OximetryRefers to determination of

percentage of oxygen saturation of the circulating arterial blood.

Oxygen saturation= [ HbO2]

[HbO2] +[Hb]

[ HbO2] is concentration of oxygenated haemoglobin.

[Hb] is concentration of deoxygenated haemoglobin.

Oxygen saturation is an indication of performance of most important cardio-respiratory functions.

Application areas of oximetry: Diagnosis of cardiac and vascular

anomalies Treatment of post operative anoxia During Anaesthesia, prevention of tissue

hypoxia.

Oximetry is now considered as a standard care in anaesthesiology and significantly reduced anaesthesia-related cardiac deaths.

Under normal condition, during each cycle through the tissues, about 5ml of oxygen is consumed by the tissues from each 100 ml of blood which passes thro tissue capillaries.

In-vitro Oximetry: When blood is withdrawn from the subject

under anaerobic conditions and measurement for oxygen saturation is made at a later time in lab, the procedure is referred to as in-vitro oximetry.

A spectrophotometric measurement of oxygen saturation can be made by transmission or reflectance method.

Transmission Oximetry: Concentration of substances held in

solution are measured by determining the relative light attenuations that the light absorbing substances cause at each of several wave lengths.

Intensity of transmitted light I is related to incident light Io is given by, Beer Lambert’s law: I=I0-KCb, where C is concentration of absorbing medium , thickness b and K is extinction coeffecient . KCb is absorbance A.

Reflection oximetry: Based on scattering of light by

erythrocytes

.

In-Vivo Oximetry:Measures the oxygen saturation of

blood while the blood is flowing through the vascular system or it may be flowing through a cuvette directly connected with the circulatory system by means of a catheter.

Reflection and transmission techniques are used.

EAR OXIMETER

transmission principle to measure the arterial oxygen saturation.

In this case, the pinna of the ear acts as a cuvette.

Blood in the ear must be made similar to arterial blood in composition.

This is done by increasing the flow through the ear without appreciably increasing the metabolism.

Maximum vasodilatation is achieved by keeping the e£ warm. It takes about 5 or 10 min for the ear to become fully dilated after the ear unit has been put up in place and the lamp turned on.

the technique involves measuring the optical transmittance of the ear at 8 wavelengths in the 650 to 1050 nm range.

A 2.5 m long flexible fibre ear probe connects the patient to the instrument.

The ear probe can be either held in position for discrete measurements or can be convenient mounted to a headband for continuous display.

The resulting light transmissions are processes digitally according to a set of empirically determined constants and the resulting oxygen saturation results are displayed in the digital form.

Analog-to-digital

converter

Digital averaging

and storageCentral processor

Fibre optic ear probe

Optical sync, pin holes

The instrument is based on the Beer-Lambert law. The mathematical statement of this law for

wavelength can be written as:Aj = E1j C1Dl + E2j C2 D2 +….+Eij Ci Di+………+ ENj CN DN

If the measurements are made at eight wavelengths, the absorbance Aj at wavelength j can be related to the transmittance Tj in the following way:

Aj = - logTj where Tj = [I/I0]j To measure the percentage of functional

haemoglobin combined with oxygen. This can be expressed as:

S02 = C0 x 100

Co+ CR

SO2= A0+ A1 log T1+……..+ A8 log T8 B0+ B1 log T1+……...+ B8 log T8

The coefficient A0 through A8 and Bo through B8 are constants which were empirically determined by making a large number of measurements on a select group of volunteers.

Having found the coefficient set, the instrument measures ear transmittance at eight wavelengths 20 times per second, performs the indicated calculations, and displays the results.

Pulse oximeter:

Pulse oximeter is based on the concept that arterial oxygen saturation determinations can be made using two wavelengths, provided the measurements are made on the pulsatile part of the waveform.

The two wavelengths assume that only two absorbers are present: [HbO2] and reduced [Hb].

