Haemodynamics Flashcards
Plasma serum and blood composition
Serum is plasma without clotting factors
Whole blood viscosity changes relatively uncommon – polycythaemia (RBCs), thrombocythaemia (platelets) or leukaemia (WBCs)
“sludgey” thick blood – leading to dry gangrene in peripheries
Minor changes to plasma viscosity typically from acute phase /plasma proteins (fibrinogen, compliment, C-reactive protein (CRP) – used to measure plasma viscosity as indicator of inflammation
CRP clinically measured
Movement of blood = haemodynamics
metabolic demands of body dictate delivery of blood - blood moves from relative high to low pressure regions
Blood flows from Artery to arterioles to capillaries to venules to veins
Pressure starts high in the arteries and ends up low
Blood flow:
Laminar flow - is smooth, maintains the energy and is present in most arteries, arterioles, venules and veins
Turbulent flow is disorganised - travelling overall in roughly the same direction but sometimes it can double back on itself - energy is lost with this blood flow
This occurs when the pressure increase is beyond flow’s ability to match it linearly (i.e. making in linear flow)
Flow and pressure
Fluid (blood) goes from high to low pressure
Flow and pressure in the circulation pulsate – to model steady flow and pressure column
Flow - volume transferred per unit time – (L/min)
Pressure – force per unit area – (mmHg used as surrogate when measuring BP – SI unit is Pascal)
Flow and resistance
Flow and resistance:
Flow = K(∆P)
K is conductance - measure of ease of flow
R is resistance – measure of difficulty of flow - reciprocal of K - (1/K)
Flow = ∆P/R – Darcy’s law – like Ohm’s law (I = V/R)
Flow and resistance:
R = ∆P/Flow – the difference in mean pressure needed to move one unit of flow in steady state – mmHg min/L
So if resistance increases and flow is maintained – pressure difference has to rise
Poiseuilles law
Primary factors are diameter, length of vessel and viscosity - physiologically diameter is most important quantitatively – vessel length (L) doesn’t change and viscosity (η) of blood regulated within narrow range
R = 8ηL πr4
Flow = ∆P/R
Poiseuille’s law = Flow = ∆Pπr4 8ηL
The radius and changes in the radius of a vessel have a large impact on flow and or resistance – 4th power - 19% decrease in radius → halve flow
The biggest determinant of flow is the diameter of the vessel - even a small change to the radius is going to have a big effect
Pressure and resistance
R = ∆P/Flow
Change in pressure indicative of a change in resistance across vessel class
Resistance in the aorta is low – large diameter, relatively short
Smallest arteries and arterioles contribute greatest component of total peripheral resistance – biggest jump in pressure across vessel class
Arterioles are the seat of total peripheral resistance
Resistance in the pulmonary circulation is much lower than systemic system as there are Shorter and wider vessels
Flow and velocity
Flow and velocity:
Velocity (V) is distance fluid (blood) moves in a given time (cm/s)
Flow (F) of blood in a vessel is related to velocity:
F = V x A where A is the cross-sectional area of the vessel
A = πr2 therefore F ∝ V x r2
At constant flow, V is inversely related to r2 V ∝ 1/r2, so if the radius increases then the velocity decreases, and if the radius decreases then the velocity increases
Velocity and area
Capillaries – cross-sectional area is vast – thousands of times greater than that of aorta or named arteries
Velocity at capillary level much slower than at aorta or large
arteries
Why is lower velocity useful in capillaries? What is happening? - as the cross sectional area is huge the velocity (as radius has technically increased) slows right down, which is very useful for nutrient and gas exchange
Velocity increases again as vessel merge into larger veins and into vena cava
Blood pressure
Pulse pressure (PP) = SBP – DBP Mean arterial pressure (MAP) = DBP + (SBP-DBP)/3) = DBP + 1/3 PP
NOT arithmetic mean of SBP and DBP – time weighted mean – (AUC/T)
Below 70 mmHg – organ perfusion becomes impaired - Usually ~90mmHg
Mean arterial pressure
R = ∆P/Flow
Total flow is the cardiac output (CO) Pressure difference (∆P) is mean aortic pressure minus central venous pressure (CVP)
R is systemic resistance – total peripheral resistance (TPR)
TPR = mean aortic pressure - central venous pressure / cardiac output - CVP is near to 0 (BP reported as pressure above atmospheric pressure)
TPR = mean aortic pressure / cardiac output
Mean arterial pressure = CO x TPR
Mean arterial pressure is determined by cardiac output and total peripheral resistance
CO = SV x HR
Pulse pressure
Volume of blood ejected and the compliance of the arterial system govern pulse pressure
Increased stroke volume during exercise with relative compliance of vessels will cause an increase in pulse pressure
Haemorrhage – change to pulse pressure?
Age – atherosclerosis and pulse pressure?
Can inform us of important CVS function – stroke volume plus heart rate allows us to measure changes in cardiac output (CO = SV x HR)
Pulse - shock wave that arrives slightly before the blood itself - strong bounding pulse - indicative of a good stoke volume
Increasing pulse pressure – “strong” pulse
Described as a “bounding” pulse
Heart block – bradycardia
Vasodilatation – decrease peripheral resistance – hot bath, pregnancy
Elite athletes – systolic increased and diastolic decreases
Measuring blood pressure
Direct measurement precise but invasive technically demanding (historically!)
Indirect measurement convenient, not invasive and can be carried out by anyone with minimal training
Principles of indirect BP measurement rely on changes in type of flow – laminar and turbulent
Blood flow Pathological turbulence (caused by atheroma)
Velocity increases and flow decreases beyond stenosis
Thrill can be felt
Bruit can be heard
Korotkoff sounds
Creating turbulent flow and auscultating
Changes from laminar to turbulent flow create sound which can be heard and used to estimate blood pressure
Korotkoff sounds
Occlude vessel completely, then slowly release pressure, so the vessel opens up gradually so we create turbulent flow - sounds you hear when you release pressure is korotkoff sound
Estimating blood pressure by auscultation
Cuff size is important
Too small – will overestimate blood pressure
Too big – will underestimate blood pressure
Positioning of the cuff
Measure in both arms, often a difference, use the higher as reference
Sat comfortably, upright with legs uncrossed and flat on the ground
Arm supported
Should be repeated several times an arithmetic mean taken of two closest values
Unless otherwise stated, assumed that measurement will be taken at the level of the heart and RESTING
Gravity on BP
Pressure below the level of the heart greater and above the level of the heart lower
Effects of gravity maintains a pressure gradient allowing blood flow from heart to foot when standing
Pooling of blood occurs below the level of the heart upon standing in the venous system
Postural hypotension – dizziness upon standing – ↓stroke volume → transient arterial hypotension
