Detailed description in blood flow measurement in single vessel and tissue floe

1. BLOOD FLOW MEASUREMENT

2. Blood flow measurement
DEFINITION: Measuring and monitoring the factors that influence the force and
flow of blood
PURPOSE: To aid in diagnosing, monitoring and managing critically ill patients
INTRODUCTION:
• Blood flow is the one of the important physiological parameter and the most
difficult to measure accurately
• In the SI system, the unit of volume flowrate is m3/s, but, in physiological
measurements, the units L/s, L/min, or mL/min are commonly used
• They are categorized into
1 invasive
2 non invasive

3. BLOODVESSELS

4. Difference between arteries and veins
Arteries Veins
oxygenated blood
away from the heart
It hold 10% -12 % of
the blood
It is thick walls
compare than veins
pressure from the heart
is so strong that blood
is only able to flow in
one direction
carry the deoxygenated blood
back to the heart
hold most of blood in body
(70%) -> “capacitance vessels”
Have thin walls & stretch easily
to accommodate more blood
without increased pressure
(=higher compliance )
Have only 0- 10 mm Hg
pressure

5. Physical Laws Describing Blood Flow
• Blood flows through vascular system when there is pressure difference (∆P)
at its two ends
Flow rate is directly proportional to difference in pressure
∆P = P1 - P2
• Flow rate is inversely proportional to resistance
𝐹𝑙𝑜𝑤 =
∆𝑃
𝑅
• Determines how much blood flows through
a tissue or organ.

6. Poiseuille's Law
• The flow of fluids through an IV catheter can be described by Poiseuille’s Law
• Poiseuille's Law describes factors affecting blood flow
𝑏𝑙𝑜𝑜𝑑 𝑓𝑙𝑜𝑤 =
∆𝑃
𝑅
=
𝑟4
∆𝑃
8
𝜋
𝐿𝑛
Where,
r - radius of vessel, ∆𝑃 - pressure gradient along vessel
L - length of vessel, n - viscosity of fluid
diameter of vessel is very important for resistance
• Tubing diameter: An important and frequently cited relationship is that of the
tubing’s radius
• Doubling the diameter of a catheter increases the flow rate by 16 fold
(r4) The larger the IV catheter the greater the flow
• Fluid Viscosity: Flow is inversely proportional to the viscosity of the
fluid Increasing viscosity decreases flow through a catheter

7. Need for blood flowmeter
• Inspection for block in blood
flow
• Testing artificial blood
vessels during organ
transplantation
• During Fistula creation in
dialysis
• Fig shows – blood flow in
different blood vessels
present in the body

8. Disease Problems due to blood flow - description
Ischemia severe blockage of a coronary artery
Atherosclerosis hardening of the arteries -> when plaque builds up on the walls of your
arteries and eventually blocks blood flow Plaque is made of cholesterol,
fat, and calcium
Coronary artery
disease (CAD)
plaque build-up in your arteries - caused the arteries to narrow and harden
Heart failure heart muscle is weakened/damaged
- no longer pump the volume of blood
Heart attacks Lack of blood reaches your heart -> due to artery blockage
It damage the heart muscle
Strokes when a blood clot blocks an artery in the brain -> reduces the blood supply
parts of the brain -> likely to be damaged
abdominal
aortic aneurism
Bulge in blood vessel If the aorta ruptures -> cause heavy bleeding
Peripheral
artery disease
(PAD)
narrowed arteries reduce blood flow to your limbs
Poor circulation can cause leg pain

9. BLOOD FLOW
• TYPES OF BLOOD FLOW
Six types of blood flow are present
Plug Flow
Laminar Flow
Parabolic Flow
Disturbed Flow
Turbulent Flow
Pulsatile Flow

10. Types of blood flow
• Plug Flow
• Plug flow is a simple model of the velocity profile of a
fluid flowing in a pipe In plug flow, the velocity of the fluid is
assumed to be constant across any cross-section of the pipe
perpendicular to the axis of the pipe
• Laminar Flow
• occurs when a fluid flows in parallel layers, with no disruption
between the layers
• Parabolic Flow
• Under parabolic flow, blood cells in the middle of the vessels
move the fastest, with a gradual decrease in flow velocity for
points farther away from the center

11. Disturbed Flow
Two types of disturbed flow Normal and abnormal
1 Normal disturbed flow
2 Abnormal disturbed flow

12. Turbulent Flow
The motion of a fluid having local velocities and pressures that fluctuate
randomly
Pulsatile Flow
In fluid dynamics, a flow with periodic variations is known as pulsatile flow
The cardiovascular system of chordate animals is very good example where
pulsatile flow is found.

