Physics and Hemodynamics

What are hemodynamics?

Study of blood and how or why it moves

What is systemic circulation?

Part of circulatory system that carries oxygenated blood from LV to rest of body (arteries, veins, venules, etc.)

What is pulmonary circulation?

Part of circulatory system that carries deoxygenated blood from RV and returns oxygenated blood to LA

Where is oxygen saturation the HIGHEST in the body?

Pulmonary veins

Where is oxygen saturation the LOWEST in the body?

SVC and IVC

What is the role of the cardiac pump during SYSTOLE?

High LV pressures exceed aortic pressures to push blood into arteries via contraction

What is the role of the cardiac pump during DIASTOLE?

Low LV pressures causes LV to fill with blood

What are the primary mechanisms that control arterial flow volume changes or BP during the cardiac cycle?

1. CO

2. Peripheral resistance

What is a normal heart rate?

60-100 BPM

1. Bradycardia < 60 BPM

2. Tachycardia > 100 BPM

What is kinetic energy?

Energy of motion or ability of blood to do work (velocity)

What is the primary form of energy driving blood flow?

Potential energy

What is potential energy?

Stored or resting energy that is influenced by hydrostatic pressure of distending vessels

What is hydrostatic or gravitational energy?

Effect of gravity on circulatory system

What is total fluid energy or total composition of blood?

Sum of potential, kinetic, and hydrostatic energy

What is a pressure gradient?

Difference in pressure between two points of a vessel that creates flow

What are the factors that affect blood flow?

1. Decreased cardiac function that causes decreased flow velocities

2. Increased cardiac function that causes increased flow velocities

3. Peripheral resistance

4. Vessel compliance

5. Tone of vascular musculature

6. Pattern of branches or collaterals

7. Vasoconstriction due to coldness, anxiety, smoking

8. Vasodilation due to exercise, heat, HTN medication, proximal stenosis, adrenaline

9. Viscosity

How does vessel compliance affect the amount of blood flow?

Stiffer or calcific vessels cause higher resistant flow

How does the tone of vascular musculature affect the amount of blood flow?

Muscular patients cause less compliant vessels and higher resistant flow

What is viscosity?

Measure of a fluid’s resistance to flow, thickness of blood, or amount of hematocrit

How does viscosity affect the amount of blood flow?

Thick blood causes higher resistant flow

How does exercise affect the amount of blood flow?

Induces vasodilation in distal vascular beds of skeletal muscles

What is autoregulation?

Ability of vascular beds to alter resistance to flow to maintain flow levels required for normal function

What is resistance?

Opposition of flow

What is the resistance equation?

What is resistance dependent on?

1. Length

2. Viscosity

3. Lumen or radius

Which property has the greatest affect on flow resistance?

Changes in vessel radius such as stenosis and vasoconstriction

An increase in resistance requires an… to maintain constant flow.

Increase in pressure

What is the sonographic appearance of a vessel with high resistance?

1. Sharp upstroke

2. Rapid deceleration

3. Minimal diastolic flow

4. Diastolic flow reversal

Where in the body does blood flow have high resistance or triphasic waveforms?

1. Muscles

2. Extremities

Where is the highest pressure located in the circulatory system?

LV at 120 mmHg

When does flow occur in vessels with HIGH resistance?

ONLY during SYSTOLE

What is the sonographic appearance of a vessel with low resistance?

1. Slow upstroke

2. Slow deceleration

3. No diastolic flow reversal

Where in the body does blood flow have low resistance or monophasic waveforms?

1. Brain

2. Kidneys

3. Spleen

4. Liver

When does flow occur in vessels with LOW resistance?

Throughout cardiac cycle or systole AND diastole

What is Poiseuille’s Law?

Demonstrates relationship between pressure, volumetric flow rate, and resistance

What is the equation for Poiseuille’s Law?

OR Q = P/R

When can we apply Poiseuille’s Law?

Straight, rigid tube with laminar flow (NOT STENOSIS)

What is Ohm’s Law?

Poiseuille’s Law in electronics

What is the equation for Ohm’s Law?

What is Bernoulli’s Law?

Where there is a decrease in pressure in regions of high flow speed (velocity)

What is the equation for Bernoulli’s Law?

What is the average pressure in veins?

2 mmHg

What is the average pressure in arteries?

100 mmHg

Flow volume is always proportional to…

BP

What is laminar flow?

Flow that travels in concentric rings or layers with various velocities

1. Center of flow = FASTEST

2. Periphery of flow = SLOWEST due to friction from vessel walls

What is parabolic flow?

Similar to laminar flow, but has a smaller velocity range and is only found in medium sized vessels

What is plug or blunt flow?

Flow that travels at one speed and is found during systole in larger vessels, arterial branch origins, and proximal to stenosis

What is arterial pulsatility?

Primary factor that determines resistance characteristics and is related to number of changes in flow direction during one cardiac cycle

What is a monophasic waveform?

When antegrade flow continues through cardiac cycle

What is a biphasic waveform?

When flow during systole is antegrade and some flow is reversed during diastole

What is a triphasic waveform?

When flow during systole is antegrade, some reversal initially in diastole, and a small amount of forward flow in end diastole

Identify this image.

1. Antegrade flow or systolic peak

2. Retrograde flow or diastolic peak

3. Antegrade flow at end diastole due to vessel wall compliance

What is turbulence?

Chaotic flow distal to stenosis

What are the anatomic causes of turbulence?

1. Bifurcation

2. Tortuosity

3. Kinking

4. Coiling

5. Eccentric change in course

What are the acquired causes of turbulence?

1. Atherosclerosis

2. Stents

3. Bypass grafts

4. Myointimal hyperplasia

5. Aneurysms

Identify this image.

