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EI 2311 / BIOMEDICAL INSTRUMENTATION VII SEM EEE

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI- 621213.

QUESTION BANK

SEMESTER : VII DEPARTMENT: EEE

SUBJECT NAME: BIOMEDICAL INSTRUMENTATION SUBJECT CODE: EI2311

UNIT 3- NON – ELECTRICAL PARAMETER MEASUREMENTS

PART A (2 Marks)

1. Define cardiac output.( NOV 2007)

Cardiac output (Q or or CO ) is the volume of blood being pumped by the heart, in particular

by a left or right ventricle in the time interval of one minute. CO may be measured in many ways,

for example dm3/min (1 dm

3 equals 1 litre). Q is furthermore the combined sum of output from the

right ventricle and the output from the left ventricle during the phase of systole of the heart. An

average resting cardiac output would be 5.6 L/min for a human male and 4.9 L/min for a female

2. What is the pH value of arterial blood & venous blood? .( NOV 2007)

Arterial Blood: 7.37 – 7.44

Venous Blood : 7.35 – 7.45

3. Give the different types of oxygenators used in heart lung machine.(NOV 2011)

Bubble oxygenator

Film oxygenator

Foam oxygenator

Screen oxygenator

Blood film over sponge

Membrane oxygenator

Liquid- liquid oxygenator

4. How does the pH value determine the acidity or alkalinity in blood fluid? (NOV 2011)

The normal pH of the extracellular fluid lies in the range of 7.35 to 7.45, indicating that the body

fluid is slightly alkaline. When the pH exceeds 7.45, the body is considered to be in the state of

alkalosis. A body pH below 7.35 indicates acidosis. Both acidosis & alkalosis are disease

conditions widely encountered in clinical medicine.

5. List the applications of biotelemetry. (MAY 2012)

Bio-telemetry helps us to record the biosignals over long periods and while the patient is

engaged in his normal activities.

The medical attendant or computer can easily diagonise the nature of disease by seeing

the telemetered biosiganls without attending the patient’s room.

Patient is in his room without any mechanical or physical disturbance during recording by

means of biotelemetry.

For future reference or to study the treatment effect, the biotelemetry is the essential one.

For recording on animals, particularly for research, the biotelemetry is greatly used.

For monitoring the persons who are in action, the biotelemetry is an ideal one.

http://en.wikipedia.org/wiki/Heart

http://en.wikipedia.org/wiki/Ventricle_(heart)

http://en.wikipedia.org/wiki/Systole_(medicine)


6. What are the advantages of MRI scan? (MAY 2012)

A conventional X ray scanner can produce an image only at right angles to the axis of the body,

whereas the MRI scanner produce the image at desired cross section, which offers a distinct

advantage to & is big boon for the radiologists. Its advantages are,

Superior contrast resolution

Direct multiplanar imaging, slices in the sagitial, coronal and oblique directions ca be

obtained easily.

No harmful radiation like X rays, Y rays, positions.

7. What is the principle of plethysmography & how many types are available? (NOV 2012)

Instrument measuring volume changes or providing output that can be related to them are called

plethysmographs & the measurement of these volume changes or phenomenon related to them

is called plethysmography. Such an instrument consists of a rigid cup or chamber placed over the

limb or digit in which the volume changes are to be measured. The cup is tightly sealed to the

member to be measured so that any changes in volume in the limb or digit reflect as pressure

changes inside the chamber. Either fluid or air can be used to fill the chamber. Types are,

capacitance plethysmograph

pseudo plethysmograph

mercury strain gauge

photoelectric plethysmograph

impedance plethysmograph

rheoencephalography

oculo pneumo plethysmography

8. List few applications of gas analysis. (NOV 2012)

To measure pH of blood

pCO2 & pO2 of blood

9. What is plethysmograph? (MAY 2013)

Same as Q7.

10. What are the automated indirect methods of blood pressure measurement? (MAY 2013)

Automatic blood pressure measuring apparatus using Korotkoff’s method

The Rheographic Method

Differential Auscultatory Technique

11. What are the limitations of capacitive sensor?

Inadequate for measuring most physiological variables because of their low frequency

components

12. What is the principle of piezoelectric sensors?

The piezoelectric material generates an electric potential when mechanically strained. Conversely

an electric potential can cause physical deformation of the materials.

