Myocardial Performance Flashcards
Explain the steps through which the parasympathetic nervous system reduces the heart rate
- PNS releases acetylcholine in the SA node
- This causes the funny Na current to be reduced, increasing the slope of phase 4 and decreasing the heart rate
Define preload and how do you measure it
It is the stretch of the myocardial fibers before contraction. We refer preload to ventricular filling.
Stretch is difficult to measure and hence use seveal other variables
- EDV
- Venous return
- End Diastolic Pressure
What is afterload and how is it measured
The ventricular tension during ejection. It is the resistance that must be overcome to eject blood.
It is given by Law of LaPlace
Wall stress = Pressure x Ventricular radius/2*wall thickness
This is not easy to measure either but one way to measure is to check for pressure so we can either look at aortic systolic or diastolic pressure
What is a fundamental different in cardiac cells and skeletal muscle cells when they produce a power stroke
When skeletal muscle cells are made to depoalrize, all of the Ca in the ER is released and binds to calmodulin that results in a power stroke. However for cardiac cells there is a submaximal level of Ca in the cytoplasm so not all of the myosin heads form due to lack of Ca. This means that anything that will increase [Ca] will increase the contractile force, termed as CONTRACTILITY, of the heart, increasing it inotropic state
What causes and increase in inotropic state and what is the mechanism for it
Norepinephrine, released from sympathetic nerves, causes an increase in inotropic state by allowing more Ca into the cadiac myocytes
Starling’s Law and what causes this
It states that stroke volume increases when preload increases. Associated with this law is the fact that there is a higher stroke volume not due to increasing [Ca] in the cardiac myocytes but because of the fact that there is an increased force of contratction due to stretching of the sarcomeres, there is more favourable overlap of thick and thin filaments.
It is important to know how this graph looks like
His explanation on increasing stroke volume and increasing force.
What is an important point to remember regarding the increase in force
The increase in force is not because of an increase in inotropic state (which is only possible due to increased [Ca]) but it is due to more favourable thick and thin filaments alignment.
What deterimines preload
- Filling time, which is determined by heart rate (ventricular diastole)
- Rate of venous return: venous tone, blood volume and gravity.
Blood volume can be reduced in hemorrhagic shock or in dehydration and it can be increased in hypertension
What causes point 1 to 3 shift and point 1 to 2 shift
Point 1 to 3 will be tachycardia or any other reason reducing venous return
Point 1 to 2 will be heart rate is slowed, higher EDV and contracting veins.
How does intracellular [Ca] changes along the Starling curve
It remains the same
How does the Stroke volume against End Diastolic volume curve look like for different afterlaods.
What conditions will increase afterload
Ejection fraction increases with an decrease in after load.
Afterload will increase if the pressure in the aorta increases and decreases if the pressure in the aorta decreases.
Afterload is a resistive force, it has to be overcome to eject the blood out
What happens to afterload when epi is injected
Arterioles dilate which causes the pressure in the aorta to decrease which then results in the afterload to decrease.
The opposite happens when you inject a vasoconstrictor
What happens to the afterload in the following
- Anaphylaxis
- Aortic valve stenosis
- Hypertension
- Giving vasoconstrictor
- Decreases
- Increases
- Increases
- Increases
Inotropic state on starling curve
What drug would cause a decrese in inotropic state
Beta blocker
How does cardiac glycosides increase the force of contraction (cause an increase in inotropic state)?
Cardiac glycosides like digoxin can cause the Na-K pump to stop working. As a result the concentation of Na ions increases inside the cardiac myocytes which causes the Na-Ca pump to activate which pumps out 3Na for every Ca. This causes an increase in intracellular Ca and hence increasing the inotropic state of the heart
Summary of Starling’s Law
Definitions for Starling Reserve and Inotropic Reserve
- Maximum increase in stroke volume by increasing preload is called the starling reserve
- Maximum increases in stroke volume by increasing the inotropic state is called the inotropic reserve
Explain this graph
We are mainly concerned with P0 graph, the blue graph. This is the maximum pressure that the ventricles can develop with ejecting any blood (when the aorta is clamped).
Increase in end diastolic volume will cause an increase in stroke volume as the reasoning associated with starling’s law
What is stroke work and how do you estimate it from the graph
The area under the pressure volume loop, it is MAP times the stroke volume
What is he trying to explain here
What is an important principle to remember here
If the diastolic pressure in the aorta is increased then the volume of the ejected blood decreases as shown by the red curve. The reasoning beind that is P (zero) - DP is decreased (as shown by the really really red dot situated vertically above the diastolic pressure).
An important relationship to remember is that the volume of ejected blood is proportional to the [P(zero) - DP]
This is similar to an increase in afterload except that here the increase in aotirc DP has veen exaggerated to show the difference in volume of ejected blood.
In a closed pressure loop what represents the stroke volume
The width of the curve
What happens to systolic pressure when the graph changes from blue to red
SP decreases as the EDV is decreasing and the CO is decreasing for the same inotropic state
What will cause this
Increase in TPR (like in hypertension)
What changes are happening over time as the blue curve is shifting towards the red
The blue curve will shift to red in higher TPR when for example in systemic hypertension. Initially the SV was larger, so a larger venous return will come to the heart. This will cause the red curve to get wider and the SV will then increase. This leads to strectching of the heart and will cause hypertrophy.
Why cant this be a hemorrhagic shock
EDV would be less
This is an example as to what happens in anaphylaxis
Are there any signs of hypertrophy here
No since the bottom curve has stayed the same, when there is hypertrophy the heart becomes stiff and the bottom curve shifts.
So the change in P zero line has to be due to an increase in inotropic state, not due to hypertrophy
Is this acute or chronic
Chronic, evidence of hypertrophy
