Cardiac Mechanics (B2: W2) Flashcards
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
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
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
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
How do you figure out the resting tension in the heart?
- Inhibit calcium channels
OR
- Remove calcium from extracellular space
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
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
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
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
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
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
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
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
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
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
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
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
How do you calculate ejection fraction?
Ejection fraction = Stroke volume/Total volume
In this example:
80/120 = 66.6%
What happens to the ejection fraction if you increase preload?
Increases ejection fraction
Pumps harder
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
What happens to contraction if you increase resting volume?
As you increase resting volume, you increase force of contraction of the heart
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
What happens to the pressure-volume loop if preload is decreased?
Decrease preload by decreasing Left ventricular volume
- Smaller pressure
- Smaller stroke volume
What happens to the pressure-volume loop whith an increase in the afterload?
Aortic pressure is higher - peripheral resistance has increased
- Decreased stroke volume
What happens to the pressure-volume loop if there is a decrease in the afterload?
Increased stroke volume
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
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
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)
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
What happens to the energy of the heart when there is no load?
Much of the energy is just basal metabolism - generates heat
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
