Arterial Hemodynamics Flashcards
What is hemodynamics?
- the study of blood moving through the circulatory system
- study of the physical principles of blood circulation
What is different about arterial and venous hemodynamics?
structure and purpose
What is the circulatory system?
a closed system that consist of the heart, blood vessels and blood
What are the two circulatory systems?
- systemic circulatory system
- pulmonary circulatory system
What is the systemic circulatory system?
- includes the left heart and all of the systemic vessels
- the circulation between the heart and aorta and the rest of the body
What is the pulmonary circulatory system?
- includes the right heart and all of the pulmonary vessels
- the circulation between the heart and lung
What is the heart?
the muscular pump that provides the driving forces (contractile forces) in order to distribute the blood throughout the body
What are the only arteries to carry deoxygenated blood?
pulmonary arteries
What ae the only veins to carry oxygenated blood?
pulmonary veins
What is the purpose of the circulatory system?
to meet the metabolic demand of all end organs in the body by:
- supplying adequate oxygenated blood and nutrient
- removal of wastes product from the body
What can a single cycle of cardiac activity be divided into?
Two basic phases:
- Systole
- Diastole
What is systole?
represents the time during which the left and right ventricles contract and eject blood into the aorta and pulmonary artery
What is diastole?
represents the period of time when the ventricles are relaxed, blood is passively flowing from the left atrium (LA) and right atrium (RA) into the left ventricle (LV) and right ventricle (RV)
What is the stroke volume (SV)? What is normal?
the volume of blood ejected by the left ventricle with each heart beat (in one contraction)
-normal stroke volume (SV) = 70-100ml
What is heart rate (HR)? What is normal? What is not normal?
the number of heart beats per minute (bpm)
- normal resting HR=60-100 bpm
- Bradycardia <60 bpm
- Tachycardia >100 bpm
What is cardiac output (CO)? What is normal?
the volume of blood pumped by the heart in one minute
-normal adult CO is 4.7 liters of blood per minute
What is the cardiac output (CO) calculation?
CO=stroke volume (SV) X heart rate (HR)
What is the arterial system?
a multibranched elastic conduit that carries blood away from the heart and outward to the most distant tissues
What do arteries do?
- transport gases, nutrients and other essential substances to the capillaries
- progressively decrease in size from the aorta (largest) to arterioles (smallest)
What do arterioles do?*
-*responsible for regulating resistance in the tissues and assist in regulating blood flow through contraction and relaxation
What does arterial hemodynamic depend on?
- heart pumping
- recoil of the large arteries
- distal vascular bed
What are capillaries?
- the smallest vessel in the body (8-10 micron (μm) in diameter and not much more than a mm long)
- only composed of intimal tissues (have walls that consist of endothelial cells, forming a layer one cell thick) to allow perfusion of the surrounding tissue and the exchange of nutrients and waste products between blood and tissue
What is the resistance of capillaries compared to arterioles?
-capillaries have a lower resistance than arterioles
What are the layer of arteries and veins*?
*BOTH have a tunica intima, tunica media, and tunica adventitia
What is the tunica intima and its *sonographic appearance?
- the inner layer of the vessel
- a single layer of endothelial cells which is supported by a basement membrane
- -appears hyperechoic
What is the tunica adventitia/externa and its *sonographic appearance?
- outer layer of the vessel
- made up of fibrous connective tissue that is arranged longitudinally
- *appears hyperechoic
What does the adventitial layer usually contain?
-the vasa vasorum-a small network of blood vessels that feed/drain the blood vessels wall
What is the tunica media and its *sonographic appearance?
- the middle muscular layer, which is the *THICKEST layer
- a layer of muscle and elastic tissue arranged in a circular pattern
- *appears hypoechoic
*Which vessel has a thicker layer? Which layer and why?
*arteries typically have a much thicker tunica media compared to veins, because of higher pressure in the arteries
What separates the 3 coats of arteries?
internal and external elastic membrane
What characteristic are unique to arteries?
-supply blood, non-collapsible, high blood pressure, thick wall, no valves, supply oxygenated blood, pulsatile, and follow the rhythm of the heart
What characteristics are unique to veins?
-return the blood, collapsible, low pressure inside, thinner wall, contain multiple valves, supply deoxygenated blood, phasic and continuous, and follow the rhythm of respiration
What are the 3 basic forms of blood flow?
- pulsatile flow
- phasic flow
- steady flow
What is pulsatile flow?
