4 - Cardiac and Venous Return

Question 1

What is Venous Return (VR)?

What is Cardiac Output (CO)?

How are these related under steady state conditions?

Answer 1

VR = Amount of blood that enters right atrium / minute

Controlled by sum of all factors regulating blood flow

CO = Amount of blood pumped into aorta / minute

Controlled by factors that regulate blod flow and pumping ability of the heart

Under steady state; VR = CO

Question 2

To diagram cardiac output, what two values would you plot against?

What does the plateu of these curves represent?

Answer 2

Left cardiac output (LV)

vs

Right atrial pressure

Plateau measures maximum cardiac output; e.g. there is a max atrial pressure that can continue to influence CO; this is about 2.5 times normal resting CO

Question 3

What are some conditions which can cause the heart to become hypereffective?

What are some conditions which can cause the heart to become hypoeffective?

Answer 3

Hypereffective: Sympathetic Stimulation, Athletic Conditionion

Hypoeffective: Decreased sympathetic stimulation, Increased atrial pressure (hypertension), Reduced ventricular filling, Heart disease, Coronary hypoxia

Question 4

Equation:

How do you calculate Cardiac Output in regards to peripheral resistance?

Answer 4

Cardiac output varies inversely with peripheral resistance!

CO = (MAP - CVP)/SVR

MAP = mean arterial pressure

CVP = central venous pressure

SVR = systemic vascular resistance (total peripheral resistance)

Increase SVR, decrease CO

Question 5

How does venous return compare to right atrial pressure?

Why does this plateau at low pressures?

Answer 5

Inverse relationship

As right atrial pressure increases, venous return decreases

At low pressur, large beins in the thoracic cavity collapse, causing venous return to level off

Question 6

What is the mean systemic filling pressure (PSF)?

What does the normal difference in this value equate to?

What is PSF dependent on?

Answer 6

Point on the Vascular Return Curve where the venous function curve crosses the X axis–Cardiac Output (CO) = 0

Difference between PSF and right atrial pressure is the driving force for venous return to the heart

PSF if depends on blood volume (increase PSF) and vascular compliance (decrease VC = increase PSF)

Question 7

What conditions directly alter the venous return curve?

Answer 7

1. Change in blood volume
2. Change in venous tone
3. Resistance to venous return

Question 8

Clinical:

How would an uncompensated loss of blood volume (hemorrhage or dehydration) shift the PSF? (and the entire venous return curve)

How would a decrease in venomotor tone (increase in venous compliance–shift blood from arteries to veins) shift the PSF?

Answer 8

Hypovolemia or increase in venous compliance will:

Decrease venous return, shifting plot to the LEFT, lowering PSF

Question 9

Clinical:

How would an increase in blood volume affect the systemic filling pressure (PSF)?

How would an increase in venomotor tone (decrease in venous compliance–shift blood to arteries) affect PSF?

Answer 9

Increase in blood volume (transfusions, mobilization of blood resevoir via sympathetic activation) or increase in venomotor tone:

Will increase PSF, shift venous return curve to the right (increase venous return)

**Hypervolemia / Decrease in Venous Compliance**

Question 10

What determines the slope of the venous return curve?

Remember–what is the Y-axis on this plot?

Answer 10

The resistance to venous return determines the slope of the venous return curve

Decrease Resistance = Increase Slope

Increase Resistance = Decrease Slope

Note: PSF DOES NOT CHANGE!

- - -

The Y-axis is Venous Return! So it’s common sense–increase resistance, you’ll have less venous return!

Question 11

How is Systemic Vascular Resistance (SVR) and viscosity related to Resistance to Venous Return?

Answer 11

Direct

Increase SVR = Resistance to Venous Return

Increase Viscosity = Resistance to Venous Return

Question 12

When plotting Cardiac Output on the same plot as Venous Return, what is the intersecting point?

What impact would an increase in sympathetic activation have on the plot?

Answer 12

Intersection = Equilibrium Point, or

Cardiac Output (CO) = Venous Return (VR)

Cardiac Output would increase, equilibrium would shift higher on the venous return curve to compensate; Right Atrial Pressure (x-axis) would DECREASE

Question 13

How does Left and Right Atrium pressure relate to Aortic Flow (especially under sympathetic stimulation)?

Answer 13

Inversely! As Aortic Flow (Cardiac Output) increases, Atrial Pressure decreases

Question 14

What effect would in increase in blood volume (hypervolemia) have on CO and RA Pressure (venous return curve)?

Answer 14

Increase Right Atrial Pressure due to increased venous return

CO curve does NOT change, but the equilibrium point will increase

Question 15

What effect will an increase in Systemic Vascular Resistance have on Cardiac Output and Venous Return?

Answer 15

Increased SVR Will:

1. More blood on arterial side (sympathetics control arterial tone, increased resistance = shrinking these vessels)
2. CO Shift down (increased afterload)
3. Venous Return rotates about PSF counterclockwise

= New equilibrium with both values decreased

Question 16

What effect will a decrease in vascular resistance have on Cardiac Output and Venous Return curves?

Answer 16

Decreased SVR will:

1. More blood in venous side of heart = clockwise rotation of VR curve
2. Decreased afterload = Increase CO Curve
3. Equilibrium point increase for both

Question 17

What is the difference between compensated and uncompensated heart failure?

Answer 17

Compensated involves sympathetic-induced vasoconstriction; decrease CO with increase in venous return—BUT decrease is CO is GREATER in UNCOMPENSATED

So it’s bad, but not AS BAD due to compensation of vasoconstriction

Question 18

What effect will SVR have on the plot of CO and VR?

Answer 18

Increase both plots