1. Haemodynamics Flashcards
What is the difference between serum and plasma?
Plasma = fluid collected from unclotted blood Serum = fluid collected from clotted blood
Serum = plasma - clotting factors (esp fibrinogen)
What is peripheral resistance? How does it increase/decrease?
The resistance of arteries to blood flow.
- increases as arteries constrict
- decreases as arteries dilate
What causes increased whole blood viscosity?
- Marked increase in plasma viscosity
2. Increase in red blood cells (polycythaemia), leukocytes (leukaemia) or platelets (thrombocythaemia)
What is the result of increased whole blood viscosity?
Sludging of blood in peripheries.
What is the commonest cause for marked increase in plasma viscosity?
Multiple myeloma (cancer of plasma cells - increased immunoglobulins)
How can the inflammatory response be measured in blood?
Inflammation increases concentration of acute phase plasma proteins (e.g. Fibrinogen, complement factors and C-reactive protein), causing MINOR changes in plasma viscosity.
So measure C-reactive protein (CRP) to assess inflammation.
What is the difference between laminar and turbulent blood flow?
- Laminar = blood flows in streamlines with each layer of blood remaining the same distance from the wall.
- Turbulent = blood flows in all directions in the vessel, and is continually mixing.
What type of blood flow is usually present in vessels?
Laminar
Where is blood velocity greatest in laminar flow?
Greater in vessel centre than in periphery due to friction of vessel walls (creates a parabolic profile).
What might cause turbulent blood flow?
- Rate of blood flow becomes too great (e.g. Increased chronotropy in anaemia)
- When blood passes by a vessel obstruction (occlusion)
- When blood makes a sharp turn
- When blood passes over rough surface (atheroma - porridge)
- Increases resistance to blood flow (stenosis)
What is the difference between blood flow and velocity?
Blood flow = volume of blood moving per unit time (e.g. L/hr)
Blood velocity = distance traveled by blood per unit time
What is the relationship between blood flow and velocity?
Flow = mean velocity x cross-sectional area of the vessel
How does blood velocity change according to total cross-sectional area? What is the implications of this in capillaries?
Blood velocity varies inversely with total cross-sectional area of blood vessels: velocity decreases as total area increases.
Velocity is slowest in capillaries, allowing time for gas and nutrient exchange.
How does blood flow change according to vessel diameter?
As vessel diameter decreases, resistance increases and blood flow decreases.
What is the effect of obstruction/stenosis on blood flow and velocity?
Decreases flow but increases velocity.
What is the effect of 2 or more stenoses in 1 blood vessel?
Completely inhibits blood flow as each stenosis reduces flow.
How can a vessel stenosis cause an aneurysm?
Stenosis increases blood velocity… causes blood vessel damage… causes post-stenotic dilation… aneurysm.
What is the terminology for the feel and sound cause by a stenosis?
Thrill (feel)
Bruit (sound in peripheral artery)
Murmur (sound across valve)
Why is it harder to feel a pulse in the elderly?
Calcification of arterial tree due to ageing reduces compliance of vessel.
How does arterial blood velocity change during systole and diastole?
Positive peak during systole, reaching 0 again by start of diastole. Negative during diastole.
Why might there be a small increase in arterial blood velocity during diastole?
Retrograde flow in the arterial system can occur when peripheral resistance is high.
How does blood pressure change during systole and diastole?
Peak in blood pressure during systole: anacrotic limb and start of dicrotic limb. Systole ends at dicrotic notch (closing of aortic valve).
Continued decrease in pressure in diastole (end of dicrotic limb).
What is mean arterial pressure represented by on a blood pressure trace?
Area under the curve
What is ‘pulse’?
Rhythmic expansion of an artery caused by ejection of blood from the ventricle.
What we feel is a shock wave that arrives slightly before the blood itself.
What is pulse pressure and how is it measured?
Pulse pressure = peak systolic pressure - end diastolic pressure
Because of the way we measure BP with a sphygomanometer: pulse pressure = systolic pressure - diastolic pressure.
So most commonly, pulse pressure is 120 mmHg - 80 mmHg = 40 mmHg
How can mean arterial pressure be estimated?
Mean arterial pressure = diastolic pressure + 1/3 pulse pressure.
What is the value of mean arterial pressure most commonly?
80 mmHg + 13 mmHg = 93 mmHg
At which point is MAP too low and what is the consequence of this?
If MAP falls below 70 mmHg then organ perfusion is impaired.
What is the strength (volume) of the pulse mainly determined by?
- Force with which the left ventricle is able to eject blood into the arterial system and thus develop a normal shock wave.
- The pulse pressure: the greater the pulse pressure, the stronger the pulse.
How is a weak or a strong pulse described?
‘Thready’ or ‘bounding’
Name 3 conditions that can cause a thready pulse.
- Left ventricular failure
- Aortic valve stenosis
- Hyopvolaemia (severe dehydration)
Name a condition that can cause a bounding pulse.
Bradychardia (such as heart block)
Why does bradycardia cause a bounding pulse?
Widense pulse pressure: diastole has more time to occur… so lower end-diastolic pressure… so increased pulse pressure.
Which 4 main factors interact to affect blood pressure?
- Cardiac output
- Blood volume
- Peripheral resistance
- Blood viscosity
Suggest everyday causes for low peripheral resistance and explain the impact of this on diastolic pressure and pulse pressure.
Low peripheral resistance (e.g. hot bath, exercise, pregnancy)… lowers diastolic pressure… and thus increases pulse pressure.
Why does aortic insufficiency increase pulse pressure?
The regurgitant flow of blood through the aortic valve back into the left ventricle… decreases the diastolic blood pressure in the aorta… and therefore increases the pulse pressure.
Why must BP be measured at the same level as the heart and when sitting/lying?
Venous and arterial blood pressure greatly increase towards the feet or decrease above the heart due to gravity.
This effect is enhanced by standing.
How are systolic and diastolic pressure determined when measuring BP?
Systolic pressure: when 1st hearing karotkoff sounds (phase 1 sounds)
Diastolic pressure: when sounds disappear (phase 5)
What is normal BP?
120/80