These observations are clinically proven: (1) Light passing thro ear or finger will be

absorbed by skin pigments, tissue, cartilage, bone, arterial blood, venous blood.

(2) Absorbance are additive and obey Beer-Lambert’s

A = -log T = logI0/ I = kCb

(3) Absorbances are fixed and do not change with time, Constant in composition and flow.

(4) only blood flow in arteries and arterioles is pulsatile.

Pulse oximeter probe:

Has two LEDs.One transmits infrared light at a

wavelength of approx. 940 nm and other at approx. 660 nm.

Absorption of these select wavelengths of light thro living tissues is significantly different for oxygenated haemoglobin and reduced haemoglobin.

Absorption is measured with a photosensor.

Most probes have a single photodetector (PIN-diode), so the light sources are generally sequenced on and off.A typical pulsing scheme of the LEDs

is shown.

No pulsation Pulsatile blood

Infrared light absorption in the finger

1 t t t t

There will be one signal that represents the absorption of red light (660 nm) and one that represents infrared light (940 nm)

The ac signal is due to the pulsing of arterial blood while the dc signal is due to all the non-pulsing absorbers in the tissue.

Oxygen saturation is estimated from the ratio (R) of pulse-added red absorbance at 660 nm to the pulse-added infrared absorbances at 940 nm.

R= ac 660/dc 660 “ ac 940/dc 940

Blood flow meters

widely used techniques for measuring the blood flow and velocity are categorized into

invasive (surgical) and non-invasive (through the skin).

ELECTROMAGNETIC BLOOD FLOWMETER

based upon Faraday's law of electromagnetic induction

which 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

Principle:

The blood stream, which is a conductor, cuts the magnetic field and voltage is induced in the blood stream.

This induced voltage is picked up by two electrodes incorporated in the magnetic assembly.

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

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

e = ClV where C1 is a constant and equal to CHd.

the flow rate Q through a tube is given by Q=VA

Therefore, V = Q/A where A is the area of cross-section of the tube,

e = C1* Q/A = C2*Q where C2 is a general constant and is given

by C1 / A.

equation shows that the induced voltage is directly proportional to the flow rate through the blood vessel.

ULTRASONIC BLOOD FLOWMETERS two types transit-time velocity meter Doppler-shift type.

Doppler-shift instruments, are available for the measurement of

blood velocity, volume flow, flow direction, flow profile and to visualize the internal lumen of a

blood vessel.

Doppler-shift Flow-velocity Meters

It is a non-invasive technique to measure blood velocity: in a particular vessel from the surface of the body.

It is 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 the frequency which the probe transmits.

The Doppler frequency 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 received by the second transducer.

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 'scatterer' and second as it leaves the scatterer.

Blood cells

If the blood is moving towards the transmitter; the apparent frequency f1 is given by

f1== f ( C- v cos θ) C

Where f = transmitted frequency C = velocity of sound in blood θ = angle of inclination of the

incident wave to the direction of blood

flow v = velocity of blood cells

Assuming that the incident and scattered radiations are both inclined at θ to the direct flow

f2 = f1 C ( C+ v cos θ)

Resultant Doppler shift is ▲f = f - f2 = f - f1 C ( C+ v cos θ)

= f- f ( C- v cos θ ) C C ( C+ v cos θ) = f 1- ( C- v cos θ ) ( C+ v cos θ)

since C>> v ▲f = 2f v cos θ C v = ▲f C 2f v cos θ

CW ultrasonic Doppler technique instrument works by transmitting a beam of high frequency ultrasound 3-10 MHz towards the vessel of interest.

A highly loaded lead zirconate titanate transducer is usually used for this purpose.

The transducer size may range from 1 or 2 mm to as large as 2 cm or more.

A separate element is used to detect the ultrasound back scattered from the moving blood.

The back-scattered signal is Doppler shifted by an amount determined by the velocity of the scatterers moving through the sound field.

Since the velocity varies with the vessel diameter to form a velocity profile, the returned signal will produce a spectrum corresponding to these velocities.

