Page 228 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Coronary Heart Disease
During physical work or psychological stress, chronic stable angina pectoris suddenly has
the myocardial oxygen demand rises, particu- stronger and more frequent anginal pain (un-
larly because heart rate and myocardial con- stable angina pectoris), it is often a premoni-
tractility will have been increased by sympa- tory sign of acute myocardial infarction, i.e.,
thetic stimulation. In response to this the cor- complete occlusion of the relevant coronary
onary vascular resistance can in the normal artery (see below).
heart drop to as low as ca. 20% of its resting However, complete coronary occlusion does
level so that, with the corresponding increase not necessarily lead to infarction (see below),
in coronary perfusion, the O 2 balance will be because in certain circumstances a collateral
restored even during this period of increased blood supply may develop as long-term adap-
demand. The capacity to increase perfusion to tation so that, at least at rest, the O 2 demand
up to five times the resting value is called cor-
can be met (→ B). The affected region will,
Heart and Circulation blood flow is due to the fact that the distal cor- hypoxemia, a drop in blood pressure, or an in-
onary reserve. The wide range in coronary
however, be particularly in danger in cases of
creased O 2 demand.
onary vessels are constricted at rest and dilate
1
Pain resulting from a lack of O 2 can also oc-
only on demand (→ A; normal vs. ⁄4 resis-
cur at rest due to a spasm (α 1 -adrenoreceptors;
tance).
→ p. 216) in the region of an only moderate
Diminished coronary reserve is characteris-
spastic, Prinzmetal’s, or variant angina). While
to O 2 supply no longer being able to meet any
increased O 2 demand. This ischemic anoxia
shortening of the arterial muscle ring by, for
7 tic of coronary heart disease (CHD) and leads atherosclerotic narrowing of the lumen (vaso-
manifests itself in pain mainly in the left chest, example, 5% increases the resistance of a nor-
arm, and neck during physical work or psycho- mal coronary artery about 1.2fold, the same
logical stress (angina pectoris; see below) shortening in the region of an atheroma that
The main cause of CHD is narrowing of the is occluding 85% of the lumen will increase
proximal large coronary arteries by athero- the resistance 300 times the normal value
sclerosis (→ p. 217 D and 236ff.). The postste- (→ D). There are even cases in which it is large-
notic blood pressure (P ps ) is therefore signifi- ly (or rarely even exclusively) a coronary
cantly lower than mean diastolic aortic pres- spasm and not the atheromatous occlusion
sure (P Ao ; → A). To compensate for this raised that leads to an episode of vasospastic angina.
resistance or reduced pressure, the coronary Another cause of diminished coronary re-
reserve is encroached upon, even at rest. The serve is an increased O 2 demand even at rest,
price paid for this is a diminution in the range for example, in hypertension or when there is
of compensatory responses, which may ulti- an increased ventricular volume load. The
mately be used up. When the luminal diame- ventricular wall tension, i.e., the force that the
ter of the large coronary arteries is reduced by myocardium must generate per wall cross-sec-
– 2
more than 60–70% and the cross-sectional tional area (N · m ) to overcome an elevated
area is thus reduced to 10–15% of normal, aortic pressure or to eject the increased filling
myocardial ischemia with hypoxic pain occurs volume, is then significant. In accordance with
even on mild physical work or stress. If syn- Laplace’s law, the wall tension (K) of an ap-
chronously O 2 supply is reduced, for example, proximately spherical hollow organ can be cal-
by a lowered diastolic blood pressure (hypo- culated from the ratio of (transmural pres-
tension, aortic regurgitation), arterial hypox- sure · radius)/(2 · wall thickness) (→ p. 217 C).
emia (staying at high altitude), or decreased Thus if, without change in wall thickness, the
O 2 capacity (anemia), O 2 balance is disturbed, ventricular pressure (P ventr ) rises (aortic valve
even when there is only mild coronary artery stenosis, hypertension; → p.198 and 208)
stenosis (→ p. 217 C). and/or the ventricular radius increases (great-
If the pain ceases when the physical or psy- er filling in mitral or aortic regurgitation;
218 chological stress is over, the condition is called → p.196 and 200), the wall tension necessary
stable angina pectoris. When a patient with for maintaining normal cardiac output and
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Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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