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99 Cards in this Set

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  • Back
What is cardiac output?
The quantity of blood pumped into the aorta by the heart
What is normal resting CO?
What factors result in variations of resting CO between patients?
1. Body size/age/metabolism
2. Level of activity
What is Cardiac Index?
Cardiac output per square meter of body surface area.
What is the normal peak CI for humans and at what age?
At age 10 - 4L/min/square meter
What is the typical CI at age 80? What does it indicate?
2.4L/min /sq meter - indicates declining activity and metabolic activity.
What is the main determinant that regulates CO?
Venous return
Ohm's law for bloodflow:
F = P/R
Ohm's law for CO:
Why doesn't Ohm's law for CO quite accurately describe it?
Because TPR is the resistance of arteries and MAP is the arterial bp; CO is determined by VENOUS characteristics.
How are the venous and arterial blood comparments different?
1. Volume (low in arteries/high in veins)
2. Pressure (high in arteries/low in veins)
3. Compliance (low in arteries, high in veins)
What is Frank-Starling's law of the heart?
Increased blood flows to the heart stretches the walls and increases its effectiveness
2 results of increased volume in the heart:
1. Increased pumping (contractility)
2. Increased heartrate
How does heartrate increase when the heart stretches?
Stretch has a direct effect on the sinus node to increase its rythmic rate by 10-15%.
What is the Bainbridge reflex?
A nervous reflex initiated by the right atrium that passes to the Vasomoter center of brain and back to the heart via SNS and Vagus nerves to incr HR.
What controls cardiac output under most normal unstressful conditions?
Peripheral factors that determine Venous Return
What is Venous Return equivalent to?
The sum of all blood flows through all segmnts of the peripheral circulation
What will make the bloodflow to most tissues increase?
Increased oxygen consumption (metabolism / activity)
What 2 variables change when tissues have increased metabolism and activity to causes CO to increase?
-Bloodflow increases
-VR increases
2 mechanisms for increasing bloodflow in local tissues:
1. Vasodilator release
2. Ischemic response - to increase oxygen levels
What 2 variables does CO change in proportion to? How?
1. TPR - oppositely but equally
2. O2 consumption = parallely
2 Components of CO REGULATION:
1. Cardiac (limiting when the amt of VR overcomes the heart's capacity to pump it)
2. Peripheral bloodflow
Normal max cardiac output:
13 L/min
Hypereffective max CO:
25 L/min
Hypoeffective heart max CO:
5 L/min
What does it mean that that is a normal maximum cardiac output?
The heart would never pump more than 13L/min without external factors increasing its effectiveness.
2 Factors that cause Hypereffective heart:
1. Nervous stimulation
2. Hypertrophy of heart muscle
What TYPE of nervous stimulation causes a hypereffective heart?
1. SNS activation
2. PNS inhibition
2 Effects of nervous stimulation of the heart:
1. Increased heartrate
2. Increased contractility
How can heart hypertrophy cause hypereffectiveness of the heart?
The increased muscle mass increases its contractile strength
What can increase the heart's effectiveness by increasing hypertrophy?
Long-term increases in workload - but not so much that it damges the heart!
What does liard call the max CO of the heart?
Maximum permissive pumping
What is the exact effect of SNS stimulation on the heartrate and contractility?
HR = 180 beats/min
Contractility is 2X normal
What does liard call "hypertrophy"?
Increased muscle mass by 50-75%
What is the effect of SNS and hypertrophy together?
Increases CO to 30-40L/min
How much higher than normal CO is 30-40 L/min?
2.5 times
8 factors that cause hypoeffective heart:
1. Inhibition of Nervous stim
2. Abnormal heart rythms/rate
3. Valvular heart disease
4. Increased MAP (HTN)
5. Congenital heart disease
6. Myocarditis
7. Cardiac anoxia
8. Diptheria/toxicity
What is the role of the nervous system in controlling CO?
It maintains arterial pressure when VR and CO increase.
Metabolic stimulant that greatly increases CO
What happens when you give a dog DNTP and its nervous system is not intact?
The VR will increase and CO will increase but b/c there are no nerves to maintain arterial tone, MAP falls profoundly and CO fails to increase adequetely.
What is the effect of giving DNTP to a dog with an intact nervous system?
CO increases 4X because the maintained tone of arteries keeps blood returning to the veins/heart.
How does the nervous system function to effect CO during exercise?
It maintains the arterial pressure as increased metabolism releases vasodilatory substances so CO remains adequete and increased.
Other than changing the heart's max permissive pumping ability what can control the heart contribution to CO regulation?
Changes in the transmural pressure of the heart.
What has the highest volume:
1. Container w 100:90 transmural pressure (inside:outside)
2. Container w 10:0 transm press
3. Container w 0:-10 trnsm press
They all have equal transmural pressures, so their volumes are equal
How does increasing pressure outside of the heart change its ability for cardiac output?
Increasing pericardial pressure compresses the heart from the outside and limits its ability to pump.
What are the axes of the cardiac output curve?
X = right atrial pressure (RAP)
Y = cardiac output
How does increasing intrapleural pressure change CO?
Increased intrapleural pressure increases the amt of right atrial pressure needed to fill the heart chabers with blood, and decreases cardiac output.
How does decreasing pleural pressure affect CO?
It decreases the required RAP for cardiac chamber filling, hence increases CO. Eg inhaling; shifts CO curve to the left.
What happens to transmural pressure if you crack open the chest?
All of a sudden the negative pressure normally in the chest cavity is lost; transmural pressure decreases so CO decreases.
