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