Blood Flow Meter

BLOOD FLOW METER

Blood flow meters are used to monitor the blood flow in various blood vessels and to measure cardiac output.

Types

·                    Electromagnetic blood flow meters

·        Ultrasonic blood flow meters

·        Laser based blood flow meters

1. ELECTROMAGNETIC FLOWMETERS

§  Electromagnetic blood flow meters measure blood flow in blood vessels

§  Consists of a probe conneected to a flow sensor box

An Electromagnetic Flow Met er is a device capable of measuring the mas s flow of a fluid. Unlike the common flow meter you can find on the market it has no moving parts, and for this reason it can be made to withstand any pressure (without leakage)and any fluid(corrosive and non corrosive). This kind of flow m eter use a magnet and two electrodes to p eek the voltage that appears across the fluid moving in the magnetic field.

The Neumann Law (or Lenz Law) states that if a conductive wire is moving at right angle through a magnetic field, a voltage E [Volts] will appear at the end of the conductor (Fig.1):

E= B*L*V

Where

B = Magnetic Induction

[Weber/m2]

L    =    Length   of   the   portion  of   the   wire   'wetted'  by    the

magnetic      field [m]

V = Velocity of the wire [m/sec ]

Now imagine you have a plastic tube with two electrodes on the diameter and Mercury flowing into it (fig.2). A voltage will appear on the electrodes and it will be

E=B*L*V

As in the previous example (L in this case is the inner diameter of the tube).Mercury as tiny conductive wires next to each other: each wire, moving in the tube, will touch the two electrodes ,and thus you can mea sure their voltage.

An  interesting  fact  is  t hat  if  you  reverse  the  flow,  you  still  get  a  voltage  but  with  reverse polarity (Fig.1). Till  n ow  we have  talked  about  a conductive  fluid ,Mercury,  but this  stuff  will  also  work  with  non conductive fluid, provided that you use an   alternating magnetic field. Two physicists, Middleman and Cushing, in an unpublished work, stated that when using a non conductive f luid, if the frequency of the alternating magnetic field is v the voltage at the electrodes will be attenuated by a factor a so that:

Measuring the flow

`A perfect axisimmetri c construction cannot be achieved and thus some magnetic flux lines will 'wet' t he connecting wires to the electrodes. The alternating magnetic field will create an of fset voltage in this wire and even if the fluid is not moving, the measured voltage will not be zero.

2. ULTRASONIC FLOWMETERS

The blood cells in the fluid scatter the Doppler signal diffusively.In the recent years ultrasound contrast agents have been used in order to increase the echoes.Th e ultrasound beam is focused by a suitable transducer geometry and a lens.

In order to know where a long the beam the blood flow data is colledted, a pulsed Doppler must be used.The flow velocity is obtained from the spectral estimation of the received Doppler signalThe ultrasound Doppler device can be either a continuous wave or a pulsed Doppler

A Continuous Wave

No minimum range

Simpler hardware

Range ambiguity

Low flow cannot be dete cted

A Pulsed Doppler

Accuracy

No minimum flow

Minimum range

(Maximum flow) x (range)= limited the power decays exponentially because of the heating of the tissue. The absorption coefficient ~ proportional to frequency the far fi eld operation should be avoided due to beam divergence.

D = Transducer diam eter (e.g. 1 – 5 mm) the backscattered power is proportional to f .The resolution and SNR are related to the pulse duration. Improving either one of the parameters always affects inversely to the other

3. LASER DOPPLER FLO WMETRY

The principle of measurement is the same as with ultrasound Doppler.The laser parameter may have the following properties:5 mWH e-Ne-laser 632,8 nm wavelength.

The moving red blood cells cause Doppler frequency 30 – 12 0 00 Hz.The method is used for capillary (microvascular) blood flow measurements

Indicator Dilution Methods

Dye Dilution Method

A bolus of indicator, a co lored dye (indocyanine green), is rapidly injected in to the vessel. The concentration is measured in the downstream The blooddis drawn through a colorimetric cuvette and the concentration is measured using the principle of absorption photometry

Thermal Dilution Method

A bolus of chilled saline solution is injected into the blood circulation system (right atrium). This causes decrease i n the pulmonary artery temperature. An artery puncture is not needed in this technique.Several measurements can be done in relatively short time .A standard technique for measuring cardiac output in critically ill patients

Photoelectric Method

A beam of IR-light is directed to the part of the tissue which is to be measured for blood flow (e.g. a finger or ear lobe)

The blood flow modulatees the attenuated / reflected light which is re corded.The light that is transmitted / reflected is collected with a photodetector

Radioisotopes

A rapidly diffusing, inert radioisotope of lipid-soluble gas ( Xe or Kr) is injected into the tissue or passively diffused

The elimination of the radioisotope from microcirculatory bed is related to the blood flow:

Thermal Convection Probe

·        This is one of the earliest techniques for blood flow measurements

·        The rate of heat removal from the tissue under probe is measured

·        The concentric rings are i solated thermally & electrically from each other

The central disk is heated 1 – 2 C over the temperature of tissue.A temperature difference of 2- 3 C is established between the disks.The method is not very common due extreme nonlinear properties and difficulties in practical use (e.g. variable thermal characteristics of skin)