Cardiac output and Venous Return

Question 1

An increase in preload will ___ cardiac ouptput and cause a __in afterload, which will __

Answer 1

Increased preload increases CO and increases afterload, which will then decrease CO

Thus, preload and afterload are ‘coupling factors’

Question 2

What’s the relationship between contractility & afterload and the Frank-Starling curve?

Answer 2

Increased contractility shifts the curve up

Increased afterload shifts the curve down

Question 3

How do ACE inhibitors restore contractility after heart failure?

Answer 3

reduces afterload

Question 4

venous compliance > arterial compliance

Under equilibrium conditions,

• Flow across resistance (Qf) =
• Arterial blood volume and venous blood volume are ____

Answer 4

Flow across resistance (Q_f) = Flow exiting the heart (Q_h)

Arterial blood volume (V_a) and Venous blood volume (V_v) are constant.

Question 5

What happens when you have a heart attack (Q_h = 0)?

Answer 5

• P_a begins to fall and P_v rises as blood flow continues
  
  • Because veins have higher compliance, a 20mmHg drop in Pa is accompanied by a 1mmHg increase in Pv

Question 6

Describe the steady state where Qh = 0

Answer 6

• Pv = Pa = static pressure
• Pmc = the y-intercept of the CVP vs CO curve when CO=0

Question 7

If we restart theheart after failure so blood flwo begins again, what happens to Q and P’s?

Answer 7

• Qh restored
  
  • depletes V_v, causing P_v to drop
  • Increases Pa
• Resistance initially prevents flow from the arterial to the venous compartment.
  
  • Flow across the resistance (Qr) =0
• Qr will rise above 0 when Pa=26 and Pv=6

Question 8

Describe the vascular function curve when

• flow is 0
• when flow restarts
• when flow is “too high”

Answer 8

• At Qh=0, central venous pressure maxes out because most of the blood is in the veins
• When flow starts again, blood leaves the veins and central venous pressure falls
• If too much blood leaves the veins (CO is too high), they’ll collapse under the external pressure

Question 9

What does increasing or decreasing blood volume do to the CVP vs CO curve?

Answer 9

Increasing blood volume (e.g. transfusions) increases the mean criculatory pressure and the CO at which the vessels collapse -> shift the curve rightward

Decreasing blood volume (e.g. hemorrhage) shifts the curve leftward

Question 10

How do changes in peripheral resistance affect the curve?

Answer 10

• The equilibrium that’s reached under Qh=0 conditions is the same, so y-intercept is the same
• Vasoconstriction means a greater P_a is needed for flow –> V_a increases –> V_v decreases –> P_v decreases, making venous collapse easier at lower CO’s
• Vasodilation is the opposite

Question 11

Plotting cardiac function (frank-starling curve) on the same graph as the vascular function curve shows how changing CO alters the equilibrium for the system.

What happens if you suddenly increase Pv?

Answer 11

Suddently increasing P_v (point A) causes an increase in CO (point B).

However, the increase in CO will be met with a drop in CVP that will then produce lower CO (point C).

Each subsequent cycle (D) will see a gradual return of both parameters to equilibrium

Question 12

If you increase sympathetic stimulation, you know this causes an upward shift in the cardiac function curve (increased CO). But what does it do the new equilibrium?

Answer 12

The new equilibrium has a higher CO!

Initial stimulation takes you to point B, but over time it equilibrates to point D

Question 13

Blood transfusions shift the equilibrium point where?

Answer 13

Up and right (point B)

It increases preload and thus increases CO.

Shifts only the vascular function curve to the right.

Question 14

How does increased peripheral resistance affect equilibrium?

Answer 14

Downward (lower CO)

Brings both curves down

Note: If you only looked at cardiac fxn curve, you might have though CVP would be increased.

Question 15

Progessing heart failure has what impact on the equilibrium?

Answer 15

Down and right

Heart failure lowers the cardiac fxn curve -> body compensates by fluid retention to increase blood volume (shift right), but eventually it reaches a point where it isn’t effective anymore.

_{<strong>A</strong>: normal equilibrium}

_{<strong>B</strong>: moderate heart failure –> lower CO}

_{–> compensated w increased fluid retention, restoring blood volume and CO (point <strong>D)</strong>}

_{As heart failure progresses, fluid retention is less effective at maintaining CO (point <strong>E)</strong>}