Human Blood flow measurement techniques explained

1. BLOOD FLOW MEASUREMENT
Ms. Divya Jenifar P
AP - BME

2. INTRODUCTION
 Blood flow is the continuous circulation of blood in cardio vascular system
 Blood flow is the volume of blood /time
 Blood flow is highest in the pulmonary artery and the aorta, where these blood
vessels leave the heart.
 The flow at these points, called cardiac output, is between 3.5 and 5 liters/min in
a normal adult at rest.
 In the capillaries, , the blood flow can be so slow that the travel of individual
blood cells can be observed under a microscope
 The stroke volume is the volume of blood, in milliliters (mL), pumped out of the
heart with each beat.
 If the volume of blood increased (waste products not being removed to the
kidneys due to kidney failure for example) then there would be a greater
quantity of blood within the system increasing the pressure within.

3.  If the blood supply to an organ is reduced by a narrowing of the
blood vessels, the function of that organ can be severely limited.
 When the blood flow in a certain vessel is completely obstructed
(e.g., by a blood clot or thrombus), the tissue in the area supplied
by this vessel may die.
 Such an obstruction in a blood vessel of the brain is one of the
causes of a cerebrovascular accident (CVA) or stroke.
 An obstruction of part of the coronary arteries that supply blood
for the heart muscle -myocardial (or coronary) infarct or heart
attack.
 Types
 Electromagnetic blood flow meters
 Ultrasonic blood flow meters

4. ELECTROMAGNETIC BLOOD FLOWMETERS
 Measures instantaneous pulsatile flow of blood
 Works based on the principle of 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.
 A permanent magnet or electromagnet positioned around the blood vessel generates
a magnetic field perpendicular to the direction of the flow of the blood.
 The blood stream cuts the magnetic field and voltage is induced in the blood stream.
 Voltage induced in the moving blood column is measured with stationary electrodes
located on opposite sides of the blood vessel and perpendicular to the direction of the
magnetic field.
 The magnitude of the voltage picked up is directly proportional to the strength of
the magnetic field, diameter of the blood vessel and the velocity of blood flow
 This method requires that the blood vessel be exposed so that the flow head or the
measuring probe can be put across it.

5. The magnitude of the voltage picked up is directly
proportional to the strength of the magnetic field, the
diameter of the blood vessel and the velocity of blood
flow, i.e.
• e = CHVd
• where
e = induced voltage
• H = strength of the magnetic field
• V = velocity of blood flow
• d = diameter of the blood vessel
• C = constant of proportionality

6. ULTRASONIC BLOOD FLOWMETERS
 A beam of ultrasonic energy is used to measure the velocity of
flowing blood
 There are basically two types of ultrasonic blood flow-velocity
meters.
 Transit time flow meter
 Doppler-shift type

7. TRANSIT TIME FLOW METER
 As the name suggests, transit time flowmeters measure the time it takes for
an ultrasonic signal transmitted from one sensor, to cross a pipe and be
received by a second sensor. One transducer transmits sound while the
other acts as a receiver.
 Upstream and downstream time measurements are compared .The pulsed
beam is directed through a blood vessel at a shallow angle and its transit
time is measured T
 The transit time is shortened when the blood flows in the same direction as
the transmitted energy. The transit time is lengthened otherwise.
 With no flow, the transmit time would be equal in both the upstream and
downstream directions. With flow, sound will travel faster in the direction of
flow and slower against the flow.
 Through fluid and bubbles, the high frequency sound will be attenuated and
too weak to transverse.

9. DOPPLER TYPE ULTRASONIC FLOW METERS
 Based on the Doppler principle
 A transducer sends an ultrasonic beam with a frequency F into the flowing blood.
 A small part of the transmitted energy is scattered back and is received by a
second transducer arranged opposite the first one.
 The reflected signal has a different frequency F + FD or F – FD due to Doppler
effect.
 The frequency shift due to the moving scatterers is proportional to the velocity of
the scatterers.
 Alteration in frequency occurs first as the ultrasound arrives at the scattered and
second as it leaves the scattered.
 The Doppler component FD is directly proportional to the velocity of the flowing
blood.
 A fraction of the transmitted ultrasonic energy reaches the second transducer
directly with the frequency being unchanged

10.  Ultrasound is beamed through the vessel walls, backscattered by the red blood cells
and received by a piezoelectric crystal
 Oscillator-should have low output impedance to drive the low impedance crystal
 The ultrasonic waves are transmitted to the moving cells, which reflect the Doppler
shifted waves to the receiving transducer.
 The amplified radio-frequency signal plus carrier signal is detected to produce an
audio frequency signal.
 The zero-crossing detector emits a fixed area pulse each time the audio signal crosses
the zero axis.
 These pulses are low pass filtered to produce an output proportional to the velocity of
blood cells