Session 1: Haemodynamics Flashcards
What does blood contain? (4)
- Plasma
- WBCs
- Platelets
- RBCs
What does an increase in plasma viscosity lead to and why?
Increased whole blood viscosity and sludging of blood in peripheries - colder in the peripheries
What is the comments cause of increased plasma viscosity?
Multiple myeloma (cancer of plasma cells)
What leads to whole blood viscosity? (4)
- Increase in plasma viscosity - caused by multiple myeloma
- Polycythaemia (increase in RBCs)
- Thrombocythaemia (increase in platelets)
- Leukaemia (increase in WBCs)
What is peripheral resistance?
Determined by the state of contraction of resistant arterioles
What can minor changes in plasma viscosity measure?
Inflammatory response
What can minor changes in plasma viscosity result from?
Raised levels of acute phase plasma proteins eg fibrinogen, complement factors, C-reactive protein
What is characteristic of blood flow in vessels? (Type of flow)
Laminar flow
What is laminar flow?
Blood flows in streamlines layer of blood remaining the same distance from the wall
Describe the velocity of blood when in laminar flow?
Velocity of blood is greater in the centre of the vessel than that toward the outer edge (parabolic profile)
When does turbulent flow of blood occur? (5)
- When the rate of blood flow becomes too great
- Occlusion/stenosis
- Sharp turn in the vessel
- Blood passes a rough surface
- Increased resistance to blood flow
What units are used to measure flow?
Volume per unit time
What is characteristic of a compliant vessel?
Its blood volume will change in response to a change in pressure
What happens to flow when there is narrowing (stenosis) in a vessel?
Decreases (remember volume per unit time)
What happens to the velocity of blood in a stenotic vessel?
Increases
What happens in a vessel when there are consecutive stenoses?
Pressure decreases after every stenosis - two consecutive stenosis could stop flow altogether
What is critical limb ischaemia?
Consecutive stenoses in arteries in the limb resulting in reduced or no blood flow
What happens to an artery following a stenosis?
Post-stenotic dilatation (aneurysm)- increased velocity and therefore energy of blood causes walls to stretch
What is clinically relevant about calcified arteries in the elderly?
Pulse cannot be felt
What is an aortic thrill?
Vibration felt near stenotic aorta
On a descending aorta pressure tracing graph, what does the dicrotic notch denote? (3)
- Aortic valve shuts
- Systole ends
- Diastole begins (ventricles start to relax - pressure drops)
What is pulse pressure?
Difference between peak systolic pressure and end diastolic pressure (120-80=40mmHg)
How can mean arterial pressure be estimated?
Diastolic pressure + 1/3 pulse pressure (usually 93 mmHg)
A patient’s mean arterial pressure was measured - <70 mmHg. What does this value signify?
Organ perfusion will impaired
When is retrograde flow in the arterial system greatest?
When peripheral resistance is high - blood ‘bounces’ back
What determines the strength of the pulse? (2)
- Pulse pressure (main determinant)
2. Force with which the left ventricle ejects blood into arterial system
What are the possible causes of a weak/’thready’ pulse? (3)
- Left-ventricular failure
- Aortic valve stenosis
- Hypervolaemia (severe dehydration, bleeding)
What is the principle behind a stronger pulse?
Increase in pulse pressure as a result of a decrease in end-diastolic pressure
What are the possible causes of a strong/’bounding’ pulse? (2)
- Bradycardia
2. Low peripheral resistance - hot bath, exercise, pregnancy
How does bradycardia result in a bounding pulse?
Each systole starts later, which gives diastole more time to occur - end-diastolic pressure goes down (seen on pressure trace)
Pulse pressure widens
How does low peripheral resistance result in a bounding pulse? (Hint: think of pressure trace)
Vasodilatation of arterioles - less resistance means blood flows out to peripheral circulation more quickly
Pressure trace - diastolic decline becomes steeper; before the next pulse comes in, diastolic pressure has gotten lower
What is the significance of using the right-sized cuff while measuring someone’s BP?
Wrong cuff can give a falsely hypertensive reading
How does gravity affect arterial and venous pressure when standing?
- Arterial pressure - if BP measured on arm placed above the heart, pressure decreases (below the heart, increased pressure)
- Venous pressure - the further away from the heart, the higher the pressure
