Phys- Coronary Flow

Question 1

Oxygen delivery equation

Answer 1

Oxygen Deliivery = Blood Flow x Oxygen Content

Question 2

Flick Equation

Answer 2

VO2= Blood Flow x (CaO2 - CvO2)

Question 3

Extraction equation

Answer 3

Difference between what goes in and what comes out CaO2-CvO2

Question 4

If coronary O2 extraction is nearly maximal at rest how does the heart match O2 delivery with increases in myocardial VO2?

Answer 4

increased blood flow

Question 5

Control of coronary blood flow

Answer 5

Primarily under local metabolic control

Question 6

Intramural arteries

Answer 6

penetrate the myocardium and are subject to high transmural pressure during systole (surround pressure)

Question 7

capillary to fiber ratio in cardiac muscle

Answer 7

(number or capillaries per muscle fiber) 1 capillary/fiber

Question 8

Cardiac Capillary density

Answer 8

VERY high (~3500 capollaries/mm^2 ) to support the metabolism of the heart

Question 9

describe cardiac capillary perfusion at rest

Answer 9

heterogeneous- not all are well perfused at rest

Question 10

Capillary recruitment

Answer 10

increased myocardial work increases blood flow and more uniform perfusion of capillaries

Question 11

How do we get more uniform perfusion of capillaries during capillary recruitment

Answer 11

Increased myocardial work causes 1.) Increased “Functional” capillary density 2.) Decreased diffusion distances 3.)Enhanced capillary exchange

Question 12

Coronary Collateral Vessels

Answer 12

Arterial-to arterial anastamoses: Coronary arteries form collateral channels to ensure myocardial perfusion and help protect the mycoardium against ischemia

Question 13

Native coronary collateral vessels

Answer 13

exist in all hearts - relatively small and few in number

Question 14

Mature (or stimulated) coronary collaterals

Answer 14

Myocardial ischemia stimulates collarteral growth. The lumen of collateral vesslesl enlarges followed by significant wall thickening.

Question 15

Myocardial Oxygen Consumption depends on what variables

Answer 15

Blood flow (Q) and coronary oxygen extraction (CaO2-CvO2) where MVO2 = Q x (CaO2-CvO2)

Question 16

In the heart, when is oxygen extraction nearly maximal

Answer 16

under resting conditions

Question 17

what is the only way to increase MVO2

Answer 17

by increasing blood flow (remember: oxygen extraction is at near max under resting conditions)

Question 18

MVO2 increases in proportion to what

Answer 18

Myocardial work- Maximal MVO2 can increase 5-6 fold from resting MVO2

Question 19

Determinants of MVO2 “The Big 3”

Answer 19

1.) Contractility 2.) Heart Rate 3.) Wall Tension

Question 20

Theory behind how increases in ionotropy increases MVO2

Answer 20

thought to be related to the metabolic cost of Ca release and uptake

Question 21

Discuss the increase in MVO2 as a result of wall tension

Answer 21

MVO2 increases linearly with peak systolic pressure (or peak systolic wall tension) developed by the left ventricle (Law of LaPlace Wall tension = (pressure x radius)/2thickness)

Question 22

Discuss tratments if there is a mismatch between MVO2 and the ability to deliver O2 to the tissue

Answer 22

1.) B-Blockers (decreases HR and Contractility= decrease Myocardial work by decreasing MVO2) 2.) Ca channel blockers (decrease contractility = decrease MVO2) 3.) Nitrates (decrease afterload by causing peripheral vasodilation and decrease preload through venodilation)

Question 23

Determinants of MVO2 “The Little 3”

Answer 23

1.) Basal requirements (cellular metabolism) 2.) Activation Energy (cost of electrical activation) 3.) Ejection of Blood (Cost of shortening muscle)

Question 24

Myocardial Metabolic Substrate Utilization

Answer 24

1.) ATP (used for contraction ) is in dynamic equilibrium with a pool of creatine phosphate 2.) Oxidative Metabolism (fatty acids) - 60% of substrate utilization- balance is glucose and lactate 3.) Glycolysis - 40% of substrate metabolism

Question 25

Discuss situations where Glycolysis predominates in myocardial substrate utilization

Answer 25

During ischemia and hypoxemia - glycolysis predominates and the heart switched from net lactate uptake to net lactate production. NOTE: more predominant but less efficient

Question 26

How does the heart meat increased O2 demands (increased MVO2)

Answer 26

almost entirely though increasing blood flow

Question 27

Does the percentage of cardiac output used changed when MVO2 is increased?

Answer 27

NO. Flow increases up to 5 fold as MVO2 increased but cadiac output remains ~5% (as it is in resting conditions)

Question 28

What is the drivig pressure for coronary flow

Answer 28

Aortic diastolic pressure

Question 29

Describe the coronary flow in ventricular systole

Answer 29

Coronary flow decreased due to "systolic squeeze" (Psurround \> Pdiastolic in coronaries) Flow begins to increase again in diastole

Question 30

When does coronary perfusion occur

Answer 30

Diastole

Question 31

Describe the phasic nature of coronary flow

Answer 31

Flow increases during early diastole and follows aortic pressure

Question 32

Where does "systolic Squeeze" have the greatest effect

Answer 32

the left ventricle (occurs to a lesser extent in the low pressure right ventricle)

Question 33

Is phasic flow more pronounced in subendocardium or subepicardium?

Answer 33

Subendoardium due to systolic squeeze (still occurs in the subepicardium but to a lesser extent)

Question 34

Effect of Aortic Stenosis and coronary perfusion

Answer 34

Increases left ventricular work but will NOT increase coronary perfusion pressure or flow - creates and unfavoralble supply/demand ratio

Question 35

What is the most important determinant of Coronary Blood Flow

Answer 35

Myocardial oxygen consumption

Question 36

Autonomic control of coronary flow

Answer 36

1.) Direct effects - innervated by both sympathetic (vasoconstriction) and parasympathetic (modest vasodilatio) 2.) Indirect effects- MORE IMPORTANT- Metabolic vasoregulation

Question 37

Coronary Vasoregulation

Answer 37

Changes in metabolic demand due to changes in HR, contractility, or blood pressure are far more important in determining vascular caliber and myocardial blood flow

Question 38

Adenosine Hypotheis

Answer 38

Adenosine causes vasodilation in hypoxia

Question 39

what substrates can induce vasodilation in coronary arteries

Answer 39

Adenosine, hyperkalemia (K) , hypercapnia (high CO2), acidosis (H) , lactate, and prostaglandins

Question 40

Vascular Reserve (Coronary Autoregulation)

Answer 40

How much we can increase flow at a given pressure (difference between the autoregulating curve and the maximally vasodilated curve)

Question 41

Conditions of reduced coronary reserve

Answer 41

1.) Lower perfusion pressures (low diastolic pressure) 2.) Chronically higher MVO2 (increased flow) 3.) Increased fixed resistance (eg: atherosclerosis)

Question 42

Which has less fow reserve, subendo or subepi? What is the risk of this?

Answer 42

Subendo has less flow reserve - greater risk of ischemia, injury, and infarction