13. Requirements for measurement ranges
1.Blood flow in a Single Vessel
• Roughly estimated by the size of the blood vessel, because the
vessel size can vary adaptively with the blood flow rate
• Assumed as a long straight tube having a circular cross section, and
flow is assumed as steady and laminar, a parabolic velocity profile
• In large artery, very high velocities can occur temporarily so that
turbulent flow appear
2.Tissue blood flow
• usually represented as the volume flow rate per unit mass of the
tissue
• If a region of a tissue is uniform-> estimated by the total blood
flow
• flow is not uniformly distributed -> estimated by the average tissue
blood flow

14. Different Blood flow measurements
Blood flow single vessel
• Electromagnetic flow meter
• Ultrasonic blood flow meter
•Transit and phase
shift ultrasound
flowmeters
•Ultrasonic Doppler
flowmeters
• Indicator dilution method
Tissue blood flow
• Plethysmography
• Laser Doppler flowmeters

15. ELECTROMAGNETIC (EM) BLOOD
FLOWMETER
• Measures instantaneous pulsatile
flow of blood
• Principle : electromagnetic Induction-
The voltage induced in a conductor
moving in a magnetic field is
proportional to the velocity of the
conductor
• The conductive blood is the moving
conductor
For an uniform magnetic field B and a
uniform velocity profile u, the induced
emf is
e = BLu
Where,
B – magnetic flux density, T, L- length
between electrodes, m, u – instaneous
velocity of blood, m/s

16. Types of Electromagnetic Blood Flow
Meters
DC Flow meters AC Flow meters
• Use DC Magnetic field.
• Cause electrode polarization and
amplifier drift.
• Output same as ECG
• Poor SNR
• Electromagnets are driven by
alternating currents.
• The transducer acts like a
Transformer and induces error
voltages that often exceed the signal
levels by several orders of
magnitude

17. Sine wave Flowmeters Square wave Flowmeters
• Probe magnet is energized with a sine
wave and the induced voltage will also
be sinusoidal
• The transformer induced voltage is 90˚
out of phase and is eliminated by
• Injecting a voltage of equal strength
and opposite phase into the signal.
• Using a gated amplifier.
• Probe magnet is energized with a
square wave and induced voltage is
only a spike.
• Separation of transformer voltage is
easy
• For the measurement action square
wave is amplitude modulated by
variation in blood flow

18. Advantage of electromagnetic
flow meter
Disadvantages of electro magnetic
flow meter
• In blood vessels, the velocity is not
uniform. But still valid by taking
the mean velocity as long as the
velocity profile is axisymmetric
about the longitudinal axis of the
vessel.
• Using mean velocity, the flow rate
Q is expressed as
𝑄 =
𝜋𝑑2 𝑈
4
=
𝜋𝑑𝑉
4𝐵
Many factors which may affect
sensitivity.
• velocity profile,
• magnetic field distribution, and
• electric conductivities of the inside
vessel wall and outside media may
affect the electromotive force(emf).
Today, the electromagnetic
flowmeter principle is replaced to a
large extent by other methods like
ultrasound and thermodilution that
require less surgical incisions and are
more convenient to use

19. ULTRASONIC FLOW METER
• Can measure instantaneous flow of blood
• The ultrasound can be beamed through the skin,
thus making transcutaneous flowmeters practical
• Advanced types of ultrasonic flowmeters can also
measure flow profiles
Generally ultrasonic flow meters works on
two principle:
1. Doppler effect ultrasonic flow meter
It uses reflected ultrasonic sound to measure the fluid velocity By measuring the
frequency shift between the ultrasonic frequency source, the receiver and the fluid
carrier In this the relative motion are measured The resulting frequency shift is
named as “Doppler effect”
2 Transit time difference ultrasonic flowmeter
With the time of flight ultrasonic flowmeter the time for the sound to travel
between a transmitter and a receiver is measured This method is not dependable on