Transmitter crystal

Block diagram of Doppler shift blood flowmeter

The piezo-electric crystal A is electrically excited to generate ultrasonic waves, which enter the blood.

Ultrasound scattered from the moving blood cells excites the receiver crystal.

The electrical signal received at B consists of a large amplitude excitation frequency component, which is directly coupled from the transmitter to the receiver, plus a very small amplitude Doppler-shifted component scattered from the blood cells.

The detector produces a sum of the difference of the frequencies at D.

The low-pass filter selects the difference frequency, resulting in audio frequencies at E.

Each time the audio wave crosses the zero axis, a pulse appears at G.

The filtered output level at H will be proportional to the blood velocity

NMR BLOOD FLOWMETER

non-invasive method for the measurement of peripheral blood flow or blood flow in various organs.

The method pertains to quantum mechanical phenomenon related to the magnetic energy levels of the nucleus of some elements and their isotopes.

For blood flow measurement work, behaviour of the two hydrogen atoms of water is studied, since blood is approximately 83% water.

Due to the magnetic moment of the hydrogen atom, the nucleus behaves as a microminiature magnet which can be affected by externally applied magnetic fields.

The hydrogen nuclei orient themselves to produce alignment to a steady magnetic field.

The nuclear magnets precess around the magnetic field lines until they become aligned.

The angular frequency ( Larmor frequency) of this precession is given by

W = 2 Π v = r B0

where r is the ratio of the magnetic

moment to the angular momentum (the magnetic gyro ratio)

B0 is the density of the steady magnetic field and

v is the frequency of radiation.

Transmitter

electromagnet

The blood vessel of interest is positioned in a uniform steady magnetic field B0.

The nuclear magnets of the hydrogen atoms, which before insertion were randomly oriented, now begin to align themselves with B0.

Some begin to align parallel, whereas others commence to align anti-parallel.

The alignment occurs exponentially with a Time constant T1 known as longitudinal relaxation time.

For blood at 37°C, there are 0.553 x 1023 hydrogen nuclei per millilitre.

For B0 = 1000 gauss, there are 1.82 x 1016 per millilitre net nuclei aligned.

The maximum magnetization M0 due to the magnetic field B0 is given by M0 = X0 B0,

where X0 is the static nuclear magnetic susceptibility equal to 3.23 x 10-9 for blood at 37°C. Thus for B0 =1000 gauss, M0 = 3.23 microgauss.

Two magnets are used, a strong permanent magnet B0 for premagnetization and a weaker, homogeneous electromagnet BD for detection.

A graph plotted between the magnetization at the centre of the receiver coil for typical distances as a function of velocity shows that the magnetization changes proportionally to velocity and thus a measurement of the magnetization can yield velocity information.

If a receiver coil is used, the voltage induced in the coil by the magnetization is proportional to the cross sectional area of the vessel carrying the blood.

The NMR signal voltage proportional to velocity V, and multiplied by area A, will give a volumetric flow rate Q.

Cardiac Output MeasurementCardiac output is the quantity of

blood delivered by the heart to the aorta per minute.

determinant of oxygen delivery to the tissues

A fall in cardiac output may result in low blood pressure, reduced tissue oxygenation, acidosis, poor renal function and shock.

cardiac output is 4 to 6 l/min.

INDICATOR DILUTION METHOD a known amount of indicator is

introduced into or removed from a stream of fluid

measure the concentration difference upstream and downstream of the injection (or withdrawal) site, we can estimate the volume flow of the fluid.

The method employs several different types of indicators.

Two methods are generally employed for introducing the indicator in the blood stream : it may be injected at a constant rate or as a bolus.

The method of continuous infusion suffers from the disadvantage that most indicators recirculate, and this prevents a maxima from being achieved.

In bolus injection method, Indicator such as a dye or isotope is injected into a large vein or into right heart .

After passing through the right heart, lungs and left heart, the indicator appears in the arterial circulation.

