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Flow & Volume of Blood
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Measurement of
Flow & Volume of Blood
A measurement of paramount importance: concentration of O2 and other
nutrients in cells Very difficult to measure
Second-class measurement: blood flow and changes in blood volumecorrelate well with concentration
Third-class measurement: blood pressure correlates well with blood flow Fourth class measurement: ECG correlates adequately with blood pressure
How to make blood flow / volume measurements? Standard flow meters,
such as turbine flow meters, obviously cannot be used!
Indicator-dilution method: cont./rapid injection, dye dilution, thermodilution
Electromagnetic flowmeters
Ultrasonic flowmeters / Doppler flowmeters
Plethysmography: Chamber / electric impedance / photoplethysmography
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Indicator Dilution with Continuous
Injection
Measures flow / cardiac output averaged over several heart beats
Ficks technique: the amount of a substance (O2) taken up by an organ /
whole body per unit time is equal to the arterial level of O2 minus the
venous level of O2 times the blood flow
va CC
dtdm
Cdtdm
dtdVF
Blood flow, liters/min
(cardiac output)
Consumption of O2 (mL/min)
Arterial and venous
concentration of O2 (mL/L of blood)
dtdV
dtdmC
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Indicator Dilution with
Rapid Injection
A known amount of a substance, such as a dye or radioactive isotope,
is injected into the venous blood and the arterial concentration of the
indicator is measured through a serious of measurements until the
indicator has completely passed through given volume.
The cardiac output (blood flow) is amount of indicator injected,divided by average concentration in arterial blood.
t
dttC
mF
0
)(
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Dye Dilution
In dye-dilution, a commonly used dye is indocyanine green
(cardiogreen), which satisfies the following
Inert
Safe
Measurable though spectrometry Economical
Absorption peak is 805 nm, a wavelength at which absorption of blood isindependent of oxygenation
50%of the dye is excreted by the kidneys in 10 minutes, so repeat
measurements is possible
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Thermodilution
The indicator is coldsaline, injected into the right atrium using a
catheter
Temperature change in the blood is measured in the pulmonary
artery using a thermistor The temperature change is inversely proportional to the amount of
blood flowing through the pulmonary artery
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Measuring Cardiac Output
Several methods of measuring cardiac output In the Fick method, the indicator is O2; consumptionis measured by a spirometer. The arterial-venous concentration difference is measure by drawingsimples through catheters placed in an artery and in the pulmonary artery. In the dye-dilutionmethod, dye is injected into the pulmonary artery and samples are taken from an artery. In the
thermodilution method, cold saline is injected into the right atrium and temperature is measured inthe pulmonary artery.
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Electromagnetic
Flowmeters
Based on Faradays law of induction that a conductor that movesthrough a uniform magnetic field, or a stationary conductor placed
in a varying magnetic field generates emfon the conductor:
When blood flows in the vessel withvelocityu and passes through the magneticfield B, the induced emf emeasured at theelectrodes is.
L
de
0
LBu
For uniform B and uniform velocity profile u,the induced emf is e=BLu. Flow can be obtainedby multiplying the blood velocityuwith thevessel cross sectionA.
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Electromagnetic
Flowmeter Probes
Comes in 1 mm increments for
1 ~ 24 mm diameter blood vessels
Individual probes cost $500 each
Made to fit snuggly to the vessel
during diastole
Only used with arteries, not veins,
as collapsed veins during diastole
lose contact with the electrodes
Needless to say, this is an
INVASIVE measurement!!! A major advantage is that it can
measure instantaneous blood
flow, not just average flow
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Ultrasonic Flowmeters
Based on the principle of measuring the time it takes for an acoustic
wave launched from a transducer to bounce off red blood cells and
reflect back to the receiver.
All UT transducers, whether used for flowmeter or other
applications, invariably consists of a piezoelectric material, whichgenerates an acoustic (mechanical) wave when excited by an
electrical force (the converse is also true)
UT transducers are typically used with a gel that fills the air gaps
between the transducer and the object examined
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Transit time flowmeters
Effective velocity of sound in blood: velocity of sound (c)+
velocity of flow of blood averaged along the path of the ultrasound ()
=1.33 for laminar flow, =1.07 for turbulent flow: velocity of blood averaged over the cross sectional area, this is differentthan because the UT path is along a single line not over an averaged of
cross sectional area
Transit time in up/down stream direction:
Difference between upstream and downstream directions
cosvelocityconduction
distance
uc
Dt
2222
cos2
)cos(
cos2
c
uD
uc
uDt
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Transit Time
Flowmeters
The quantity T is typically very smalland very difficult to measure,
particularly in the presence of noise.
Therefore phase detection techniques
are usually employed rather then
measuring actual timing.
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Doppler
Flowmeters
The Doppler effect describes the change in the frequency of a
received signal , with respect to that of the transmitted signal, when
it is bounced off of a moving object.
Doppler frequency shift
c
uf
f
o
d
cos2
Speed of sound in blood
(~1500 m/s)
Angle between UT beam
and flow of blood
Speed of blood flow(~150 cm/s)
Source signalfrequency
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Directional Doppler
Directional Doppler borrows the quadrature phase detector
technique from radar in determining the speed and direction of an
aircraft.
Two carrier signals at 90 phase shift are used instead of a single
carrier. The +/- phase difference between these carriers after thesignal is bounced off of the blood cells indicate the direction,
whereas the change in frequency indicate the flowrate