Cardiac Mechanics (B2: W2) Flashcards Preview

Physiology > Cardiac Mechanics (B2: W2) > Flashcards

Flashcards in Cardiac Mechanics (B2: W2) Deck (36):
1

What is the relationship between the radius of the cavity of the heart and its volume?

A 50% reduction in the radius would lead to an 87% decrease in volume

 

2

What is the relationship between wall tension and pressue inside of the heart? What about between tension and radius of the ventricular cavity? 

Wall tesnion is directly proportional to the pressure and the radius

Imagine a balloon as it is filled with air

3

What is the relationship between wall stress and wall thickness?

Wall stress is inversely related to wall thickness

Tension is alleviated by wall thickness! 

When the heart contracts, wall thickness increases, relieving the tension produced by the pressure of ventricular contraction

4

At what sarcomere length do the cardiac muscles have optimal tension for contraction? 

~2 micrometers

B and C in the figure

Too stretched or too condensed: lose ability of polar heads to interact with actin

5

How do you figure out the resting tension in the heart?

  • Inhibit calcium channels

OR

  • Remove calcium from extracellular space

6

How does stretching muscle relate to the force of contraction?

As you stretch, you increase force of contraction

A large component of this is passive - a smaller component is active tension

If you stretch too much, you lose force 

7

Which is more stiff: cardiac or skeletal muscle? 

Heart muscle is stiff relative to skeletal muscle - Force increases rapidly

 

Heart is working near the peak of the action tension curve in this figure

8

How do you evaluate cardiac contractility? 

  • Changes in ventricular pressure (P)
  • Rate of changes in ventricular pressure (±dP/dt)

 

Figure: 

A) Steady state

B) ß adrenergic stimulation 

9

What is the relationship between force of contraction and velocity of contraction? (How fast does the muscle contract as a function of load)

  • At no load - maximum velocity
  • More load - slower velocity
  • Until the load becomes so heavy that the velocity is zero 

10

What happens to the blood being pumped by the heart if you increase the afterload?

Afterload is the force that the heart has to work against

As you increase the afterload, the amount of blood pumped per unit decreases exponentially

11

What happens to the velocity and force of contraction in the case of congestive heart failure? In the case of ß adrenergic stimulation?

  • With CHF
    • The maximum velocity is lower
    • The amount of blood that the heart can pump is lower
  • With ß1 adrenergic receptor agonist
    • Increase maximum velocity
    • Increase maxmum load

12

How do you analyze the mechanical function of the heart?

Pressure-volume loop tells us how effective the contraction of the heart is

Intraventricular pressure as a function of Left ventricular volume

Puple curves: range of possible contraction expressed as a volume

13

What does curve D of the pressure-volume loop represent?

Phase I - period of filling 

  • At the end of this diastole there is a rest 
  • Pre-load at this point determines the amount of contraction

14

What does curve A on the pressure-volume loop represent?

Phase II - Isovolumic contraction

  • Rapid elevation in pressure
  • No change in volume
  • Ends in afterload - the amount of resistance that ejection has to overcome

15

What does curve B of the pressure-volume loop represent?

Phase III - Period of ejection

  • Pressure in ventricle exceeds that of the aorta
  • Increase in force and pressure
  • Ends point is end-systole

16

What does curve C of the pressure-volume loop represent?

Phase IV - Isovolumic relaxation

  • Pressure in ventricles is less than that of the aorta
  • No change in volume

17

How do you calculate stroke volume from the pressure-volume loop?

Subtract end systolic volume from end diastolic volume

In this example: 120 - 40 = 80 mL

18

How do you calculate ejection fraction?

Ejection fraction = Stroke volume/Total volume

In this example:

80/120 = 66.6%

19

What happens to the ejection fraction if you increase preload?

Increases ejection fraction

Pumps harder

20

What is Starling's Law of the heart?

As you increase end diastolic volume, you increase contraction of the heart

Increase ability to generate pressure and eject blood into aorta

21

What happens to contraction if you increase resting volume?

As you increase resting volume, you increase force of contraction of the heart

22

What is preload? What does an increase in preload cuase?

Preload is the volume at the end of diastole

Increasing preload can

  • Increase stroke volume
  • Increase venous return
  • Return more blood to the left atrium
  • Create more stretch in the left ventricle --> more pressure

23

What is afterload?

Afterload is the force of resistance that you have to fight against

Aortic pressure

24

What happens to the pressure-volume loop if you increase preload?

You increase preload by increasing Left ventricular volume

  • Increase pressure
  • Stroke volume is increased

25

What happens to the pressure-volume loop if preload is decreased?

Decrease preload by decreasing Left ventricular volume

  • Smaller pressure
  • Smaller stroke volume

26

What happens to the pressure-volume loop whith an increase in the afterload?

Aortic pressure is higher - peripheral resistance has increased

  • Decreased stroke volume

27

Why does stroke volume decrease with an increase in afterload?

  • Less ejection time
  • Opens later to overcome afterload
  • Closes faster because pressure in ventricle is going to be lower than in aorta sooner 

28

How would you try to balance out the change in afterload?

Increase preload!

If this situation goes on chronically for years due to hypertension, it builds mass

29

What happens to the pressure-volume loop if there is a decrease in the afterload?

Increased stroke volume

30

How does increased contractility of the heart by stimulation of ß adrenergic receptors impact the pressure-volume loop?

Upward shift

  • Heart contracts more powerfully
  • Increase in pressure and stroke volume 

This is autonomic regulation

31

How does a decrease in ionotropy (contractility) affect the pressure-volume loop?

Lower end systolic pressure-volume relationship

  • Expands to try to increase preload 
    • Tries to generate more force
    • Trying to maintain constant stroke volume

This is seen in heart failure 

32

How do you calculate stroke work?

Stroke Work = Pressure x Stroke Volume

or

Stroke Work = P x (EDV - ESV) 

This is the external work accomplished by the heart (yellow area of the pressure-volume loop)

33

Which ventricle exhibits more stroke work? 

Left ventricle exhibits more stroke work than right

  • This is due to pressure
  • Right side has to build less pressure to eject blood into the pulmonary circulation

34

Does atrial pressure affect stroke work?

Yes 

Increase in stroke work with an increase in mean atrial pressure

More atrial pressure leads to more volume in the ventricle, produces more external work 

35

What happens to the energy of the heart when there is no load?

Much of the energy is just basal metabolism - generates heat

36

What happens as you increase the load of the heart?

Heart contracts 

Optimal range occurs where work increases and heat decreases

As you increase afterload, there is no work and just heat