Hemodynamics Flashcards
What is the main function of the heart?
~ Return blood from the capillaries back to the heart
~ Act as a reservoir to maintain homeostasis
At any given time what is the percent of blood volume moving through the veins?
80%
How large is the resistance of flow against veins? Why?
Little resistance to flow because of their large diameters
What is needed for blood to flow from 1 point to another?
A pressure gradient
What is another name for pressure gradient?
Energy gradient
Veins rely on a variety of mechanisms to do what?
- Prevent bidirectional flow
2. Aid in the return of blood against gravity
What mechanisms do veins rely on to prevent bidirectional flow and aid in the return of blood against gravity? (6)
~ cardiac function ~ respiratory function ~ skeletal muscles ~ pressure gradients ~ venous valves ~ compliance
What is capacitance?
The ability to collapse and expand and adapt to changes in the blood volume
What type of blood does the LV send through the arteries when it contracts?
High pressure pulsatile stream
What happens to the blood as it flows through the high resistance arterials and capillaries?
Looses pressure
By the time the arterial blood enters the venules what has the pressure been reduced to?
15mmHg
What is hydrostatic pressure?
The weight of a column of blood
Describe hydrostatic pressure
It is the pressure exerted by a fluid in a closed system and is equivalent to the weight of a column of blood extending from the heart to the level where the pressure is being measured
What causes hydrostatic pressure?
Gravity
What varies hydrostatic pressure?
Patient position
Standing = more pressure
Height
What must be overcome for blood to flow into the lower extremity veins?
Venous pressure
What type of person has more hydrostatic pressure?
Tall
What is hydrostatic pressure also know as?
Gravitational pressure
What is the primary factor in determining intravascular pressure within the venous system?
Hydrostatic pressure
What is the hydrostatic pressure in a supine person? Why?
15mmHg - negligible because all parts are at the same level as the RA
What happens to the leg veins when standing?
Dilate to accept the blood draining into them
When standing what is the venous pressure?
Rises to 110mmHg
What happens to the veins when standing?
Stiffen
What is needed for blood to flow?
Pressure gradient
What is the hydrostatic pressure in the capillaries?
15mmHg
What part of the body has the highest hydrostatic pressure?
Lower portion of the body
What happens to the force of gravity when you get further from the RA?
The force of gravity is greater
What happens to the capillary pressure in a raised arm?
Capillary pressure increases so the veins don’t collapse
Why is the pressure in the raised arm negative?
Due to the raised arm being above the RA pressure of 0
How does hydrostatic pressure affect the LE when stationary? (7)
~ increase transmural venous pressure distally ~ increase venous distension ~ increase venous pooling ~ decrease capillary perfusion ~ decrease venous return ~ decrease cardiac output ~ hypotension
What is the hydrostatic pressure when standing for an average person?
Roughly 80 mmHg
What is the hydrostatic pressure of the average person when walking?
Roughly 25 mmHg
What does venous wall shape depend on? (3)
Pressure
Volume
Flow
What is compliance?
The veins ability to permit a large increase in venous flow without significant an increase in the venous pressure
What effects the veins compliance? (2)
Intramural or interstitial pressure
What is intramural pressure?
Internal pressure
What is interstitial pressure?
External pressure
What does blood capacitance vary and depend on? (4)
- Limb position
- Muscle pump activity
- Venous valve function
- Blood volume
What happens to the vein in the event of surplus fluid?
The vein will expand and increase in pressure
What does compliance allow to happen?
Allows for a range of venous volume changes without affecting the central venous pressure
What causes compliance to decrease?
Higher pressures and volumes
What is the difference between elasticity and compliance?
Elasticity is the ability to expand and recoil
Compliance is the ability to with stand a large amount of volume without affecting pressure
How do veins compare to arteries in reference to their elasticity/compliance?
Veins are less elastic then arteries but more compliant
Arteries have greater elasticity therefore having the ability to expand and snap back/recoil to original shape
Veins are more expandable and don’t have an original shape
What determines the cross sectional shape of a vein?
