Biomedical Engineering Reference
In-Depth Information
and nerves coming to the heart only accelerate or slow down
the automatic action. Thus, the myocardium itself is a source
of electrical signals required for depolarization of myocar-
dial contractile cells. Cells of the conduction system have
larger and more rounded nuclei than myocardial contractile
cells. The conduction system (Fig.
3.1
) consists of the sinoa-
trial node, internodal pathways, the atrioventricular node, the
atrioventricular bundle of His, right and left bundle branches,
and Purkinje fibers.
The principal part of the conduction system is the sinoa-
trial (SA) node. This is where electric impulses that determine
the heart's rate are primarily generated. The SA node is an
elongated structure, 10-20 mm long, about 3 mm wide, and
1 mm thick. It is located in the wall of the right atrium beside
the opening of the superior vena cava near the upper border of
the atrial appendage. It is situated deep within the myocar-
dium, approximately 1 mm from both the endocardium and
the epicardium. An impulse arises in central nodal myocytes
and is transmitted by slow myocyte-to-myocyte conduction
to the subsequent segments of the conduction system.
The atrioventricular (AV) node is a structure located at the
boundary between the atria and the ventricles, approximately
10 mm from the opening of the coronary sinus. It is roughly
oval in shape, 7-8 mm long, about 1 mm wide, and 3 mm
thick. It is made up of myocytes with a network of collagen
fibers. Its purpose is to delay the conduction of impulses,
which is referred to as AV delay and is thought to be achieved
by accumulating a large number of myocytes with a slow
conduction velocity. An impulse from the SA node is con-
ducted to the AV node via internodal pathways. They go
through the right atrial wall and enter the AV node or as far
as the AV bundle. These pathways include the anterior inter-
nodal pathway, middle internodal pathway, posterior inter-
nodal pathway, interatrial bundle, collateral fibers, and
accessory AV bundles.
Arising from the anterior margin of the AV node is the AV
bundle of His. It is a compact band of myocytes surrounded
by a layer of vascularized connective tissue. At transition to
the muscular part of the septum, it divides into the right and
left bundle branches. The right bundle branch is a slender,
round fascicle that passes down on the right side of the sep-
tum toward the apex. It first runs in the myocardium, then
below the endocardium, and it continues to divide, forming
numerous Purkinje fibers. The left bundle branch is com-
posed of many fine fascicles with a fibrous sheath.
The fascicles gradually divide toward the apex, where
they branch along the ventricular walls into Purkinje fibers
composed of typical large myocytes. Branching of these
fibers proceeds proximally from the apex of the heart. As a
consequence, contraction of the ventricular myocardium also
proceeds from the apex to the base of the heart as well as
from the internal (the endocardium) to the external layer.
3.4
Heart Vessels
Oxygenated blood is brought to the heart walls by the right
and left coronary arteries that arise immediately at the origin
of the aorta. Deoxygenated blood is removed from the heart
walls by cardiac veins. With respect to the pacing method,
the anatomy of the coronary arteries is of minor importance
only. However, the anatomy of the cardiac veins, into which
left ventricular pacing leads are placed, is important.
The coronary sinus that empties posteriorly into the right
atrium is the main venous outflow. The great cardiac vein
that begins at the apex of the heart and encircles the left heart
empties into the coronary sinus. It drains blood from the left
and right anterior sides of the left ventricle. The posterior left
ventricular vein that collects blood from the posterior wall of
the left ventricle empties into it at the posterior side. The
middle cardiac vein passes from the apex of the heart directly
to the coronary sulcus and drains into the coronary sinus near
its entry into the right atrium. The cardiac veins contain no
valves. However, valves sometimes do occur at the openings
of the major veins into the coronary sinus. The veins contain
numerous anastomoses, sometimes even extracardial ones.
3.5
X-Ray Projections of the Heart
Pacing and defibrillation leads are implanted under X-ray
skiascopic guidance. For this reason, it is advisable to be
familiar with the basic projections used in the imaging of the
heart and cardiac veins.
When implanting leads into the right-sided heart cham-
bers, the heart is most commonly viewed in the anteroposte-
rior (AP) projection. In the case of a venogram and left
ventricular lead placement, oblique projections are used:
right (right anterior oblique) and left (left anterior oblique)
under an angle of 30°-60°. On an AP projection, margins of
the cardiac shadow are identifiable. On oblique projections,
dimensions of the heart chambers are visible and, after
obtaining a venogram, a clear image of the left ventricular
venous bed is available on a left anterior oblique projection.
3.6
Cell Electrophysiology
The cell body is enclosed by the cell membrane. Functionally,
the membrane is an important cell organelle involved in
maintaining the intracellular environment and composing
the extracellular environment.
The intracellular and extracellular environments are made
up of electrolytes, a solution of various concentrations of
ions [ 22 ] . The predominant intracellular ion is K
+
, having a
