Biomedical Engineering Reference
In-Depth Information
damage can be done to the lung, leading to the weakening
of the valve.
2.2.7.4 Left ventricular hypertrophy
The heart responds to prolonged inadequate myocardial
contraction by enlarging to keep pace with the re-
quirements for increased cardiac output ( Hori et al.,
1989 ). This increased workload causes an increase in the
wall thickness, a condition commonly referred to as left
ventricular hypertrophy (shown in Fig. 2.2-40 ).
This inward expansion of the ventricular wall reduces
both the rate and amount of relaxation during diastole,
leading to a decrease in the wall compliance. The de-
crease in compliance (increase in wall stiffness), coupled
with the concomitant increase in pressure, leads to a re-
duction in the size of the left chamber. If the left
chamber is too small, it cannot fill efficiently, which leads
to blood backing up into the vessels of the lungs and less
blood circulating to the vital organs. At times, hyper-
trophy is itself the primary disease; but more commonly
it is the consequence of another disorder.
Frequently, chronic hypertension is found to be the
underlying cause of left ventricular dysfunction in humans.
When operating correctly, the aortic and pulmonary valves
are responsible for creating the pressure differences that
lead to proper blood flow and circulation. Abnormal
functioning of the valves is the underlying factor that
produces either (1) pressure overloading due to restricted
opening; or (2) volume overloading due to inadequate
closure (Legato, 1987).
A narrowing of the aortic or pulmonary valves induces
a decrease in blood flow (by Ohm's Law). Assuming
constant resistance in the blood transport vessels, the
Figure 2.2-39 Right heart volume.
volume in the pulmonary arteries causes the pressure in
the pulmonary circuit to increasedthis increased pres-
sure in the lung is called hypertension. The plots in Figs.
2.2-38 and 2.2-39 show that the blood volume in both the
right and left heart are greater than normal.
VSD is a problem that affects the pressure in the heart
because of the blood flow between the left and right
ventricles through the septum. If the hole is large, the
blood from the left ventricle blood flows into the right
ventricle because of the pressure difference. The addi-
tional blood volume also causes more blood in the lung,
causing extra pressure that sometimes results in hyper-
tension. If VSD is not treated, the arteries in the lungs
can thicken up under the extra pressure and permanent
Normal heart
Left ventricular
hypertrophy
Right ventricle
Left ventricle
Hypertrophy
Figure 2.2-40 Overwork of the left ventricle causes an increase in wall thickness, a condition called left ventricular hypertrophy.
Picture courtesy of the Mayo Educational Foundation.
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