Principles of Biomedical Systems

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Principles of Biomedical Systems Devices
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WEEK 10 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 volume ? correlate 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 ?

Change in due to continuously added
indicator m to volume V
Consumption of O2 (mL/min)
Blood flow, liters/min(cardiac output)
Arterial and venousconcentration of O2 (mL/L of
blood)
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Ficks technique

• How is dm/dt (O2 consumption) measured?
• Where and how would we measure Ca and Cv?
  (Exercise)

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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.

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Indicator Dilution Curve
After the bolus is injected at time A, there is a
transportation delay before the concentration
begins rising at time B. After the peak is
passed, the curve enters an exponential decay
region between C and D, which would continue
decaying alone the dotted curve to t1 if there
were no recirculation. However, recirculation
causes a second peak at E before the indicator
becomes thoroughly mixed in the blood at F. The
dashed curve indicates the rapid recirculation
that occurs when there is a hole between the left
and right sides of the heart.
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An Example
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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 is independent of oxygenation
• 50of the dye is excreted by the kidneys in 10
  minutes, so repeat measurements is possible

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Thermodilution

• The indicator is cold saline, 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 consumption
is measured by a spirometer. The arterial-venous
concentration difference is measure by drawing
simples through catheters placed in an artery and
in the pulmonary artery. In the dye-dilution
method, 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
in the pulmonary artery.
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Electromagnetic Flowmeters

• Based on Faradays law of induction that a
  conductor that moves through a uniform magnetic
  field, or a stationary conductor placed in a
  varying magnetic field generates emf on the
  conductor

When blood flows in the vessel with velocity u
and passes through the magnetic field B, the
induced emf e measured at the electrodes is.
For uniform B and uniform velocity profile u, the
induced emf is eBLu. Flow can be obtained by
multiplying the blood velocity u with the vessel
cross section A.
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ElectromagneticFlowmeter 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, which generates 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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Near / Far Fields

• Due to finite diameters, UT transducers produce
  diffraction patterns, just like an aperture does
  in optics.
• This creates near and far fields of the UT
  transducer, in which the acoustic wave exhibit
  different properties
• The near field extends about dnfD2/4?, where D
  is the transducer diameter and ? is the
  wavelength. During this region, the beam is
  mostly cylindrical (with little spreading),
  however with nonuniform intensity.
• In the far field, the beam diverges with an angle
  sin?1.2 ?/D, but the intensity uniformly
  attenuates proportional to the square of the
  distance from the transducer

Higher frequencies and larger transducers should
be used for nearfield operation. Typical
operating frequency is 2 10 MHz.
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UT Flowmeters
Zero-crossingdetector / LPF
Determine direction
High acoustic impedance material
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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 (û) û1.33u for
laminar flow, û1.07u for turbulent flowu
velocity of blood averaged over the cross
sectional area, this is differentthan û 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
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Transit Time Flowmeters
The quantity ?T is typically very small and 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

Speed of blood flow (150 cm/s)
Source signal frequency
Angle between UT beamand flow of blood
Speed of sound in blood (1500 m/s)
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Doppler Flowmeters
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Problems Associated withDoppler Flowmeters

• There are two major issues with Doppler
  flowmeters
• Unlike what the equations may suggest, obtaining
  direction information is not easy due to very
  small changes in frequency shift that when not in
  baseband, removing the carrier signal without
  affecting the shift frequency becomes very
  difficult
• Also unlike what the equation may suggest, the
  Doppler shift is not a single frequency, but
  rather a band of frequencies because
• Not all cells are moving at the same velocity
  (velocity profile is not uniform)
• A cell remains within the UT beam for a very
  short period of time the obtained signal needs
  to be gated, creating side lobes in the frequency
  shift
• Acoustic energy traveling within the beam, but at
  an angle from the bam axis create an effective
  ??, causing variations in Doppler shift
• Tumbling and collision of cells cause various
  Doppler shifts

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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 90º phase shift are used
  instead of a single carrier. The /- phase
  difference between these carriers after the
  signal is bounced off of the blood cells indicate
  the direction, whereas the change in frequency
  indicate the flowrate

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Directional Doppler
(a) Quadrature-phase detector. Sine and cosine
signals at the carrier frequency are summed with
the RF output before detection. The output C from
the cosine channel then leads (or lags) the
output S from the sine channel if the flow is
away from (or toward) the transducer. (b) Logic
circuits route one-shot pulses through the top
(or bottom) AND gate when the flow is away from
(or toward) the transducer. The differential
amplifier provides bi-directional output pulses
that are then filtered.