7- Cardiac Output Regulation: Vascular Mechanisms

Question 1

what determines cardiac output (CO)

Answer 1

-cardiac output is proportional to right atrial pressure (in steady state, venous return is equal to CO- amount of blood moved out in periphery or being absorbed)

• heart and vasculature
• if we know preload and “state” of heart then we know CO

Question 2

Preload

Answer 2

• end diastolic volume (related to right atrial pressure)

- determined by volume of blood in circulation and resistance and compliance of blood vessels

Question 3

cardiac tamponade

Answer 3

filling of pericardium with fluid that can compress the heart and impact right atrial pressure

Question 4

normal interpleural pressure

Answer 4

-4 mmHg w/ right atrial pressure starting around -2 mmHg

Question 5

what happens when right atrial pressure increases?

Answer 5

amount of blood returning to heart decreases

-during respiratory inspiration the venous return increases because of a decrease in right atrial pressure

Question 6

Right atrial pressure vs.

• Cardiac Output
• Venous return

Answer 6

• right atrial pressure and cardiac output have a direct relationship
• RAP and venous return have an indirect relationship

Question 7

Key points of the venous return curve

Answer 7

1) Mean systemic pressure – depends on blood volume and vasculature compliance
2) Slope of the curve–depends on vascular resistance
3) Plateau–depends on the collapse of the thin walled veins in the chest

Question 8

mean systemic filling pressure (Psf)

Answer 8

7 mmHg (right amount of blood for body in system)

-fullness of systemic circulation (depends on blood volume)

Psf = total volume / total compliance

Question 9

Cardiac Output regulation (2 primary factors)

Answer 9

1. pumping ability of heart
   - hypereffective: increased intrapleural pressure (increase in contractility)
   - hypoeffective: reduced intrapleural pressure (decrease in contractility)
2. venous return
   - if you increase resistance it decreases venous return and if you decrease resistance you get more venous return

Question 10

sympathetic stimulation and inhibition is on what side of systemic circulation

Answer 10

mainly on venous side of circulation

Question 11

REVIEW THIS LECTURE IN TOTAL CAUSE OF ALL THE GRAPHS

Answer 11

REVIEW THIS LECTURE IN TOTAL CAUSE OF ALL THE GRAPHS

Question 12

To think about cardiac output think about the aspects of the circulation that can have a big effect.

Answer 12

1) Cardiac contractility–changes the slope and “maximum permissive level” of the cardiac function curve
2) Arterial resistance-changes the slope of the Venous return curve– more resistance decreased negative slope (i.e. shifts counter clockwise)
3) Venous compliance–changes the intercept of the Venous return curve – more compliance = decreased pressure of the intercept
4) Blood volume–changes the intercept of the Venous return curve– more blood = increased pressure of the intercept

Question 13

Arteriovenous fistula

Answer 13

A-V fistula or A-V shunt

-connection b/w arteriole and venous side of circulation (can happen in heart and peripheral system)

• very low resistance pathway from the arteries to the veins
• It can happen at any level in the periphery including in the large vessels or in the organs.
• In order to understand what happens think about blood shifting from the arterial to the venous side as the resistance decreases

Question 14

Fick principle for determining CO

Answer 14

O2 consumption / [O2 in pulmonary vein - O2 in pulmonary artery]

Question 15

blood volume determined by?

Answer 15

difference between water and sodium intake (drinking) and excretion (urine formation)

• has a direct impact on cardiac filling
• cardiac filling is preload that helps determine CO

Question 16

Atrial pressure =

Answer 16

cardiac output x total peripheral resistance

TPR is determined by nerves, heroines and local mechanisms

Question 17

what happens when venous return increases

Answer 17

end-diastolic volume (preload) increases and stretches or lengthens the ventricular muscle fibers

Question 18

frank-starling relationship

Answer 18

describes increases in stroke volume and cardiac output that occur in response to increase in venous return or end-diastolic volume

Question 19

increases in contractility cause

Answer 19

increase in cardiac output for any level of right atrial pressure or end-diastolic volume (preload)

Question 20

decreases in contractility cause

Answer 20

decrease in cardiac output for any level of right atrial pressure of end-diastolic volume (preload)

Question 21

length-tension relationship in ventricle

Answer 21

increases in end-diastolic volume (preload) cause an increase in ventricular fiber length, which produces an increase in developed tension.

Question 22

venous return and cardiac output

Answer 22

the greater then venous return the greater the cardiac output