13. What are the applications of piezoelectric sensors?

in cardiology

In phonocardiology

in blood pressure measurement

in measuring physiological accelerations

14. What are the different thermal sensors?

Thermocouples

Thermistors

Radiation sensors

Fiber optic detectors

15. What are the different radiation sources?

Tungsten lamp

Fluorescent lamp

LED s


LASERS

16. What are the different radiation sensors?

thermal sensors

Quantum sensors

Photo emissive sensors

Photo conductive cells

Photo junctions sensors

Photo voltaic sensors

17. What is a filter?

A filter is often a frequency selective circuit that passes a specified band of frequencies and

blocks or attenuated signal of frequencies outside this band.

18. List the different types of filters.

Analog or digital filters

Passive or active filters

Audio (AF) or radio (RF) filters.

19. Specify the advantages of an active filter

Gain and frequency adjustment flexibility

No loading problem

Low cost

20. Mention the factors considered while selecting a transducer.

Operating range

Sensitivity

Frequency response & resonant frequency

Environmental compatibility

Minimum sensitivity

Accuracy

Usage and ruggedness

Electrical parameters

21. What is meant by POT?

POT is a resistive potentiometer used for the purpose of voltage division. It consists for a resistive

element provided with a sliding contact called as wiper.

22. Explain the working principle of a strain gauge.

Strain gauge works on the principal that the resistance of a conductor or a semiconductor changes when

strained. This property can be used for measurement of displacement, force and pressure.

23. Name the different types of strain gauges.

Un-bonded metal strain gauge

Bonded metal wire strain gauge

Bonded metal foil strain gauge

Vacuum deposited thin metal film strain gauge

Sputter deposited thin metal strain gauge

Bonded semiconductor strain gauge

Diffused metal strain gauge.

24. Write notes on LVDT?

It is the linear variable differential transformer which is used to translate the linear motion into

electrical signals. It consists of a single primary winding and secondary winding.

25. List the advantages of LVDT?

High range of displacement measurement

Friction & electrical isolation , Ruggedness


Immunity from external effects

High I/p and high sensitivity

Low hysterisis & low power consumption.

26. Define operational amplifier?

It is the high gain dc differential amplifier

It is normally used in circuits that have characteristics determined by external negative

feedback networks.

27. What are the different applications of op-amp?

integrator

Differentiator

summing amplifier

Differential amplifier

Rectifier

log amplifier

28. What is heart block?

If he normal heart conduction system is disturbed, then the beat rate will be slower than the

normal rate. This state is known as heart block.

29. Classify the different types of heart block?

First degree AV block

Second degree AV block

Third degree AV block

Adam –stokes attack

Bundle block

Atria fibrillation

Ventricular fibrillations

30. Name the parts of heart conduction system?

Sino arterial node

Atria ventricular node

Bundle of his

Purkinje fibers

31. What is the color coding of the differential leads?

White – RA

Black – LA

Green - RL

Red - LL

Brown – chest

PART B (8,16 marks)

1. Draw the block diagram of automated electro sphygmomanometer for blood pressure

measurement & explain its operation. .( NOV 2007)

INDIRECT METHODS OF BLOOD PRESSURE MEASUREMENT

The classical method of making an indirect measurement of blood pressure is by the use of a cuff

over the limb containing the artery. This technique was introduced by Riva- Rocci for the

determination of systolic & diastolic pressure. Initially, the pressure in the cuff is raised to a level

well above the systolic pressure so that the flow of blood is terminated completely. Pressure in

the cuff is released at a particular rate. When it reaches a level, which is below the systolic

pressure, a brief flow occurs. If the cuff pressure is allowed to fall further, just below the diastolic

pressure value, the flow becomes normal & uninterrupted.


The problem here finally reduces to determining the exact instant at which the artery opens &

when it is fully opened. The method given by Korotkoff & based on the sounds produced by flow

changes is the one normally used in the conventional sphygmomanometers. The sounds firat

appears when the cuff pressure falls to just below the systolic pressure. They are produced by

the brief turbulent flow terminated by a sharp collapse of the vessel & persist as the cuff pressure

continues to fall. The sounds disappear or change in character at just below the diastolic pressure

when the flow is no longer interrupted. These sounds are picked up by using a microphone

placed over an artery distal to the cut off. The sphygmomanometric technique is an ausculatory

method, it depends upon the operator recognizing the occurrence & disappearance of the

Korotkoff sounds with variations in cuff pressure.

