medical instrumentation- blood flow rate and plethysmograph

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UEEG002
Medical
Instrumentation
Blood Flow Rate, Plethysmography
D.Poornima, AP(Sr.Gr)/EEE,
Sri Ramakrishna Institute of Technology, Coimbatore

01
Slide sub title
Blood Flow Rate
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• Blood flow
• important physiological parameter
• most difficult to measure accurately.
• Instruments for measurement have to meet stringent specifications
• Sensitivity and stability requirements depend upon the magnitude of flow,
location and the diameter of the individual vessels.
• The average velocities of blood flow also vary over a wide range in blood vessels.
• Measuring system must be least traumatic.
• Techniques for measuring the blood flow and velocity are categorized into
• invasive (surgical)
• non-invasive (through the skin).
• The most accurate method----sever the vessel and time the blood flow into a
calibrated beaker.
• But this is too radical for most protocols.
Blood Flow

Blood Flow Meters
01. Electromagnetic Blood Flowmeter
02. Ultrasound Blood Flow Meter

• The most commonly used instrument for the measurement of blood flow
• It can be measured in intact blood vessels without cannulation
• Requires the blood vessel be exposed so that the flow head or the measuring probe
can be put across it
Electromagnetic Blood Flow Meters

• The operating principle underlying all electromagnetic type flowmeters is Faraday’s law of
electromagnetic induction
• States that when a conductor is moved at right angles through a magnetic field in a direction at
right angles both to the magnetic field and its length, an emf is induced in the conductor.
• In the flowmeter, an electromagnetic assembly provides the magnetic field placed at right
angles to the blood vessel in which the flow is to be measured.
• The blood stream is a conductor, cuts the magnetic field and
voltage is induced in it.
• Induced voltage is picked up by two electrodes incorporated in
the magnetic assembly.
• The magnitude of the voltage is directly proportional to
• the strength of the magnetic field
• the diameter of the blood vessel
• the velocity of blood flow

E=CHVd
e = induced voltage
H = strength of the magnetic field
V = velocity of blood flow
d = diameter of the blood vessel
C = constant of proportionality
Magnitude of voltage
shows that the induced
voltage is directly
proportional to the
flow rate through the
blood vessel.

• The induced voltage picked up by the electrodes is amplified and displayed/recorded
on a suitable system.
• The system is calibrated in terms of volume flow as a function of the induced voltage
• The diameter of the blood vessel is held constant by the circumference of the hole in
the probe that surrounds it

Types of flow meters
– Sine wave flow meters
– Square wave flow meters
Sine wave flow meters
– the probe magnet is energized with a sine wave
– Induced voltage is also sinusoidal in nature.
– The major problem encountered --- blood vessel and the fluid contained in it act as
the secondary coil of a transformer when the probe magnet is excited.
– So, in addition to the induced flow voltage, there is an induced artefact voltage
generally referred to as ‘transformer voltage’
– The ‘transformer voltage’ is much larger than the flow induced voltage
– 90° out of phase with flow induced voltage
– Causes baseline drift

• Transformer voltage can eliminated by injecting into the signal a voltage of equal
strength, but having an opposite phase.
• The artefact signal is thus cancelled and only the flow induced voltage is left
behind for display.
• Another method is using a gated amplifier.
• Gated Amplifier amplifies only the signals during the portion of the cycle where
flow induced voltages are maximum
• Thus artefact voltage is prevented from getting amplified.
• This type of instrument is known as a ‘gated sine wave flowmeter’.
• require complicated electronic circuitry for the removal of transformer voltage
• As both the waveforms are of the same type, complete elimination becomes extr
emely difficult.
• Yields good signal-to-noise ratio
• Imposes stringent phase stability requirements with increasing frequency.

