B. Murienne - Master Project Thesis - Infoscience - EPFL

2. Theory
2.1 The heart
2.1.1 Generalities
The heart is a vital organ found in all vertebrates, which is responsible for pumping blood
throughout the body, thus bringing oxygen to all tissues and taking away metabolic byproducts.
The cardiac muscle tissue is an involuntary muscle tissue, meaning its contraction is not
controlled by the nervous system, although its beating frequency and contraction strength are.
The ability of the heart to independently initiate its own beats and the regularity of its pacing
activity are called automaticity and rhythmicity respectively [2].
2.1.2 Initiation of cardiac contraction
Several structures of the heart play an important role in the initiation of cardiac contraction.
The sinoatrial node (SA node) and two or three sites located next to it are responsible for
the initiation of impulses that induce cardiac contraction. This spontaneous depolarization of the
cardiac tissue is due to the plasma membranes of the SA node cells which have pacemaker
channels. These channels are hyperpolarization-activated and have a reduced permeability to
potassium ions but allow the passive transfer of calcium and sodium ions, which induces the
creation of a net charge [2]. The impulses generated are propagated from the SA node to the atria
before it finally reaches the atrioventricular node (AV node). The wave of excitation then travels
very slowly through the AV node as it is composed of slow-response fibers. The delay between
atrial and ventricular depolarization is the time needed for the atrial contraction to fill the
ventricles. When the AV junction is unable to conduct the cardiac impulse from atria to
ventricles, pacemakers in the Purkinje fiber network initiate the ventricular contractions.
However, these contractions occur at low frequencies and are usually not sufficient for the heart
to pump the required quantity of blood out to the body. The electrical impulses are transmitted
from the AV node to the Purkinje fibers through the His-bundle, also called AV bundle [2].
Ectopic pacemakers, which are automatic cells different from the ones of the SA node
found in the atrium, AV node, or His-Purkinje system, constitute a safety mechanism when
normal pacemaking centers stop functioning. They have the ability to create propagated cardiac
impulses when normal rhythmic pacemaker cells are suppressed. However, in some cases, the
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