Automatic blood pressure measuring apparatus using Korotkoff’s method:

The method consists in putting a cuff around the upper part of the patient’s arm & applying a

microphone over the brachial artery. The compressed air required for inflating the cuff is provided

by a pumping system incorporated in the apparatus. Usually the inflating is done to a preset

pressure level, well beyond the systolic value at the rate of approximately 30mmHg/s. the

pressure in the cuff is then decreased at a relatively slow pace at the rate of 3-5mmHg/s. the cuff

is to be applied in such a way that the veins are not occluded.

While air is allowed to leak from the cuff, the korotkoff sounds are picked up by a special

piezoelectric microphone. The corresponding electrical signals are fed to a preamplifier. The

amplified signals are then passed on to a band pass filter having a bandwidth of 25 – 125 Hz.

With this pass band, a good signal to noise ratio is achieved when recording korotkoff sounds

from brachial artery beneath the lower edge of the cuff. The system is so designated that tye

appearance of the first korotkoff sound switches in the systolic manometer & locks the reading on

the indicating meter. In the similar way, diastolic value is fixed by the last korotkoff sound. The

cuff is completely deflated, automatically, after an interval of 2 – 5s after the determination of the

diastolic value.

Instruments operating on this principle are prone to error due to artefacts. On e methods of

avoiding this is to design a control system in such a way that when pressure is registered, the first

sound must be followed by the second one within a pre set interval. If this is not the case, then


the recorded value is automatically cancelled & the measurement starts again with subsequent

sounds.

A complete cycle of measurement consists of cuff pumping, controlled deflation, picking up &

evaluation of the korotkoff sounds, fixing of systolic & diastolic pressure & then a complete

deflation of the cuff. The cycle is initiated by the time delay & then operation is controlled by the

command pulse.

2. Explain with relevant equations the working & measurement procedure of plethysmograph.

( NOV 2007)

PLETHYSMOGRAPHY

Instrument measuring volume changes or providing output that can be related to them are called

plethysmographs & the measurement of these volume changes or phenomenon related to them

is called plethysmography. Such an instrument consists of a rigid cup or chamber placed over the

limb or digit in which the volume changes are to be measured. The cup is tightly sealed to the

member to be measured so that any changes in volume in the limb or digit reflect as pressure

changes inside the chamber. Either fluid or air can be used to fill the chamber.

Plethysmograph can be designed for constant pressure or constant volume within the chamber.

In either case some form of pressure or displacement transducer must be included to respond to

pressure changes within the chamber & to provide a signal that can be calibrated to represent the

volume of the limb or digit. This type of plethysmograph can be used in two ways. If the cuff,

placed upstream from the seal, is not inflated, the output signal is simply a sequence of

pulsations proportional to the individual volume changes with each heartbeat.

The plethysmograph illustrated above can be used to measure the total amount of blood flow into

the limb or digit being measured. By inflating the cuff to a pressure just above venous pressure,


arterial blood can flow past the cuff, but the venous blood cannot leave. The result is that the limb

or digit increases its volume with each heartbeat by the volume of the blood entering during that

beat. The output tracing for this is shown in the figure. The slope of the line along the peaks of

these pulsations represents the overall rate at which the blood enters the limb or the digit from the

accumulation of blood that cannot escape.

Another device quite close to the true plethysmograph is called the capacitance

plethysmograph. In this device, which is generally used either on arm or leg, the limb in which

the volume is being measured becomes one plate of the capacitor. The other plate is formed by a

fixed screen held at a small distance from the limb by an insulating layer. Pulsations of the blood

in the arm or leg cause variations in the capacitance, because the distance between the limb &

the fixed screen varies with these pulsations. Since the length of the cuff is fixed, the variations in

capacitance can be calibrated as volume changes.

Another type is the pseudo plethysmograph, which measures changes in diameter at the cross

section of a finger, toe, arm leg or other segment of the body. Since volume is related to

diameter, this type of device is accurate for many purposes.