Transmural pressure
What is transmural pressure?
The difference between intraluminal and interstitial pressures
What is intraluminal pressure?
Pressure within the vein, the force occurring within the walls of the vessel
How does an increase in intraluminal pressure affect transmural pressure and the shape of the vein?
Increases transmural and makes the vein rounder
What is interstitial pressure?
Pressure exerted against the vein by surrounding tissue and fluid, the force from outside the vein
How does low transmural pressure affect the vein?
As the volume and pressure decrease the vein wall collapses and becomes elliptical in shape
How does high interstitial pressure with low intraluminal pressure affect the vein?
It flattens the vein (low transmural)
What type of pressure is transmural pressure directly related to?
Intraluminal
What creates the pressure gradient in the IVC?
On inspiration the intrathoracic pressure is less then the abdominal pressure
Describe how the blood moves from the abdomen to the chest
Inspiration- intrathoracic pressure decreases and the intraabdominal pressure increases causing blood to flow into the chest
What happens to the outflow from the peripheral veins upon inspiration?
Blood flow stops
When is inflow allowed from the upper extremities?
During inspiration
What is the result of CHF and why?
It increases venous pressure as the blood backs up
What is the effect of the valsalva maneuver? (4)
- Increases both intrathoracic and intraabdominal pressure
- Respiratory changes are increased
- Pressure in the upper and lower extremities increase
- Flow stops/diminishes in both UE and LE
What happens when a breath is released following valsalva?
Flow volume is increased and augmentation shown
What’s an abnormal response to the valsalva maneuver?
Reversal of flow
What type of pt should not be asked to preform the valsalva maneuver?
Pts with cardiac disease
What is dynamic pressure?
Muscular contraction of the heart
What causes a suction effect on venous blood flow?
Cardiac contraction and relaxation
What are the components of the venous wave? (4)
A - atrial systolic wave
S - systolic wave
V - atrial diastolic wave
D - diastolic wave
Draw and label the venous wave
…pg 6
What are the 2 periods of increased venous flow during the cardiac cycle?
- Ventricular systole
2. When the AV valves open
What events take place during ventricular systole?
~ atrial volume increases causes atrial pressure to decrease
~ increased flow from the veins into RA
~ venous pressure decreases
What events take place when the AV open?
Venous blood enters the ventricles and atrial pressure decreases allowing venous flow into the heart
What allows back flow happen during atrial contraction?
No valves in the IVC or atrial junction
Where are changes in pressure and flow more evident? Large central veins or peripheral veins?
Large central veins and upper extremity
Does cardiac contraction affect the blood flow from the lower extremities? Why?
No, because the strong respiratory variation in the leg (lower extremities are further away from the heart therefore have physics flow not pulsatile)
How do cardiac conditions such as CHF or tricuspid insufficiency effect?
Increase central venous pressure creating pulsatile waveforms in the lower extremities due to the heart backing up and not effectively emptying
What creates large changes in the intrathoracic and intraabdominal pressures?
Respiration
What is phasicity?
Refers to the ebb and flow that occurs in the veins in response to respiration
What is a major indicator of normal flow in the deep veins?
Phasicity
Venous blood flow is affected by the movement of what structure?
Diaphragm
How does inspiration stop/decrease venous flow from the lower extremities?
Intraabdominal pressure increases by lowering of the diaphragm causing the IVC to collapse
How does inspiration result in increased flow (return) from the upper extremity veins?
Intrathoracic pressure decreases
How does expiration affect the venous flow in the lower extremities?
Intraabdominal pressure decreases as the diaphragm rises and the IVC opens, venous flow resumes its phasic pattern
How does expiration affect the venous flow in the upper extremity veins?
Intrathoracic pressure increases and results in decreased return flow
Does respiratory function have greater affect on the lower or upper extremity veins?
Lower extremity veins
Is the upper or lower extremity more pulsatile and why?
The upper extremity flow is more pulsatile due to the close proximity of the heart
When does respiration have a small effect on venous flow?
