Nordgren: Central Venous Pressure Flashcards
What components of the CVS system are crucial factors that determine how well the CV system will be operating?
- Ventricular EDV is VERY sensitive to small changes in filling pressure
- Cardiac filling pressure
How much volume does it take to fill the circulatory system if there is no pressure or stretching?
3.56 L
About 20 mmHg
What is our total systemic volume?
4.5 L
What does the extra volume in our system do?
The extra volume causes steretching and in turn pressure to build inside the system and …about 7 mmHG
What are the two major variables that affect mean circulator filling pressure?
- circulating blood volume
2. state of peripheral venous tone*
What does constriction of the venous vessels do?
Increase pressure throughout the system
What happens when there is increased tone in the other compartments?
- Arterioles don’t have much volume—very small effect
2. arteries and capillaries essentially do not change their volume (no effect)
How does cardiac pumping affect flow?
It shifts the location of some blood from venous to arterial.
What happens to veins and arteries when flow changes?
Veins- are VERY compliant, so pressure doesn’t change much in them when this happens
Arteries- NOT compliant (added blood causes pressure to rise above the mean circulatory pressure)
What is central venous pressure?
An indicator of circulatory status
How would you describe the CV system?
It’s a CLOSED hydraulic system. Changes in one area WILL cause changes in pressure, flow and volume throughout the circuit.
What is cardiac output?
The rate at which blood is LEAVING the central venous compartment (RA)
What is venous return?
The rate at which blood is COMING BACK to the central venous compartment (RA)
How are venous return and cardiac output related in a stable situation?
They’re equal!!
What happens when venous return and CO aren’t equal?
This produces a CHANGE in central venous pressure
What effects central venous pressure?
- Increased stroke volume
- decreased venous return
THEREFORE CVP is always driven to a value that makes CO = VR
What governs the flow between PV and CV compartments?
Q= change in pressure (drop between peripheral and central venous compartments) / resistance (small resistance associated with peripheral veins)
What does a crop in central venous pressure do?
It increases the pressure drop across the venous resistance and consequently causes an elevation in venous return
What does the venous function curve demonstrate?
As central venous pressure DECREASES, venous return INCREASES (to RA)
True or false…An increase in peripheral venous pressure can be just as effect at increasing venous return as a drop in central venous pressure.
True
What are two components that influence peripheral venous pressure?
- changes in blood VOLUME
2. changes in venous TONE
What does an increase in blood volume do to veins?
Increase in volume > increase in pressure (veins are elastic)
What does an increase in sympathetic activity do to venous tone?
Increase in sympathetic activity > increased vasoconstriction > increase pressure (external compression of veins does the same thing)
What happens to the CVS when there is significant blood loss?
- Increased sympathetic nerve activity (subnormal CO evokes compensatory mechanisms)
- Venous constriction after hemorrhage
Do cardiac sympathetic nerves affect the venous function curve?
NO
How does peripheral venous constriction affect CO after a significant blood loss?
It increases CO by raising the central venous pressure and moving the heart’s operation upward along a fixed cardiac function curve.
What is indicated by a pt with abnormally HIGH central venous pressure?
Depressed cardiac function, a RIGHT shifted venous function curve or both.
What is a hallmark of patients with CHF?
VERY high central venous pressure
What are the clinical implications of a pt with abnormally LOW CVP?
Must have an elevated cardiac function curve, a LEFT shifted venous function curve, or both.
How do you estimate CVP?
By observing the external jugular veins.
- Gravity keeps veins in head/neck collapsed when in upright position.
- When recumbent, w/ jugular at about 7 cm above RA can visualize distension.
How is CVP monitored in critical care settings?
It’s monitored continuously via a catheter inserted in a peripheral vein and advanced until it is in the central venous compartment.
What IS vascular control?
Change in the TONE of smooth muscle lining the walls of the vasculature.
What part of the CVS DON’T have smooth muscle?
capillaries
Why is the vascular smooth muscle unique?
- can maintain tension for a long time w/ little energy cost
- contract and relax slowly
- activated by stretch
When an organ responds to an increase in metabolic activity with a decrease in arteriolar resistance, this is known as?
Active hyperemia
What are the key components of the venous system?
Central
Peripheral
What is the volume that can be held in the peripheral venous compartment?
2.5 L
That’s a lota blood!
If you gave a blood transfusion to a pt who had recently experienced a severe hemorrhage you would expect?
To EXPAND venous volume–there will be the MOST compliancy in the venous vasculature
What happens when you get below a venous pressure of 0?
Veins in the thorax compress and you get NO flow
How does an increase vs a decrease in blood volume affect the venous function curve?
- Increased blood volume/venous tone increases venous return
- Decrease in blood volume or venous tone decreases venous return
When are CO and VR equal?
ONLY when central venous pressure is 2 mmHg
According to Starlings Law, cardiac output always decreases when central venous pressure decreases. True or False?
False