A common method of sensing diameter changes is through use of the mercury strain gauge,

which consists of a segment of small diameter elastic tubing, long enough to wrap round the limb

or digit being measured. When the tube is filled with mercury, it provides a highly compliant strain


gauge that changes its resistance with changes in diameter. With each pulsation of the

blood that increases the diameter of the limb or digit, the strain gauge elongates & in stretching

becomes thinner thus increasing the resistance.

Another type is photoelectric plethysmograph. This device operates on the principle that volume

changes in a limb or digit results in changes in optical density through & just beneath the skin

over a vascular region. A light source in an opaque chamber illuminates the small area of the

finger tip or the other region to which the transducer is applied. Light scattered & transmitted

through the capillaries of the region is picked up by the photo cell, which is shielded from all other

light. As the capillaries fill with blood, the blood density increases thereby reducing the amount of

light reaching the photocell. The result causes resistance changes in the photocell that can be

measured on a Wheatstone bridge & recorded.

The more reliable type is the impedance plethysmograph, in which volume changes in a segment

of a limb or digit are reflected as impedance changes. The impedance changes are primarily due

to the changes in conductivity of the current path with each pulsation of blood. This is done by

using either a two or four electrode system. The electrodes are either conductive bands wrapped

around the limb or digit to be measured or simple conductive strips of tape attached to the skin. In


either case, the electrodes contact the skin through a suitable electrolyte jelly or paste to form an

electrode interface & to remove the effect of skin resistance. In two electrode system a constant

current is forced through the tissue between the two electrodes & the resulting volume changes

are measures. In the four electrode system, the constant current is forced through two outer or

current electrodes & the voltage between the two inner electrodes is measured.

Several theiry explains the actual cause of the measured impedance changes. One is that the

presence of blood filling segment of the body lowers the impedance of that segment. The second

theory is that the increase in diameter due to additional blood in a segment of the body increases

the cross sectional area of the segment’s conductive path & thereby lowers the resistance of the

path.

A special form of impedance plethysmography is rheoencephalography, measurement of

impedance changes between the electrodes placed on the scalp. This technique provides

information related to cerebral blood flow & is sometimes used to detect circulatory differences

between the two sides of the head.

Another special type of plethysmograph is the oculo pneumo plethysmography. This instrument

measures every minute volume changes that occur in the eye with each arterial blood pulsation.

A small eye cup is placed over the sclera of each eye & is connected to a transducer positioned

over the patient’s head by a short section of flexible tubing. A vaccum, which can be varied from

zero to 300mmHg is applied to hold the eye cups in place. Pulsations are recorded on two

channels of a three channel pen recorder, one for each eye. The third channel is used to record

the vacuum. By periodically allowing the vaccum to build up to -300mmHg and deplete to zero,

the instrument can also be used as recording suction opthalmodynamometer, an instrument used

for measuring arterial blood pressure within the eye.

3. What are the methods for measuring blood pressure? Draw a typical set up & explain. (NOV

2011)

Same as Q1


4. Explain the measurement of heart sound with suitable diagram. (NOV 2011)

MEASUREMENT OF HEART SOUND

The technique of listening to sounds produced by the organs & vessels of the body is called

auscultation. The heart sound heard by the physician through his stethoscope occurs at the time

of closure of major valves in the heart. This timing could easily lead to false assumption that the

sounds which are heard are primarily caused by the snapping together of the vanes of these

valves. The principal cause for heart sound is due to vibrations set up in the blood inside the

heart by sudden closure of the valves. These vibrations together with eddy currents induced in

the blood as it is forces through the closing valves, produce vibrations in the walls of the heart

chambers & in adjoining blood vessels.

With each heart beat, the normal heart produces two distinct sounds that are audible in the

stethoscope, often described as LUB- DUB. The lub is caused by the closure of atrioventricular

valves. Normally this is called first heart sound 7 it occurs approximately at the time of the QRS

complex of the ECG & just before ventricular systole. The dub part of the heart sound is called

the second heart sound & is caused by the closing of semilunar valves. These valves close at the

end of systole, just before atrioventricular valves reopen. This second heart sound occurs about

the time of the end of the T wave of the ECG.

