Biomedical Engineering Reference
In-Depth Information
3
Heart Anatomy and Physiology
To provide comprehensive information on pacing and
defibrillation methods, the fundamentals of the anatomy and
physiology of the heart need to be discussed. An overview of
cardiac rhythm disorders and their pharmacological manage-
ment will follow in Chap. 4 . In terms of the pacing method,
one has to be familiar with the general structure of the heart
and, in more detail, with the right atrium and the right ven-
tricle, where endocardial leads are placed. It is also neces-
sary to understand the anatomy of the conduction system and
coronary veins as well as imaging projections.
The basis of the heart's electrical activity is the action
potential of cardiac cells. There are two types of cardiac cells:
cells of the conduction system and myocardial contractile
cells. The function of the cells of the conduction system is to
conduct impulses to myocardial contractile cells, which con-
tract to pump blood. They also differ in how the action poten-
tial passes along their membranes. For optimal cardiac output,
and thus blood supply to the whole body, it is essential to
ensure not only contractions of the heart chambers, but also
their correct timing. The purpose of pacing is to cause, by sup-
plying external electrical energy, the generation of an action
potential in the immediate surroundings of the pacing elec-
trode that is further propagated by biophysical mechanisms.
forward, its base backward and upward. At the base, where
the atria are found, large veins enter and arteries - the aorta
and the pulmonary artery - exit. An appendage arises from
the edge of each atrium. This is the site where endocardial
atrial pacing leads are attached. The right and left ventricles
extend from the atria to the apex of the heart. Under physio-
logical conditions, the left ventricle is larger and has a stron-
ger wall than the right one. This results from the physiological
difference in their functions: the greater circulation powered
by the left ventricle requires higher pressure. Each ventricle
has an inflow part and an outflow part.
The right atrium receives deoxygenated blood from the
greater circulation. The superior vena cava enters posteriorly
and superiorly; the inferior vena cava enters from the left and
inferiorly through the diaphragm. By means of internal struc-
tures, the flow of blood is directed in such a manner so as to
prevent turbulent flow. The coronary sinus, where the coro-
nary veins enter, is located at the posterior atrial wall. This is
where pacing leads are implanted in the left ventricle. This
orifice is partly covered by the semilunar valve. The left
atrium is separated from the right atrium by the atrial sep-
tum. Toward the apex, the right atrium meets the right ven-
tricle, and the orifice between them is guarded by the tricuspid
valve. During systole, cusps of this valve close together, pre-
venting regurgitation from the right ventricle into the right
atrium. The walls of the inflow part of the right ventricle are
covered by muscular trabeculae and elongated ridges. In
contrast, the outflow part of the right ventricle is smooth. The
orifice of the outflow tract is guarded by the pulmonary
valve. The left atrium begins where the pulmonary veins
empty at its posterior wall. The walls are approximately
3 mm thicker than those of the right atrium, and they are
smooth. In a downward and forward direction, the left atrium
opens into the left ventricle through the bicuspid (mitral)
valve. The inflow part of the left atrium is larger than that of
the right atrium, whereas the outflow part is shorter. The
right and left ventricles are separated by the ventricular
(interventricular) septum. It is as thick as the whole left ven-
tricular wall and arches into the right ventricle.
3.1
Heart Anatomy
The heart is a hollow muscular organ that rhythmically con-
tracts to push blood through the bloodstream [21]. The heart
of an adult weighs approximately 230-340 g; the weight of
the heart in women is, on average, about 15 % lower than
that in men. The weight of the heart depends on the volume
of the heart muscle, which varies individually. The average
dimensions of the heart are 13 × 9 × 6 cm. The heart is situ-
ated behind the sternum, in the mediastinum, with one third
located to the right of the middle and two thirds located to
the left. It lies in the pericardium, which encloses it.
The external shape of the heart resembles an inverted
irregular cone (Fig. 3.1 ). Its apex is directed downward and
 
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