1 - Haemodynamics

Question 1

What is haemodynamics

Answer 1

the physical factors that control blood flow
- metabolic demand of the body dictate blood flow flow delivery to different organs
- blood moves down a pressure gradient
- ie blood flows from areas of relatively high to relatively low pressure regions

Question 2

What is the difference between serum and plasma

Answer 2

serum is plasma without clotting factors. Serum is the fluid collected from clotted blood.
plasma is the fluid collected from unclotted blood. Ie an anticoagulant has been added to stop the blood clotting, and therefore the plasma still contains clotting factors

Question 3

Instances when plasma viscosity changes

Answer 3

whole body viscosity changes are uncommon
Ie polycythaemia, thrombocytopenia + leukaemia.
☞ leads to sludgey blood, causing gangrene in peripheries

minor changes are due to
Typically from acute phase plasma proteins eg fibrinogen + c-reactive protein
☞ indicates inflammation

Question 4

What is flow (definition + units)

Answer 4

the volume transferred per unit time
Usually measured in mL/min

Question 5

What is pressure (definition + units)

Answer 5

force per unit area
Usually measured in mmHg
SI unit is Pascal

Question 6

What is Darcy’s law (don’t need to memorise but understand the concepts)

Answer 6

flow = ΔP/R

Where ΔP is a measure of the pressure gradient / pressure difference
Where R is resistance (ie measure of difficulty of flow)

This equation is analogous to Ohm’s law (I=V/R) from basic physics
Where current is equal to the voltage divided by resistance

Question 7

What is resistance and conductance

Answer 7

resistance = the measure of the difficulty of flow. Ie the difference in mean pressure need to move one unit of flow in steady state (mmHg min/mL)
conductance = the measure of the ease of flow of blood

Therefore they are the opposites of one another

Question 8

What is the relationship between flow and resistance

Answer 8

R = ΔP / Flow
Flow and resistance are reciprocally related
Therefore if one goes up, the other comes down at any given ΔP
At any given flow if ↑R then ↑ΔP too

Question 9

Turbulent vs laminar flow

Answer 9

laminar
- Smooth and silent
- Organised flow of blood
- Maintains constant energy
- Typical blood flow in most vessels most of the time
- Blood flows fastest in the centre of the lumen

turbulent
- noisy
- Disorganised flow
- Most commonly found in changing direction of vessels (eg branching), in stenosed arteries and stenotic heart valves
- Ie in stenosed vessels – stenosis will narrow the vessel and therefore disrupt the smooth flow of blood, and therefore the blood flow will be turbulent after the stenosis
- Turbulent flow is induced when energy is lost

Question 10

What are the factors that affect resistance to flow

Answer 10

• Diameter
• Length of vessel
• Viscosity

Question 11

Pressure and resistance across vessel types

Answer 11

the radius and the changes in the radius of a vessel have large impact on flow and/or resistance
☞ therefore any changes in radius of a vessel have a massive impact on flow
☞ drop in pressure is indicative of a change

▶︎ Ie resistance in the aorta is lower as it has a large diameter and is relatively short
▶︎ smallest arteries and arterioles contribute the greatest component of total peripheral resistance

Question 12

What is velocity (definition + units)

Answer 12

Distance fluid (blood) moves in a given time (cm/s)

At a constant flow, V is inversely related to the radius (squared) of the vessel

Question 13

What is the relationship between flow + velocity + area

Answer 13

Flow (F) = V x A

Where V is velocity
A is the cross sectional area (A = πr2)

Therefore, at a constant flow, V is inversely related to the radius (squared) of the vessel
☞ velocity at capillary level is much slower than at aorta or large arteries

Question 14

What is the dicrotic notch

Answer 14

On a cardiac cycle graph (ie pressure against time) the dicrotic notch is where there is a slight drop in pressure before the pressure increases again.
☞ this is the point at which the AV node closes