When standing
What happens to pressure and flow with inspiration? (4)
- Intrathoracic pressure decreases
- Diaphragm moves up
- Increased intraabdominal pressure increased abdominal venous outflow
- Flow from the peripheral veins decrease
What happens to pressure and flow on expiration? (4)
- Intrathoracic pressure increases
- Diaphragm moves up
- Decreases intraabdominal pressure decreases abdominal pressure outflow
- Outflow from the peripheral veins increases
Does the UE increase or decrease with respiration?
Increase
Does the LE increase or decrease with respiration?
Decrease
What 2 things does the venous system rely on for the movement of blood?
- Action of the muscle
2. Competence of valves
How will the blood flow in normal functioning valves?
In one direction- towards the heart
What is the effect of contracting voluntary muscles?
Propels blood towards the heart
What is the effect of skeletal muscle activity on the venous pressure? Why?
Decreases because it interrupts the hydrostatic column of blood
What occurs when there is a decreased hydrostatic pressure? (3)
- Decreased venous pooling
- Decreased capillary pressure
- Increased blood flow
What occurs to the veins when the body is at rest and there is no muscle activity?
Veins act as reservoirs for blood collection
What direction does blood flow in the deep and superficial veins with muscle contraction?
Flow moves towards the heart
What occurs in the veins with muscle relaxation?
Small amounts of blood flow occurs in the perforators from the superficial system
What is the most efficient pump in the body? (Besides heart)
Contraction of the gastrocnemius and soleus muscles
What is the capacitance in the calf muscle during contraction?
200mmHg per contraction
What percent of blood volume is ejected from the calf in a single contraction?
40-60% of venous blood volume
What is another name for the calf muscles?
Venous heart
What does the calf muscle dependent on? (3)
- Ability of calf muscles to contract
- Competency of the venous valves
- Patency of the outflow valves
What venous pressure aids in the return of blood to the heart?
Lower venous pressure which reduces venous volume aid in the return
How does chronic venous insufficiency occur?
Venous hypertension or failure to decrease venous pressure during exercise
What is ambulatory venous hypertension?
Abnormally high venous pressure when standing
What happens to the blood flow with ambulatory venous pressure when the calf muscle pumps?
Since the valves are dysfunctional blood is expelled in all directions on contraction, therefore pressures do not decrease normally
What other conditions can occur as a result of ambulatory venous hypertension?
Edema and varicose veins
What is the main function of the venous valves?
Direct blood flow towards the heart
Prevent retrograde flow (reflux)
What layer of the vein do the valves arise from?
Intimal layer
How many cusps does a valve have? What is this called?
2 cusps
Bicuspid
What is the function of perforating veins?
Direct blood flow from superficial to deep veins
Why are there more valves in the calf veins?
Due to the high hydrostatic pressure in the distal limb
Why does the UE have less valves then the LE?
Gravity has less effect on the UE flow due to the shorter column of blood and closer proximity to the heart
What is the venous valve sinus?
Normal dilation of the vein at the site of the valve and is wider then the valve segment
What occurs to the venous valve segment during increased pressure?
The vein segment will expand
Where is the most frequent site of thrombi? Why?
At the venous valve sinus due to stagnant flow
How many valves are in the greater saphenous?
10-12
How many valves are in the soleal sinus?
0
How many valves are in the calf veins?
9-12 each
How many valves are in the SFV?
1-3
How many valves are in the EIV?
1 or none
How many valves are in the IVC?
0
How many valves are in the lesser saphenous?
6-12
How many valves are in the perforators?
1 each
How many valves are in the popliteal?
1-3
How many valves are in the CFV?
1
How many veins are in the common/internal iliac?
0
How many valves are in the jugular?
1
What phases of the cardiac cycle cause an increase in venous return?
When the AV open or during ventricular contraction during early systole
In a supine pt, what is the typical hydrostatic pressure at the ankles?
10-15mmHg
What is phasicity?
Ebb and flow in the blood as a response to respiration
How can you correct for aliasing on a spectral waveform?
Decrease the baseline
Increase scale
What effect does CHF have on venous pressure?
Increases the venous pressure