- occurs when blood moves w/ a variable velocity
- blood accelerates and decelerates as a result of cardiac contraction so *commonly appears in arterial circulation
What is phasic flow?
- also occurs when blood moves w/ a variable velocity
- blood accelerates and decelerates as a result of respiration so *often appears in venous circulation
What is steady flow?
- occurs when fluid moves at a constant speed or velocity
- present in the venous circulation when individuals stop breathing for a brief moment
What are the spatial categories of flow?
- laminar flow
- parabolic flow
- plug (blunted) flow
- disturbed flow
- *turbulent flow
What is laminar flow?
a flow condition in which streamlines are aligned and parallel
- *highest velocities in the center of the vessel
- *the most common type of flow and found in smaller arteries
What is parabolic flow?
- has a bullet shaped profile
- flow speed is maximum at the center of the tube and minimum at the tube walls
- identified in medium sized vessels and the layers of flow have a more narrow range of velocities than laminar flow
- *type of laminar flow
What is plug (blunted) flow?
occurs when all of the layers and blood cells travel at the same velocity (constant flow velocity)
- can be found in large blood vessels such as the aorta and at the entrance of vessels
- *type of laminar flow
*What flows are associated with normal physiology?
*Both plug and parabolic patterns are laminar and associated with normal physiology
What is disturbed flow?
- occurs when the parallel streamlines are altered from their straight form
- i.e. the region of a stenosis or at a bifurcation
What is turbulent flow?
the flow pattern is random and chaotic, with particles moving at different speeds in many directions, even in circles called eddies, with forward net flow still maintained
What transition causes turbulence?
transition from high flow speed in a narrow channel to slow blow in a broad stream
What is often associated with turbulent flow?
pathology (downstream from a significant stenosis in a vessel), elevated blood velocities, and from tortuous or kinked vessels
Where can turbulence be found in normal vessels?
in areas such as the carotid bulb
What is Reynolds number?
- predicts whether flow is laminar or turbulent
- *laminar flow is less than 1500
- *turbulent is over 2000
- (Re)
What is the equation for Reynolds number?
Re= Vp2r/n Re: reynolds number V: velocity p: density of the fluid r: radius of the tube n: viscosity of the fluid
What is required for the movement of any fluid between two points?
a difference in energy (pressure) levels between these two points
What are the three forms of energy in hemodynamics?
- pressure energy
- kinetic energy
- Gravitational energy
What is the Total Energy contained at a specific location within circulation?
it is the sum of all three forms
What is pressure energy?
a form of stored or potential energy, such as a hair spray can
-*pumping action of the heart provides the potential energy in the cardiovascular system
What provides the potential energy in the cardiovascular system?
-the pumping action of the heart
What is kinetic energy?
the energy of motion
- is it determined by an object’s mass and the speed at which it moves
- greater mass=more kinetic energy=greater velocity
What is gravitational energy (hydrostatic pressure)?
stored energy related to elevated position
-expressed in mmHg
What factors affect blood flow?
- cardiac function: decreased function=decreased flow
- elasticity of the vessel walls (compliance): stiffer or more calcified vessels=higher resistance flow
- tone of vascular musculature: more muscular the patient=less compliant vessels=higher resistance flow
- peripheral resistance: determines the rate of flow in the arteries and is regulated by the arterioles
- branching patterns, dimensions (stenosis), interconnections
- vasoconstriction: tightening of vessels wall due to stimuli (cold, anxiety)
- vasodilation: expansion of vessels wall due stimuli (heat)
- viscosity
- exercise: induces vasodilation in large vessels supplying the skeletal muscles
- autoregulation: vascular beds alter resistance to flow to maintain the flow levels needed for normal function
*What is the best single vasodilator of high resistance vessels within skeletal muscle?
*exercise
What is resistance?
- determines the rate of flow in the arteries and is regulated by the arterioles
- “push-back” from vascular beds against forward flow of blood during the cardiac cycle
- the greater the pathway resistance and/or energy loss, the lower the flow
What lowers flow?
-greater pathway resistance and/or energy loss
*What happens to flow if resistance is low?
higher flow rate
*What happens to flow if resistance is high?
lower flow rate
What causes increased resistance?
- stenosis
- vasoconstriction
- organ supplied not functioning properly
*What is resistance directly related to ? inversely related to?
- directly related to length of the vessel and fluid viscosity
- inversely related to the vessel radius
*What is the resistance equation?
*R=8Ln/πr^4 R: resistance L: length of vessel n: viscosity of blood r: radius of vessel
What is relatively constant when it comes to resistance?
vessel length and viscosity
*What does increased viscosity do to resistance?