A third heart sound is sometimes heard in young adults. This sound occurs for 0.1 – 0.2 sec after

the second sound due t the rush of blood from the atria into the ventricles. This sound actually

preceeds atrial contraction, which means that the inrush of blood to the ventricles causing the

sound is passive, pushed only by the venous pressure at the inlets to the atria.

An atrial heart sound, which is not audible but visible on graphic recording occurs when the atria

actually do contract, squeezing the remainder of the blood into the ventricles. The inaudibility of

this heart sound is a result of the low amplitude & loe frequency of the vibrations.

The fig shows the time relationships between the first, second,& third heart sound with respect to

ECG.


In abnormal hearts, additional sounds called murmurs are heard between the normal heart

sounds. Murmurs are generally caused either by improper opening of valves or by regurgitation,

which results when the valves do not close completely & allow some backward flow of blood. The

sound is due to high velocity blood flow through small opening. Another cause of murmurs can be

due to small opening in the septum, which separates the left side & right side of the heart. In this

case the pressure difference between the two sides of the heart force blood through the opening,

usually from the left ventricle into the right ventricle, bypassing systemic circulation.

Normal heart sound are quite short in duration, approximately on eteh=nth of a second for each,

while murmurs extend between the normal sounds.

There is also a difference in frequency range between normal & abnormal heart sounds. The first

sound is composed primarily of energy in the 30-45 Hz range. The second heart sound is usually

of higher in pitch than the first with maximum energy in the 50 – 70 Hz range. The third heart

sound is extremely weaker in vibration, with most of the energy below 30 Hz. Murmurs on the

other hand produce much higher pitched sounds with energy in 100 – 600 Hz. Although

asculcation is the method of detecting 7 analyzing heart sounds, the graphic recording of heart

sounds is called phonocardiogram.

An entirely different waveform is produced by the vibrations of the heart against the thoracic

cavity. The vibrations of the side of the heart as it thumps against the chest wall form the

vibrocardiogram, whereas the tip or apex of the heart hitting the rib cage produces the apex

cardiogram.

Sounds & pulsations can also be detected & measured at various locations in the systemic

arterial circulation system where the major arteries approach the surface of the body. The most

common one is the pulse, which can be felt with finger tips at certain points on the major arteries.

The waveform of this pulse can also be measured 7 recorded. In addition, when an artery is

partially occluded so that the blood velocity through the constriction is increased sufficiently,


identifiable sounds can be heard downstream through the stethoscope. These sounds are called

as Korotkoff sounds commonly used in measurement of blood pressure. Ballistocardiogram is

related to the measures of direct result of dynamic forces of the heart as it beats & pumps into the

major arteries.

5. Explain the measurement of blood PO2 & PCO2. (NOV 2011)

BLOOD GAS ANALYZERS

Blood gas analyzers are used to measure the pH, partial pressure of CO2 (pCO2), partial pressure

of O2(O2) of the body fluids with special reference to human blood. The measurements of these

parameters are essential to determine the acid base balance.




conditions widely encountered in clinical medicine. Any tendency of the pH of the blood to deviate

towards these conditions is dealt with the following three physiological mechanisms: (i) buffering

by chemical means, (ii) respiration and (iii) excretion, into the urine by kidneys.

The blood & tissue fluids contain chemical buffers, which react with added acids & bases &

minimize the resultant changes in hydrogen ions & they respond to changes in carbon dioxide

concentration in seconds. Respiration can adjust sudden changes in carbon dioxide back to

normal in few seconds. Kidney requires many hours to readjust hydrogen ion concentration by

excreting highly acidic or alkaline urine to enable body conditions to return to normal. In order to

maintain pO2, pCO2 & pH within normal limits, the rate & depth of respiration vary automatically

with changes in metabolism.

pCO2 Measurement:


the emf generated by a pCO2 electrode is a direct logarithmic function of pCO2. It is observed

that a ten fold change in pCO2 causes the potential to change by 58 ±2mV. The pH versus log

pCO2 relationship is linear within ±0.002pH unit from 1 to 100% carbon dioxide. Since 0.01 unit

pH change corresponds to a 2.5% change in pCO2 or 1 mmHg in 40mmHg, for achieving an

accuracy of 0.1mmHg, it is desirable to read 0.001pH unit. This order of accuracy can be read

only on a digital readout type pH meter or on an analog meter with expanded scale. The

instrument should have a very high degree of stability & a very low drift amplifier. It is essential to

maintain the temperature of the electrode of the electrode assembly constant within close limits.


