Myocardial Oxygen: Demand, Supply and Flow Reserve

Question 1

Name three important topics that relate to the oxygen supply of the heart.

Answer 1

oxygen content (blood and lung dependent)
oxygen extraction (maximized in coronary arteries)
coronary blood flow (determined by resistance and pressure diff)

Question 2

How do you determine the O2 consumption of the myocardium?

Answer 2

simple estimates multiply the HR and peak blood pressure (double product)
a more exact method is calculating wall stress= (BP*LV radius)/ 2x wall thickness; this take into account chamber radius and wall thickness which make the measurement more accurate measure of after load

Question 3

2. Describe how the heart’s geometry acts as the heart pumps, how does it generate force?

Answer 3

unlike a muscle strip that generates force in a line, the heart functions as more of a spherical pump that generates tangental force in the walls to pump blood up and out of the chamber

Question 4

2. How does the radius of the chamber effect its ability to generate force?

Answer 4

when generating force tangental to the desired vector, the closer to parallel your action is to the vector, the more efficient you will be at generating force

Question 5

2. How does the wall thickness affect the chamber’s ability to generate force?

Answer 5

increased thickness leads to less wall stress because there are more cardiomyocytes contributing to force generation, less stress on individual cells

Question 6

2. Use an equation to describe O2 supply to the heart.

Answer 6

O2 supply ~ CBF * (O2% sat going into arterial- O2%sat coming out of the coronary sinus)

Question 7

3. How do the flow and resistance (arterial, myocardial and epicardial) vary with the cardiac cycle?

Answer 7

flow into the osmium varies with the heart beat
epicardial resistance is assumed to be 0
myocardium resistance increases during systole
arterial resistance varies to match the O2 needs

Question 8

3. How does systole and diastole effect blood flow in the right and left coronary arteries?

Answer 8

systole effects the the left coronary vessels more than the right because of the greater pressure generated by the left ventricle during pumping

Question 9

Compare the resistance in the subendocardium and the epicardium.

Answer 9

note particularly that the subendocardium resistance is much greater than epicardial resistance and is at greater danger for ischemic damage

Question 10

2. Heart rate is directly proportional to ____ _____

Answer 10

HR is proportional to O2 demand

Question 11

2. What local factors work to adjust the arterial resistance in coronary auto regulation?

Answer 11

adenosine, prostaglandins, NO, endothelia, potassium, oxygen are all factors that signal greater required O2 flow (autoregulation occurs over a range)

Question 12

4. Describe the concept of coronary flow reserve.

Answer 12

at a given pressure, it is the ability of the coronary arteries to vasodilator to modulate diameter and facilitate a favorable pressure

Question 13

5. What is the impact of epicardial disease on coronary flow reserve?

Answer 13

as epicardial resistance increases with disease, arterial resistance adjusts to maintain appropriate flow and pressure, limiting the CFR. (degree of obstruction is directly related to the decrease in CFR)

Question 14

Why is decreased CFR significant?

Answer 14

lower CFR limits peak supply, so lower CFR limits heart’s ability to increase demand without ischemia (if stenosis gets tight enough, there will be ischemia at rest)