The presence of an indicator in the peripheral artery is detected by a suitable (photoelectric) transducer and is displayed on a chart recorder called the cardiac output curve or the dilution curve.

During the first circulation period, the indicator would mix up with the blood and will dilute just a bit.

When passing before the transducer, it would reveal a big and rapid change of concentration.

This is shown by the rising portion of the dilution curve.

The run of the dilution curve

Had the circulation system been an open one, the maximum concentration would have been followed by an exponentially decreasing portion so as to cut the time axis shown by the dotted line.

The circulation system being a closed one, a fraction of the injected indicator would once again pass through the heart and enter the arterial circulation.

A second peak would then appear. When the indicator is completely mixed up with blood, the curve becomes parallel with the time axis.

The amplitude of this portion depends upon the quantity of injected indicator and on the total quantity of the circulating blood.

Calculating the cardiac output from the dilution curve, assume that M = quantity of the injected indicator in mg Q = cardiac output

Q = M / average concentration of indicator per litre of blood for duration of curve * curve duration in seconds Q = M x 60 l/min area under the curve

Suppose 10 mg of indicator was injected and average concentration from the curve is 5mg/l for curve duration of 20s, Calculate cardiac output.

DYE DILUTION METHOD commonly used indicator substance is

a dye. Fox and Wood (1957) suggested the

use of Indocyanine green (cardiogreen) dye which is usually employed for recording the dilution curve.

This dye is preferred because of its property of absorbing light in the 800 nm region of the spectrum where both reduced and oxygenated haemoglobin have the same optical absorption.

The procedure consists in injecting the dye into the right atrium by means of a venous catheter.

Usually 5 mg of cardiogreen dye is injected in a 1 ml volume.

The quantity used may be 2.5 mg in the case of children.

A motor driven syringe constantly draws blood from the radial or femoral artery through a cuvette.

The curve is traced by a recorder attached to the densitometer.

After the curve is drawn, an injection of saline is given to flush out the dye from the circulating blood.

Diagrammatic representation of a densitometer for quantitative measurement of dye concentration

The photometric part consists of a source of radiation and a photocell and an arrangement for holding the disposable polyethylene tube constituting the cuvette.

An interference filter with a peak transmission of 805 nm is used to permit only infrared radiation to be transmitted.

In order to avoid the formation of bubbles, the cuvette tubing should be flushed with a solution of silicone in ether.

A flow rate of 40 ml/min is preferred in order to get as short a response time as possible for the sampling catheter.

The sampling syringe has a volume of 50 ml/min.

The output of the photocell is connected to a low drift amplifier.

It has a high input impedance and low output impedance.

The amplification is directly proportional to the resistance value of the potentiometer R.

A potentiometric recorder records the amplifier signal on a 200 mm wide recording paper and a paper speed of 10mm/s.

THERMAL DILUTION TECHNIQUES A Thermal indicator of known volume

introduced into either the right or left atrium will produce a resultant temperature change in the pulmonary artery or in the aorta respectively, the integral of which is inversely proportional to the cardiac output.

Cardiac output = "a constant" x (blood temp. - injectate

temp.) area under dilution curve

SWAN-GANZ CATHETER - A 4-LUMEN CATHETER

Cardiac output thermal-dilution set-up

A solution of 5% Dextrose in water at room temperature is injected as a thermal indicator into the right atrium.

It mixes in the right ventricle, and is detected in the pulmonary artery by means of a thermistor mounted at the tip of a miniature catheter probe.

The injectate temperature is also sensed by a thermistor and the temperature difference between the injectate and the blood circulating in the pulmonary artery is measured.

The reduction in temperature in the pulmonary artery (due to the passage of the Dextrose) is integrated with respect to time .

The blood flow in the pulmonary artery is then computed electronically by an analog computer which also applies correction factors.

A meter provides direct reading of cardiac output after being muted until integration is complete so as to avoid spurious indications during a determination.