How does the Cardiac output curve change when both transmural pressure AND heart effectiveness are changed?
1. Hypereffective/increased intrapleural pressure will increase CO plateau even more than 15L/min!
2. Hypoeffective/decreased intrapleural pressure decrease CO Plateau to ~5 L/min
What are the 3 Factors that affect Venous Return?
1. RAP - right atrial pressure
2. Psf - Mean systemic filling pressure
3. RVR - resistance to venous return
What is RVR?
Resistance to the flow between peripheral vessels and the right atrium.
What is Psf?
Mean systemic filling pressure - the pressure on the arteries when the heart is not pumping and the blood is just sitting there in equilibrial stasis.
How is Psf different from mean circulatory filling pressure?
Mean CIRCULATORY filling pressure includes pulmonary circulation; they're not that different so we just use Psf.
What are the axes of the VR curve?
Y = VR
So what does the venous return curve show us?
The flow of blood from the veins into the heart at different levels of right atrial pressure.
How does VR change as RAP increases? Why?
VR decreases due to the backward force of rising atrial pressure on the veins flowing in.
Why does VR fall to zero if RAP reaches 7 mm Hg?
Because that is equal to Psf and at that pressure there is no pressure gradient causing flow to continue.
What would happen if RAP increased beyond 7 mm Hg?
Blood would actually dam up in the veins instead of returning to the heart.
Why is there a plateau of venous return when RAP is greatly reduced below zero?
Because as the pressure in the right atrium goes lower, it creates almost a vacuum effect that causes the veins to collapse; at that point Venous return is max and can't increase anymore.
What does the downslope of the venous return curve represent?
The inverse of RVR - the steeper it is, the easier blood flows into the heart; the shallower it is, RVR is higher.
What does Psf take into account?
-The low compliance and high pressure of arteries
-The high compliance and low pressure of veins
Normal Psf
7 mm Hg
Equation for VR
RAP - Psf
VR = ---------------
What is the effect of the pressure gradient on VR?
ESSENTIAL - there has to be a difference between RAP and Psf in order for blood to flow - that's why at RAP of 7 VR is 0
Steep slope of VR curve indicates:
low resistance to venous return (RVR)
What is normal RAP?
0 mm Hg
What does Psf reflect?
The degree of filling - fullness of the systemic circulation
2 main determinants of Psf:
-Blood volume
-Compliance (nervous stim)
How does nervous stimulation affect Psf?
-SNS stimulation increases Psf by reducing all vessels capacity
-SNS inhibition has opposite effect
How do SNS stimulation and inhibition affect the volume pressure curve for the entire circulatory system?
-SNS stim shifts it to the left so even at lower blood volumes higher Psf will be seen
-SNS inhibition shifts it to the right
How steep is the volume pressure curve for the circulatory system?
Very steep; even little changes in blood volume cause significant changes in Psf.
At normal blood volume how much does maximal SNS stimulation increase Psf?
Up to 17 mm Hg - that's 2.5 times the normal value of 7!
How much does complete inhibition of the SNS decrease Psf?
Down to about 4 mm Hg
Equation for Psf:
Total volume (stressed)
Psf = -------------------------
Total compliance
Why do we push fluids in order to increase cardiac output?
Because it increases Psf which increases the pressure gradient for bloodflow and increases VR.
How does increasing blood volume (i.e. transfusion) alter the VR curve?
It increases Psf (x axis) so shifts the curve to the right and increases VR (y axis)
What is normal VR?
About 6 L/min
How much does VR increase when Psf goes up to 14?
To about 12 - doubles
How much does VR decrease when Psf goes down to about 3.5?
VR also goes down to about 4
Give the VR formula once more:
Where is the most RVR? How mch? Why?
In the veins - ~2/3; b/c they have high capacitance.
Where is the most TPR?
On the arterial side
How do you calculate RVR?
Rveins + Rarteries/26
Why do we divide arterial resistance by 26 to calculate RVR?
Because that is the ratio of compliance of arteries vs veins
The resistance component for venous return is dominated by:
The resistance of veins
How does changing RVR affect the venous return curve?
Increasing RVR makes the slope much more SHALLOW and decreases VR; decreasing RVR has opposite effect.
What is the effect of increasing RVR by 2X?
It decreases VR by 1/2
What is the effect of decreasing RVR by 1/2?
It increases VR by 2X
Highest level to which RAP can rise:
7 mm hg - it simply can't go any higher because that is Psf.
What is the best way to increase venous return?
-Decrease RVR (increase slope)
-Increase Psf (give blood)
What is a less prominent but also good way to increase VR?
Decrease RVR - even if Psf is decreased (SNS inhibited) the VR will be higher.
What will cause a decreased VR?
-Loss of blood or SNS inhibition -> results in a reduced Psf
-Increased RVR
What must be the relationship between CO and VR?
They MUST be equal or else blood accumulates in or is removed from the chest.
What would be the case if LV output was 7 (CO) and VR were 6?
Blood would be shifted to the periphery
Why does giving a blood transfusion increase CO?
Because it greatly increases VR and VR always equals CO.
What would happen if you a transfused a blood volume equal to 20% of the blood volume?
CO would increase to 13 L/min which is the max permissive pumping ability of the normal heart, and Psf would increase to 16 mm Hg.
On the combined curve what does the equilibrium point indicate?
-Right atrial pressure
-Cardiac output
-Venous return
What are the normal CO/VR and RAP values?
CO = 5
VR = 5
RAP = 0