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The Coronary Circulation

Guyton Chapter 21

Coronary Circulation

� Heart blood

flow second only to kidney

on per gram

basis

� Heart weighs ~300 gms

� CBF ≈

250cc/min

Cardiac Hemodynamics

Region Mass

(kg)

Blood

flow

mL/m

Blood

flow mL/100

gm/m

O2

mL/100

g/m

%

Cardiac Output

% O2

Use

Brain 1.5 750 50-55 3.5 15% 20%

Kidneys 0.3 1240 420 6.0 25% 7%

Liver 2.6 1500 58 2.0 28% 20%

Heart 0.3 250 84 9.7 5% 12%

Whole

Body

63 5500 8.6 0.4 100% 100%

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Coronary Arteries

� Two main coronary arteries arise from root of aorta

� Large, low resistance

� Lie on surface of the heart (epicardium) –penetrate into cardiac muscle

Coronary Arteries

Left coronary a. supplies the LA, the anterior and lateral LV, and the right and left bundle branches

1. Left coronary artery

� anterior descending

� circumflex

Coronary Arteries

Right coronary a. supplies the RA, RV, posterior LV, SA node (60%), AV node (90%), and AV bundle

2. Right coronary artery

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Coronary Venous Blood

� Coronary Sinus

� Collects 75% of total

CBF; empties into RA

� Anterior Cardiac Vein

� Collects venous blood from RV; empties into

RA

� Thebesian Veins

� Collect blood from all

over the heart; empty directly into chambers

� Contributes to venous

admixture

Coronary Perfusion Pressure (CPP)

� Coronary arteries are perfused during diastole

Normal

Why does coronary perfusion occur

during diastole?

� CBF to LV falls during systole (opposite all other vascular beds)

� Why? Strong compression of LV muscle around coronary a. during contraction

� During diastole, cardiac muscle relaxes completely ⇒ ↑ blood flow

Note, still some blood flowduring systole i.e. flow doesnot decline to 0

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Epicardial vs. Subendocardial Blood

Flow

� During systole a pressure gradient develops within the heart muscle ⇒ highest pressure at endocardium and lowest pressure at epicardium

� Endocardium gets full force of all muscle layers and subendocardial vessels are compressed to the greatest degree

Subendocardial arterialplexus most extensive

blood vessel network

Epicardial vs. Subendocardial Blood Flow

� Subendocardial vessels are most vulnerable to decreases in coronary blood flow

Epicardial vs. Subendocardial Blood Flow

What factors impair subendocardial blood flow?

1. Low diastolic BP2. High LVEDP

• Direct compression of subendocardial vessels

• Decreased CPP due to higher opposing pressure to diastolic

pressure

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Control of Coronary Blood Flow

� Primary controller of coronary vessel resistance is locally generated metabolites

� Coronary blood flow ∝∝∝∝ metabolism

Cardiac Muscle

Metabolism

� Heart has the highest

metabolic rate (and ATP requirements) in body

� Normal basal myocardial O2

consumption 8-10

mL/100gm/min

� Normal O2 delivery 8-10 mL/100gm/min

� ↑ myocardial O2 demand met

by ↑ coronary blood flow, not by ↑ O2 extraction

Cardiac O2 supply vs. O2 Demand

O2 supply determined by:

� Diameter of coronary arteries

� LVEDP

� Aortic diastolic pressure

� Arterial O2 content

� Heart rate

� Diastolic time

O2 demand determined by:

� Heart rate

� Wall tension

� Preload (“volume work”)

� Afterload (“pressure work”)

� Myocardial contractility

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Cardiac O2 Supply vs. O2 Demand

Cardiac O2 Supply vs. O2 Demand

Anesthetic considerations

� Volatile agents in general are considered “cardioprotective”

� Coronary artery vasodilators (Isoflurane most)

� Decrease myocardial O2 requirements (↓metabolism)

� Vasodilation: ↓ preload, ↓ afterload

� May precondition the heart and reduce postoperative ischemia