*increased resistance
*What does increased vessel length do to resistance?
*increases resistance
*What does increased vessel radius do to resistance?
*decreases resistance
*What has a higher resistance–extremities and muscles or organs?
*extremities and muscles have much higher resistance to blood flow than organs
What are the three forms of energy loss?
- Viscosity
- Friction
- Inertial energy loss
What is viscosity?
- can be described as thickness of a fluid
- *increased viscosity=decreased velocity
- *decreased viscosity=increased velocity
What is the unit of viscosity?
poise
What is hematocrit and its relation to viscosity?
-the percentage of blood made up of RBCs. With anemia, hematocrit is reduced and blood has reduced viscosity
What is friction?
layers of blood sliding against each other and across the vessel walls, energy lost in the form of heat
What is inertial energy loss?
energy is lost when the speed of a fluid is changed regardless of whether the fluid speeds up or slows down and when it changes direction. (relate to the tendency of fluid to resist changes in its velocity)
When does inertial energy loss occur?
- pulsatile flow (usually in arterial circulation)
- phasic flow (usually in venous circulation)
- velocity changes at a stenosis
What is peripheral resistance?
-the resistance of the arteries to blood flow. as the arteries constrict, the resistance increases and as they dilate, resistance decreases
*What accounts for about 1/2 of the total resistance in the systemic system?
-*the resistance of the arterioles
What can arterioles do?
their muscular walls can constrict or relax, producing dramatic changes in flow resistance
What is vasoconstriction?
narrowing of the vessel, increased resistance
What is vasodilation?
widening of the vessel, decreased resistance
What is low-resistance/ low pulsatile flow?
a flow on continuous nature throughout systole and diastole, feeding a dilated vascular bed w/ significant amount of blood during diastole and have broad systolic peaks
What arteries have a low resistance flow?
internal carotid, vertebral, renal, celiac, splenic, and hepatic arteries
What word is used to describe low-pulsatility waveforms?
*monophasic
What does monophasic mean?
- flow is always forward, and the entire waveform is either above or below the Doppler spectrum baseline (depending on the orientation of the ultrasound transducer)
- low pulsatile waveforms
What are moderate-pulsatility Doppler waveforms?
- in between the low- and high-resistance patterns
- the systolic peak is tall and sharp but forward flow is present throughout diastole (perhaps interrupted by early-diastolic flow reversal known as dicrotic notch)
What is an example of a moderate-pulsatility Doppler waveform?
external carotid artery
What is a high resistance/ high pulsatile flow?
- flow is forward in systole and reversed in diastole
- have tall, narrow, and sharp systolic peaks and reversed or absent diastolic flow
- triphasic flow pattern seen in extremity artery of a resting individual
- sharp systolic peak (first phase) is followed by brief flow reversal (second phase) and then by brief forward flow (third phase)
What arteries are examples of high resistance flow?
aorta, external carotid, subclavian, iliac, extremity
What is Poiseuille’s Law?
-describes relationship between flow volume (Q), pressure (P), and resistance (R)
-law states that flow volume is equal to the difference in pressure divided by the resistance to flow in the vessel
Q=ΔP/R
Q=(P1-P2)/R
Q=ΔPπr^4/8Ln
What does Poiseuille’s law help answer?
how much fluid (blood) is moving through a vessel
What is directly and inversely related in Poiseuille’s law?
- the radius is directly proportional to the volume flow, while the length of the vessel or the viscosity of the blood are inversely proportional to flow
- *an increase in length or viscosity decreases flow, while an increase in radius increases flow
What has the greatest effect on flow resistance? (Poiseuille’s law)
changes in vessel radius b/c the radius in the equation is with power 4; so small changes in radius may result in large changes in flow
What is the collateral effect?
- in an extremity at rest, total blood flow may be fairly normal even in presence of severe stenosis or complete occlusion of the main artery b/c of the development of collateral network, as well as the compensatory decrease in peripheral resistance
- in the body, if a major artery is blocked it becomes the path of highest resistance, while the previously collapsed collateral become the path of least resistance
What causes the collaterals to open up and the blood follows the lower-resistance path?
the higher pressure proximal to a stenosis
What happens to the collateral, distal to the stenosis?
it eventually rejoins the main artery
-in chronic disease, if the collateral network is extensive enough, the net flow distal to the stenosis may actually be normal
Are collaterals found in acute obstructions?
no, they are formed over time and in response to chronic change
Why do collaterals have a higher resistance than normal flow channels?
because they have a small lumen diameter and greater length
What is a stenosis?
a narrowing in the lumen of a vessel
What is a critical stenosis (hemodynamically significant stenosis)?
one in which there is decreased distal flow
*What percentage(s) are considered to be hemodynamically significant in a stenosis?
