■ FIGURE 7.7 Pulsatile nature of coronary blood flow measured in the left coronary artery. Flow is lower
during systole because of mechanical compression of intramuscular coronary vessels. Flow is maximal
early in diastole as the heart is relaxing, and then it falls as aortic pressure declines.
reason that coronary flow is influenced by the
cardiac cycle is that during systole, the contrac-
tion of the myocardium compresses the micro-
vasculature within the ventricular wall, thereby
increasing resistance and decreasing flow. Dur-
ing systole, blood flow is reduced to the great-
est extent within the innermost regions of the
ventricular wall (i.e., in the subendocardium)
because this is where the compressive forces
are greatest. (This results in the subendocar-
dial regions being more susceptible to ischemic
injury when coronary artery disease or reduced
aortic pressure is present.) As the ventricle
begins to relax in early diastole, the compres-
sive forces are removed and blood flow is per-
mitted to increase. Blood flow reaches a peak
in early diastole and then falls passively as the
aortic pressure falls toward its diastolic value.
Therefore, it is the aortic pressure during dias-
tole that is most crucial for perfusing the coro-
naries. This explains why increases in heart rate
can reduce coronary perfusion. At high heart
rates, the length of diastole is greatly shortened,
which reduces the time for coronary perfusion.
This is not a problem when the coronary
arteries are normal, because they dilate with
increased heart rate and metabolism; however,
if the coronaries are diseased and their vasodi-
lator reserve is limited, increases in heart rate
can limit coronary flow and lead to myocardial
ischemia and anginal pain.
The mechanical forces affecting coronary
flow are greatest within the left ventricle
because this chamber develops pressures that
are severalfold greater than those developed
by the right ventricle (see Chapter 4). The
right ventricle and, to a lesser extent, the atria
show some effects of contraction and relaxa-
tion on blood flow within their musculature,
but it is much less apparent than that observed
in the left ventricle.
Mean coronary blood flow (averaged over
several cardiac cycles) can range from 80 mL/
min per 100 g of tissue at resting heart rates
to over 400 mL/min per 100 g during exercise
(see Table 7-1). Therefore, the coronary vas-
culature normally has a relatively high vasodi-
lator reserve capacity.
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