Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 34Circulation Through Special Regions 579

patients with stenotic aortic valves because the pressure in the
left ventricle must be much higher than that in the aorta to eject
the blood. Consequently, the coronary vessels are severely com-
pressed during systole. Patients with this disease are particularly
prone to develop symptoms of myocardial ischemia, in part be-
cause of this compression and in part because the myocardium
requires more O 2 to expel blood through the stenotic aortic valve.
Coronary flow is also decreased when the aortic diastolic pres-
sure is low. The rise in venous pressure in conditions such as con-
gestive heart failure reduces coronary flow because it decreases
effective coronary perfusion pressure (see Clinical Box 34–4).
Coronary blood flow has been measured by inserting a cath-
eter into the coronary sinus and applying the Kety method to

the heart on the assumption that the N 2 O content of coronary

venous blood is typical of the entire myocardial effluent. Coro-

nary flow at rest in humans is about 250 mL/min (5% of the

cardiac output). A number of techniques utilizing radionu-

clides, radioactive tracers that can be detected with radiation

detectors over the chest, have been used to study regional blood

flow in the heart and to detect areas of ischemia and infarct as

well as to evaluate ventricular function. Radionuclides such as

thallium-201 (^201 T1) are pumped into cardiac muscle cells by

Na, K ATPase and equilibrate with the intracellular K+ pool.

For the first 10–15 min after intravenous injection,^201 T1 distri-

bution is directly proportional to myocardial blood flow, and

areas of ischemia can be detected by their low uptake. The

uptake of this isotope is often determined soon after exercise and

again several hours later to bring out areas in which exertion

leads to compromised flow. Conversely, radiopharmaceuticals

such as technetium-99m stannous pyrophosphate (99mTc-PYP)

are selectively taken up by infarcted tissue by an incompletely

understood mechanism and make infarcts stand out as “hot

spots” on scintigrams of the chest. Coronary angiography can

be combined with measurement of^133 Xe washout (see above)

to provide detailed analysis of coronary blood flow. Radiopaque

contrast medium is first injected into the coronary arteries, and

x-rays are used to outline their distribution. The angiographic

camera is then replaced with a scintillation camera, and^133 Xe

washout is measured.

VARIATIONS IN CORONARY FLOW

At rest, the heart extracts 70–80% of the O 2 from each unit of

blood delivered to it (Table 34–1). O 2 consumption can be in-

creased significantly only by increasing blood flow. Therefore,

it is not surprising that blood flow increases when the metab-

olism of the myocardium is increased. The caliber of the cor-

onary vessels, and consequently the rate of coronary blood

flow, is influenced not only by pressure changes in the aorta

but also by chemical and neural factors. The coronary circula-

tion also shows considerable autoregulation.

CHEMICAL FACTORS

The close relationship between coronary blood flow and myo-

cardial O 2 consumption indicates that one or more of the

products of metabolism cause coronary vasodilation. Factors

suspected of playing this role include O 2 lack and increased lo-

cal concentrations of CO 2 , H+, K+, lactate, prostaglandins, ad-

enine nucleotides, and adenosine. Likely several or all of these

vasodilator metabolites act in an integrated fashion, redun-

dant fashion, or both. Asphyxia, hypoxia, and intracoronary

injections of cyanide all increase coronary blood flow 200–

300% in denervated as well as intact hearts, and the feature

common to these three stimuli is hypoxia of the myocardial fi-

bers. A similar increase in flow is produced in the area sup-

plied by a coronary artery if the artery is occluded and then

released. This reactive hyperemia is similar to that seen in the

TABLE 34–4 Pressure in aorta and left and
right ventricles (vent) in systole and diastole.

Pressure (mm Hg) in

Pressure Differential

(mm Hg) between

Aorta and

Aorta

Left

Vent

Right

Vent

Left

Vent

Right

Vent

Systole 120 121 25 –1 95

Diastole 80 0 0 80 80

FIGURE 34–13 Blood flow in the left and right coronary
arteries during various phases of the cardiac cycle. Systole occurs
between the two vertical dashed lines. (Reproduced with permission from
Berne RM, Levy MN: Physiology, 2nd ed. Mosby, 1988.)

120

100

80

100

60

40

20

0

15

10

5

0

80

0.2 0.4 0.6 0.8 1.0

Time (s)

Left coronary

Right coronary

Phasic coronary blood flow

(mL/min)

Aortic pressure

(mm Hg)