-*cross-sectional area loss of 75% which corresponds to a 50% decrease in diameter
What is the stenotic profile of a hemodynamically significant arterial stenosis?
- proximal to the stenosis: the flow velocities are usually dampened
- as flow enters the stenosis: there is increase in velocities
- at the exit from the stenosis: a post stenotic turbulence is typically seen
How to calculate diameter of stenosis?
=[(vessel diameter - true lumen diameter)/vessel diameter] x 100
*50% diameter stenosis = 75% area stenosis
How to calculate area of stenosis?
=[(vessel area-true lumen area)/ vessel area] x 100
What is the continuity rule?
- a narrowing of the lumen of a vessel, or stenosis, produces disturbed and possibly turbulent flow
- the avg flow speed in the stenosis must be greater than the avg flow speed proximal and distal to the stenosis
- *Volumetric flow rate (Q) indicate the volume of blood moving during a particular time. it must be constant in all 3 regions-proximal to stenosis, at stenosis, and distal to stenosis–b/c blood is neither created nor destroyed while it flows through a vessel
What is the continuity equation?
- ties together the relationship between the vessel area, the velocity of blood, and the volume of blood flow
- volume of blood that enters a blood vessel must be the same at any point along the length of the blood vessel. Blood flow is constant
- Q=VA
- flow (Q) is equal to the product of velocity and area
According to the continuity rule, what happens if the area of the vessel decreases (diameter)?
-velocity must increase b/c the size of a vessel is inversely proportional to the velocity of the blood flow. (V=Q/A)
What is the driving force behind fluid flow?
pressure
How is pressure distributed?
it is equally distributed throughout a static fluid and exerts its force in all directions
What is a pressure gradient?
a pressure difference required for flow to occur (fluid flows in a tube in response to a pressure difference at the ends)
What is proportional to pressure gradient?
flow rate, the greater the pressure difference, the greater the flow rate
What is the relationship between flow, resistance , ad pressure gradient?
Pressure gradient= flow x resistance
How can a pressure difference be generated?
by a pump (the heart) or by the force of gravity
When does pressure gradient increase?
flow increases or resistance increases
When does flow increase?
when pressure gradient increases or resistance decreases
What is the direction of flow in a pressure gradient?
-liquid will flow from the higher pressure to lower pressure
What is the Bernoulli principle?
- *describes the relationship between velocity and pressure in a moving fluid
- *derived from the principle of conservation of energy, which states that with a steady flow, the sum of all forms of energy is the same everywhere
- when fluid flows at a constant velocity from one point to another, the total energy remains constant
- if the pressure changes, the velocity will also change to maintain total fluid energy
- slow moving blood will exert more pressure than faster moving blood
Describe pressure and velocity in the stages of stenois.
- Pressure is higher proximal to the stenosis causing lower velocity
- There is a reduction in pressure seen with an increased blood velocity at a stenosis (decrease pressure = increase velocity)
- After the stenosis, the velocity decreases causing an increase in pressure
Do multiple stenotic lesions have a greater effect on volume flow and distal pressure than a single lesion of equal total length?
YES
What is antegrade?
moving forward in the normal direction
What is retrograde?
backward flow, or against normal direction of flow
What is turbulence?
disrupted flow caused by a stenosis, tortuosity, or bifurcation; appears at the exit point of stenosis
What is a murmur?
abnormal blood flow sound in the heart, usually valvular regurgitation
What is bruit?
abnormal (auscultatory consequences) of blood flow sound in a blood vessel; can be due to stenosis; or with vessel branching or tortuosity
What is thrill?
abnormal blood flow sensation in a blood vessel (vibration), can be due to stenosis, also seen with pseudoaneurysm and hemodialysis grafts
What is pulsatility?
continuous throbbing or beating, flow in the arteries pulsates due to continuous cardiac contractions
How is flow often described?
by the shape of its waveform on spectral Doppler as triphasic, biphasic (multiphasic) or monophasic
The higher resistance the vessel..
-the more pulsatile the flow
What waveform is normally seen in a high resistance vessel (extremities)?
-triphasic or biphasic
What waveform is normally seen in a low resistance vessel (organs)?
monophasic
