1 - Haemodynamics Flashcards
What is haemodynamics
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
What is the difference between serum and plasma
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
Instances when plasma viscosity changes
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
What is flow (definition + units)
the volume transferred per unit time
Usually measured in mL/min
What is pressure (definition + units)
force per unit area
Usually measured in mmHg
SI unit is Pascal
What is Darcy’s law (don’t need to memorise but understand the concepts)
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
What is resistance and conductance
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
What is the relationship between flow and resistance
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
Turbulent vs laminar flow
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
What are the factors that affect resistance to flow
- Diameter
- Length of vessel
- Viscosity
Pressure and resistance across vessel types
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
What is velocity (definition + units)
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
What is the relationship between flow + velocity + area
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
What is the dicrotic notch
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
What is the pulse pressure
systolic BP – diastolic BP
- Represents the force that the heart generates each time it contracts
- Can suggest cardiac output
What are some factors that affect pulse pressure
- 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)
Bounding vs thready pulse
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.
What conditions can cause changes in pulse pressure
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
What is the mean arterial pressure equations
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)
at what mean arterial pressure does organ perfusion become impaired
if it falls below 70mmHg
what is the difference between thrill and bruit
thrill can be felt (when palpating)
bruit can be heard (when listening)
How is auscultating done
- 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
Important things to consider when auscultating
- 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
Effects of gravity on blood pressure
- 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
What is the equation for cardiac output
CO = heart rate x stroke volume
The volume of blood the heart pumps per minute
generally in L/min
What are the factors that determine stroke volume
- 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