20. Transit time Flow meter
Transit time between the pulses of ultrasound propagating into and against
the direction of flow
Transit time in the upstream and downstream direction is,
𝑡 =
𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
=
𝐷
𝑐 ± 𝑢 𝑐𝑜𝑠𝜃
Where,
c – velocity of sound
u – Blood flow velocity
D- distance between the transducers
t – transit time

21. Doppler-shift flow-velocity meters
• It is an non invasive method
• It is based on the analysis of echo signals from erythrocytes(RBCs) in blood
• The incident ultrasound is scattered by the blood cells and scattered wave is received by the
second receiver
• The frequency shift of the scattered wave gives idea about velocity of scatterers
• The Doppler frequency shift is a measure of size and direction of the flow velocity
• For small changes, the fractional change in frequency is equals to the fractional change in velocity
𝑓 𝑑
𝑓0
=
𝑢
𝑐
Where,
fd –Doppler frequency shift
f0 – source frequency
u - target velocity
C- velocity of sound
Doppler frequency 𝑓𝑑 =
2 𝑓0 𝑢 𝑐𝑜𝑠𝜃
𝑐

22. Continuous DopplerVs Pulsed Doppler
CONTINUOUS DOPPLER PULSED DOPPLER
 Separate crystal for transmitting
and receiving
 Can measure high velocities
 Range ambiguity
 Single crystal transmits and
receives
 Cant measure high velocities
 Range resolution

23. Transit timeVs Doppler ultrasonic flow
meter
TRANSIT TIME METHOD DOPPLER METHOD
 Flow rate is measured by detecting
the time difference generated with
the flow
 Suitable for clean fluid
 Meets IEC 41 requirements
 Very high measurement accuracy
 Flow velocity profile and flow rate
is measure by using Doppler
frequency shift
 Suitable for opaque fluid
 Latest intelligent fourth generation
technology

24. FICK PRINCIPLE
• FICK principle – “gold standard”
• Fick Principle relies on the total uptake of a substances by peripheral tissue is
equal to the product of blood flow to the peripheral tissue and arterial – venous
concentration difference of the substances
• Fick cardiac outputs are infrequently used because difficulties in collecting and
analysing exhaled gas concentration in critically ill patients because may not
have normal 𝑉𝑂 value
• 𝑂2 concentration - spirometer
• arterial-venous concentration
catheters placed in an artery and
in the pulmonary artery

25. INDICATOR DILUTION METHOD
• A method in which a definite amount of indicator is injected into the
blood stream, and the averaged flow rate is estimated from the time
course of the concentration of the indicator at the downstream.
• Color dyes, radio isotopes, electrolytes, or heat - indicators

26. Dye Dilution Method
A bolus of indicator, a colored dye (indocyanine green), is rapidly injected in
to the vessel. The concentration is measured in the downstream
The blood is drawn through a colorimetric cuvette
and the concentration is measured using the
principle of absorption photometry
  dttC
m
F t
 
 1
0
Avg.
flow
amount of
dye
1% peak C

27. THERMO DILUTION
• A bolus of chilled saline solution is
injected into the blood circulation
system (right atrium). This causes
decrease in the pulmonary artery
temperature.
 dttTc
Q
F t
bbb  
 1
0

An artery puncture is not needed in this technique
Several measurements can be done in relatively short time
A standard technique for measuring cardiac output in critically ill patients
density of blood
( kg/m3)
specific heat of blood
(J/(kg*K)
heat content
of injectate

28. Thermo Dilution…
A special four-lumen catheter is floated through the brachial vein into place
in the pulmonary artery
1. A syringe forces a gas through one lumen; The gas inflates a small,
doughnut-shaped balloon at the tip
2. The cooled saline indicator is injected through the second lumen into the
right atrium
->The indicator is mixed with
blood in the right ventricle
->The resulting drop in
temperature of the blood is
detected by a thermistor located
near the catheter tip in the
pulmonary artery
3. The third lumen carries the
thermistor wires
4. Fourth lumen -> Used for
withdrawing blood samples