1. Normal flow proximal to stenosis

2. Elevated velocities and spectral broadening at stenosis

3. Monophasic and low resistance waveform with distal to stenosis

What is a stenosis?

Narrowing of a vessel lumen

What are the factors that determine the hemodynamic significance of a stenosis?

1. Shape and degree of stenosis

2. Diameter of stenosis

3. Length of stenosis

4. Contour of stenosis

5. Distal peripheral resistance

6. Pressure gradient

7. Presence of collaterals

What are stenoses in a series?

Multiple areas of stenosis in same vessel that cause a higher effect on resistance to flow than parallel stenoses

What are stenoses in parallel?

Stenosis in different vessels coursing in same direction that cause a lower effect on resistance to flow than series stenoses

What is considered a hemodynamically significant stenosis?

1. 50% diameter

2. 75% area

Identify this image.

Distal obstruction due to newly high resistance waveform

Identify this image.

Proximal obstruction due to newly low resistance waveform

Identify this image.

Low resistance flow in distal arm and calf arteries due to vasodilation in distal vascular beds

1. Relaxation shows newly low resistance waveform

2. Clenching hands or toes increases resistance and returns flow pattern to normal

What is the most common reason for underestimation of an arterial stenosis?

Improper sample volume location

What are eddy currents or vortices?

Turbulent and swirling blood found distal to stenosis

What is a murmur?

Abnormal blood flow sound in heart most commonly from valvular regurgitation or stenosis

What is a bruit?

Abnormal blood flow sensation in a blood vessel due to a stenosis, branching, or tortuosity

What is a thrill?

Abnormal blood flow sensation or vibration in a blood vessel due to a stenosis, pseudoaneurysm, and NORMAL hemodialysis grafts

What factors cause a reduced systolic velocity in arteries?

1. CHF

2. CAD

3. Diastolic dysfunction

4. AS

5. MS

6. Dilated CM

7. Pericarditis

8. Increased blood viscosity

9. Coldness

10. Reduced HR

11. Hypocalcemia

What factors cause an elevated systolic velocity in arteries?

1. Arrhythmias

2. Volume overload due to liver and renal disease, pregnancy, or obesity

3. Systemic HTN

4. AR

5. Hypertrophic CM

6. Reduced blood viscosity

7. Warmness

8. Increased HR

9. Hypercalcemia

What is spectral broadening?

Widening of spectral waveform or filling of spectral window

What is bandwidth?

Difference between highest and lowest frequencies or velocities

What is acceleration time (AT)?

Time from onset of systole to point of maximum systolic peak and is used to differentiate inflow from outflow disease

What is resistive index (RI)?

Comparison between systolic and diastolic velocities to maximum velocity of vessel segment and is used to evaluate tissue resistance to blood flow

What is the equation for resistive index (RI)?

RI = PSV - EDV / PSV

What is pulsatility index (PI)?

Degree of resistance in a vessel measured by determining velocities during cardiac cycles and is used to distinguish proximal from distal disease

What is the equation for pulsatility index (PI)?

(T/F) PI decreased the further the blood travels from the aorta.

False; PI increases the further blood travels from the aorta

What is the effect of exercise on NORMAL arterial flow?

1. Increases flow

2. Increases pressure gradient

3. Decreases resistance

What are collaterals?

Numerous, smaller vessels that provide an alternative pathway for CHRONICALLY diseased vessels

What are the flow characteristics of collaterals?

Higher resistance than native arteries due to decreased vessel diameter and increased vessel length

What is hydrostatic pressure?

Weight of column of blood inside vessels

What is the equation for hydrostatic pressure?

Fluid density X Gravity X Height of column

When standing, what is the hydrostatic pressure above the heart?

Negative or lower than true circulatory pressure

*Average = - 30 mmHG at head

When standing, what is the hydrostatic pressure below the heart?

Positive or higher than true circulatory pressure

*Average = 100 mmHg at ankle

When supine, what is the hydrostatic pressure both above and below the heart?

0 mmHg due to arteries and veins being at same level as heart

What is ambulatory venous pressure?

Lowest pressure level in limbs during exercise

What is transmural pressure?

Pressure of blood against vessel walls that determines shape of vessel

When will a vessel assume a dumbbell or elliptical shape?

When transmural pressure is low or there is a low blood volume

When will a vessel assume a circular shape?

When transmural pressure is high or there is a high blood volume

How does venous flow occur?

Calf muscle pump

What is the function of the calf muscle pump during contraction?

1. Contraction in calf muscles force venous blood movement

2. Deep venous valves open to allow blood flow up towards heart

3. Perforator valves close to prevent caudal flow

What is the function of the calf muscle pump during relaxation?

1. Relaxation in calf muscles

2. Deep venous valves close

3. Perforator valves open to allow blood to flow from superficial to deep system

What are causes of calf muscle pump failure?

1. Muscle wasting

2. Obstruction

3. Incompetent valves

What is spontaneous flow?

When flow is detected in veins without augmentation

What is respiratory phasicity?

When flow moves from areas of high pressure to low pressure during breathing

What happens to pressure in the body upon inspiration?

1. Diaphragm descends

2. Increase in intra-abdominal pressure

3. Decrease in intrathoracic pressure

4. Decreased flow in lower extremities through IVC

5. Increased flow in upper extremities toward heart

What happens to pressure in the body upon exhalation?

1. Diaphragm raises

2. Decrease in intra-abdominal pressure

3. Increase in intrathoracic pressure

4. Increase in flow from lower extremities through IVC

5. Decreased flow in upper extremities toward heart

What is cardiac pulsatility?

Ripples of motion caused by adjacent cardiac contraction that creates pulsatile flow in proximal vena cava, upper arms, and hepatic veins