Square wave flow meters
– energizing voltage given to the magnet is a square wave
– Induced voltage is also a square wave.
– Has less stringent requirements of phase stability than the sine wave type
– It can suppress the quadrature voltages relatively easily.
– Easier to control the magnitude and wave shape of the energizing current
– The transformer induced voltage in this case is only a spike
– Separation of these two voltages becomes easier as the amplifier can be gated only for
a very short period.
– The square wave is amplitude modulated by the variation in blood flow and requires to
be demodulated before it can be fed to a recorder.

Transducer: The flow transducer consists of
an electromagnet - which provides a magneti
c field perpendicular to the direction of flow
a pair of pick-up electrodes- lying within the
field are whose axis is perpendicular to both the field
and the flow axis.
Preamplifier: The induced voltage picked up
by the electrodes is given to a low noise differen
tial amplifier through a capacitive coupling.
The preamplifier must have a very high commo
n-mode rejection ratio and input impedance.
The preamplifier also must incorporate the faci
lity for ‘probe balance’ by which signals in phase
with the magnet current can be selected to ba
lance background voltages in phase with flow
voltages

Gating Circuit: A gating amplifier helps to rem
ove spurious voltages generated during magnet
current reversal.
The gating action is controlled by the circuit w
hich provides an excitation current to the elect
romagnet.
Bandpass Amplifier: active RC bandpass amp
lifier selectively passes the amplified square
wave signal.
The shape of the wave after this amplifier is a
distorted sinusoid.
Detector: A phase sensitive detector is use to
recover the signal, which is an analogue of the
flow rate being measured.
Low-Pass Filter and Output Stage: Demodula
ted signal is given to an active RC low-pass filt
er - provides a uniform frequency response

Magnet Current Drive: The excitation current
supplied to the electromagnet is a one amper
e peak square wave current.
Given from a source of high impedance to ens
ure that it remains constant
Zero-Flow Reference Line: Before measureme
nt can be made for blood flow with electroma
gnetic flowmeters, it is essential to accurately
establish the signal corresponding to zero-flo
w.

• two types of ultrasonic blood flow-velocity meters.
• transit time velocity meter
• Doppler-shift type.
• For routine clinical measurements, the transcutaneous Doppler instrument has, by
far, superseded the transit-time type.
Ultrasonic Blood Flow Meters

Doppler Shift Flow Velocity Meters
– Non-invasive technique to measure blood velocity in a particular vessel from the surface
of the body.
– Based on the analysis of echo signals from the erythrocytes in the vascular structures.
– Because of the Doppler effect, the frequency of these echo signals changes relative to th
e frequency which the probe transmits.
– Shift is a measure of the size and direction of the flow velocity.
– The incident ultrasound is scattered by the blood cells and the scattered wave is receive
d by the second transducer

02
Plethysmography

• Plethysmography - used to measure changes in volume in different parts of the
body
• Done to check for blood clots in the arms and legs and also to measure how mu
ch air you can hold in your lungs
• Measures the changes with blood pressure cuffs or other sensors
• Sensors are attached to a machine called a plethysmograph
• Especially effective in detecting changes caused by blood flow
• Plethysmography is done if
• the patients show signs of blood clots in legs o
• when you have symptoms of upper respiratory problems

• A limb plethysmography can be performed in a doctor’s office or hospital.
• Blood pressure cuffs are placed on the leg and arm.
• These will measure the patients systolic blood pressure which is the
pressure of blood in the arm and leg when the heart contracts.
• Limb plethysmography is not associated with any risks or side effects.
Limb Plethysmography
• Normally, the systolic blood pressure
in your arm and leg are similar.

• Is performed in a specialist’s office or in
a hospital
• Patient sits in a small, airtight room and
nostrils are closed using clips.
• The patient is asked to breathe against
a mouthpiece.
• Measures how much air you can hold in
your lungs.
• Normal range depends on age, gender,
body size, and level of fitness.
• An abnormal result confirms that there’s
a problem with your lung capacity
Lung Plethysmography

Thank you

medical instrumentation- blood flow rate and plethysmograph

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