29. FLOWVELOCITY MEASUREMENTS BY HEAT DISSIPATION
• The rate of heat dissipation from a heated element placed in the blood stream depends on the flow
velocity
• Although it also depends on many other factors, such as the
• temperature difference between the element and the surrounding fluid,
• the size and the shape of the element,
• viscosity,
• thermal conductivity,
• specific heat and
• density of the fluid, and
• the state of the flow, i e , if the flow is laminar or turbulent,
In such a situation, the rate of heat dissipation H can be approximated a
𝐻 = 𝑎 + 𝑏𝑈 𝑚
where U is the flow velocity a, b, and m are constants
• This method is NON-LINEAR, with large sensitivity at low velocities and a small sensitivity at high
velocities
• To maintain the element at a constant temperature, electric heating is used

30. THERMISTORVELOCITY PROBE
• The thermistor is a convenient device for use in thermal velocity probes
because it has a large temperature coefficient
• Two thermistors are commonly used in the flow probe
1. One measures the fluid temperature, and
2. The other is heated to a higher temperature,
• The temperature difference between the heated thermistor and the fluid is
kept constant

31. HOT FILMVELOCITY PROBE
• A thermal probe consisting of a thin metal film has been used for local flow
measurements, and is called a “hot film velocity probe”, or “hot film
anemometer”
• The sensing element consists of platinum, platinum-silver, or gold fused on a
Pyrex glass or quartz rod
• Hot film velocity probes have been used in
cardiovascular studies such as dynamic measurement of the velocity profile,
measurement of the velocity wave form,
detecting flow reversal or measurement of turbulence

32. IMPEDANCE CARDIOGRAPHY
• Impedance cardiography is a technique in which stroke volume or cardiac
output is estimated by the waveforms of transthoracic electric impedance
• To record transthoracic electrical impedance, the tetra polar electrode
arrangement generally been used
• An a c current (range of 20–100 kHz at a current level within the range of 10
μA–10 mA) is supplied through current electrodes -> placed at the top of the
neck and at the end of the rib cage or distal to it
• Voltage electrodes are placed at the base of the neck and at the level of the
xiphisternal joint
• The induced voltage between voltage electrodes is measured
• 𝑆𝑡𝑟𝑜𝑘𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 =
𝜌 𝑏 𝐿2
𝑍0
2 ∆𝑍
Where,
∆𝑍 – thoracic impedance, 𝜌 𝑏 - blood resistivity
L- distance between voltage
Electrodes, Z0 – average thoracic impedance

33. TISSUE BLOOD FLOW
MEASURMENT

34. Laser Doppler Flowmetry
• The principle of measurement is the same as
with ultrasound Doppler
• The method is used for capillary
(microvascular) blood flow measurements
• It consists of a photo detector and a LASER
source
• The laser parameter may have e.g. the
following properties: 5 mW He-Ne-laser 632.8
nm wavelength
c
v
ff cd 2

35. Photo Plethysmography
• Plethysmography means the methods for recording volume changes of an organ or a
body part (e.g. a leg or a hand)
• Light can be transmitted through a capillary bed.
• As arterial pulsations fill the capillary bed, the changes in volume of the vessels modify
the absorption, reflection, and scattering of the light.
• output of a light-emitting diode (transmitter) is altered by tissue absorption to modulate
the phototransistor(receiver/photosensor)
• In the photosensor, an infrared filter is used to prevent visible light interference

36. Electric-Impedance Plethysmography
• It is simple to attach electrodes to a segment of tissue and measure the
resulting impedance of the tissue.
• Principle : As the volume of the tissue changes in response to pulsations of
blood as happens in a limb.
• Different tissues in a body have a different resistivity. Blood is one of the
best conductors in a body
• A constant current is applied via
skin electrodes
• The change in the impedance is
measured
• Measured volume prportional to
impedance and thus rate of
volume change can be measured

37. Summary
• Usually more invasive methods are used than with blood pressure
measurements
• Used for understanding physiological processes (e.g. medicine
dissolution). Also used for locating clots in arteries
• Indirect measurements are done by using ultrasound or
plethysmographic method
• Normal velocity is 0.5 - 1 m/s
• Direct measurements are done by dilution methods (dye / thermal)