of a third lead epicardially on the
left ventricle. If a patient is endangered by a fast heart rhythm
(tachycardia), an implantable cardioverter-defibrillator (ICD)
is used. Again, it is possible to select only one lead (in the
right ventricle) or two leads (one in the atrium and the another
in the ventricle). The system with three leads for resynchro-
nization therapy in the case of heart failure can also be used
with a defibrillator (a cardiac resynchronization therapy
defibrillator [CRT-D]). These devices always are equipped
with a sensor for the adaptation of the paced rate (frequency)
according to the patient's needs (Fig. 2.1 ).
Cardiac pacing principles are based on the creation of an
electrical field between the electrodes and surrounding myo-
cardium by means of an electric stimulus. For the creation of
an action potential and its subsequent spontaneous propaga-
tion, it is necessary to ensure that a difference of potentials
between extracellular and intracellular domains on ecto-
plasm fall to the value of the threshold potential - from the
value of about −80 to about −60 mV. The intracellular domain
is charged relatively negatively. The extracellular domain,
however, is charged positively. The electrode fixed to the
endocardium is compared with practically all cellules in the
extracellular domain. The closest surroundings of the elec-
trode are on the same potential as the electrode. So, under an
electric stimulus, the extracellular domain is polarized in
compliance with the stimulus. The purpose is to induce an
action potential on the membranes by changing the electric
potential to above the value of the membrane potential.
Because the intracellular domain is charged relatively nega-
tively, a decrease in potential of the membrane can be
achieved by decreasing the potential of the extracellular
domain using a negative pulse. A positive electric stimulus
can also be used for pacing. However, the amplitude must be
a little higher. The action potential created on the cellules
depolarized directly by the electrode is propagated by bio-
physical mechanisms on the surrounding cellules, and they
also are depolarized. The highest current density from the
electrodes is at the edge between the electrode and the tissue.
It decreases as the distance from the electrode increases.
A minimal value of a certain physical quantity at which
a consistent cardiac depolarization has been safely created
and propagated is called a pacing threshold. The pacing
 
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