Question 15

What is the pulse pressure

Answer 15

systolic BP – diastolic BP
- Represents the force that the heart generates each time it contracts
- Can suggest cardiac output

Question 16

What are some factors that affect pulse pressure

Answer 16

• exercise causes increased stroke volume, and therefore increase in puslse pressure
• haemorrhage will decrease pulse pressure as there is less blood circulating around the body
• age can cause athlerosclerosis. This will reduce compliance (ie ability for vessels to accommodate changes in blood volume being pumped out at any given cycle)

Question 17

Bounding vs thready pulse

Answer 17

thready felt in patient where end systolic pressure is lower, leading to a lower pulse pressure. This is felt as weak pulse
bounding felt in patients where diastolic pressure is lower, felt as a strong pulse.

Question 18

What conditions can cause changes in pulse pressure

Answer 18

raising the arm above the level of the heart drains the venous blood in the arm quicker, lowering peripheral resistance. This increases the difference between the diastolic + systolic pressures
→ increased pulse pressure

bradycardia means time between beats is longer, allowing more time for diastole. Diastole pressure drops
→ increased PP

left ventricle failure means heart cannot beat as strongly
→ decreased PP

atrial fibrillation as random atrial + ventricular contraction causes irregularly irregular heart rate. Means blood volume ejected varies from beat to beat
→ PP varies from beat to beat

ventricular ectopics there is early beat in the cycle, caused by spontaneous ventricular depolarisation. Causes a longer pause before the next beat. Means ventricle has more time to fill, so more blood pumped out
→ increased PP

Question 19

What is the mean arterial pressure equations

Answer 19

mAP = (1/3 of systolic pressure) + (2/3 of diastolic pressure)

or

mAP = diastolic + (1/3 of pulse pressure)

and

mAP = (cardiac output) x (total peripheral resistance)

Question 20

at what mean arterial pressure does organ perfusion become impaired

Answer 20

if it falls below 70mmHg

Question 21

what is the difference between thrill and bruit

Answer 21

thrill can be felt (when palpating)
bruit can be heard (when listening)

Question 22

How is auscultating done

Answer 22

• Creates a change from turbulent → laminar flow
• This creates a sound that can be heard and used to estimate blood pressure
• Korotkoff sounds (laminar → turbulent)
• Put sphygmomanometer / BP cuff around brachial artery, which is then inflated, completely occulding brachial artery and then gradually releasing releasing it. This causes turbulent flow
• Hear sounds (like clicking/tapping) at the start of turbulent flow when pressure gradually being released from cuff = estimated systolic
• Silence when reverts back to laminar flow = estimated diastolic

Question 23

Important things to consider when auscultating

Answer 23

• cuff size is important (too big = underestimate BP and too small = overestimate BP)
• Positioning of the cuff
• Measure in both arms, and use the higher as the reference arm
• Sat comfortably, upright with legs uncrossed + flat on ground
• Arm supported (ie on table)
• Repeat several times and take a mean value
• Unless otherwise stated, assumed that measurement will be taken at the level of the heart + resting

Question 24

Effects of gravity on blood pressure

Answer 24

• BP higher if below the level of the heart
• BP lower if above the level of the heart
• Effects of gravity maintain pressure gradient allowing blood flow from heart → foot when standing
• Pooling of blood occurs below the level of the heart upon standing
• postural hypertension is dizziness upon standing, decreased stroke volume and blood pooling in feet

Question 25

What is the equation for cardiac output

Answer 25

CO = heart rate x stroke volume
The volume of blood the heart pumps per minute
generally in L/min

Question 26

What are the factors that determine stroke volume

Answer 26

• preload is initial stretching of the cardiac myocytes prior to contraction
• contractility is the intrinsic strength of the cardiac muscle independent of preload
• afterload is the force or load against which the heart has to contract to eject the blood

Stroke volume: the amount of blood ejected from the ventricle with each cardiac cycle.
generally in L