# Blood Flowmeters

Blood Flowmeters

Blood flow-meters are the devices that monitor the blood flow in various blood vessels and measure the cardiac output.

In the blood vessels, the blood flow rate is maximum along the axis of the vessel and decreases with the square of the distance from the axis, reaching zero at the wall of the vessel.

### Types of Blood Flowmeters

1. Electromagnetic Blood Flowmeter
2. Ultrasonic Blood Flowmeter
3. Range Gated Pulsed Doppler Blood Flowmeter
4. NMR Blood Flowmeter
5. LASER Doppler Blood Flowmeter

1. Electromagnetic Blood Flowmeter

This type of meter is widely used to measure pulsatile blood flow. The fundamental quantity measured by the flowmeter is blood velocity. It is suitable for determining instantaneous flow rates in vessels.

Basic Operating Principle

The operating principle of all electromagnetic type blood flowmeters is based upon Faraday’s law of electromagnetic induction.

This law states that when a conductor is moved at a right angle both to the magnetic field and its length, an EMF is induced in the conductor.

In this case, Blood is a conductor of electricity. When a magnetic field is applied to a blood vessel, the blood flow in the vessel causes an electric field to be induced in the direction mutually perpendicular to the direction of the applied magnetic field and the blood velocity.

 Electromagnetic Blood Flowmeter

So, induced EMF is given by

e = cHvd

where, e= induced emf
c = constant of proportionality
H = magnetic field strength
v = blood flow velocity
d = blood vessel diameter

If the magnetic field strength and blood vessel diameter remain unchanged then the induced emf will be directly proportional to the blood flow velocity.

The flow rate (Q) is given by
Q = v A
v = Q / A
where, A = cross sectional area of the tube.

Therefore, induced emf is directly proportional to the flow rate. The induced voltage is picked up the electrodes is amplified and displayed or recorded on a suitable system.

A blood flowmeter consists of an alternating current generator, probe assembly, series of capacitance coupled amplifiers, demodulator, dc amplifier and a recording device.

Types of Electromagnetic Flowmeters

Depending upon the shape of the energizing current waveform from the generator, electromagnetic flowmeters are of two types

1. Sinewave Flowmeters
2. Squarewave Flowmeters

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