6. Explain the Rheographic method of blood pressure measurement. (MAY 2012)

The Rheographic Method:

A fully automated apparatus for measuring systolic & diastolic blood pressure has been

developed using the ordinary Riva- Rocci cuff & the principle of rheographic detection of an

arterial pulse. Here, the change in impedance at the two points under the occluding cuffs forms

the basis of detection of the diastolic pressure

In this method, a set of three electrodes which are attached to the cuff are places in contact with

the skin. A good contact is essential to reduce the skin electrode contact impedance. Electrode B

which acts as a common electrode is placed slightly distal from the midline of the cuff. Electrode

B& C are placed at a certain distance from the electrode B, one distally & the other proximally. A

high frequency current source of 100KHz is connected to the electrodes A& C. When we

measure the impedance between any two electrodes before pressurizing the cuffs, it shows

modulation in accordance with the blood flow pulsations in the artery. Therefore the arterial

pulses can be detected by the demodulation & amplification of the modulation

When the cuff is inflated above the systolic value, no pulse is detected by the electrode A. the

pulse appears when the cuff pressure is just below the systolic level. The appearance of the first

distal arterial pulse results in an electrical signal, which operates a valve to fix a manometer value


on the systolic value. As long as the pressure in the cuff is between the systolic & diastolic

values, differential signal exists between electrode A & C. this is because the blood flow is

impeded underneath the occluding cuff & the pulse appearing at the electrode A is time delayed

from the pulse appearing at electrode C. when the cuff pressure reaches the diastolic pressure,

the arterial blood flow is no longer impeded & the differential signal disappears. A command

signal is then initiated & the diastolic pressure is indicated on the manometer.

7. Explain with functional diagram the working of spirometer. (MAY 2012)

Same as Q12

8. Explain the measurement methods of Galvanic skin response (GSR) & Basal Skin Resistance

(BSR). (MAY 2012)

9. Explain in detail with neat diagram, differential ausculatory technique of blood pressure

measurement. (MAY 2012)

Differential Auscultatory Technique:

The differential auscultatory technique is a non invasive method for accurately measuring blood

pressure. A special cuff mounted sensor consisting of a pair of pressure sensitive elements,

isolates the signal created each time the artery is forced open.

The figure shows how high frequency pulses are created each time, the intra arterial pressure

exceeds the cuff pressure. As long as the cuff pressure exceeds the pressure in the artery, the

artery is held closed & no pulse is generated. However, as soon as the intra arterial pressure

rises to a value, which momentarily exceeds the cuff pressure, the artery snaps open & a pulse

is created. Once this artery is open, blood flows through it giving rise to the low frequency

pressure wave signal, which lasts until the arterial pressure again drops below the cuff pressure.

The process is repeated until the cuff pressure drops to a value below the diastolic.

The figure is a cut way view of an arm with the partially occluding the brachial artery. Each time

the artery opens, the signal shown in the fig is created. The signal consists of a slow rising. Low

frequency component with a fast pulse superimposed on it., this signal is marked by arrows

marked A in the fig transmitted from the artery to both the sensor & the airbag in the cuff.


Due to air bag characteristics, the high frequency component is g=highly attenuated, leaving only

the low frequency signal is transmitted to the side of the sensor facing the airbag, as denoted by

arrow marked B in the fig.

The systolic pressure is determined as the pressure at which the first opening of the artery

occurs, shown by the first pulse in the figure. Because this pulse is created the first time the

artery is forced open by intra arterial pressure. Similarly, diastolic value is determined as the

pressure at which the differential signal essentially disappears, because this corresponds to the

last time the artery is forced open. The differential sensor subtracts side B signal from the side A

signal, therby cancelling out the pressure wave component & the motion artifact signals, 7 the

higher frequency Korotkoff signals are isolated.

10. Explain the fick’s method for cardiac output measurements. (NOV 2012)

11. Describe the need for blood pH measurement. (NOV 2012)

BLOOD GAS ANALYZERS

Blood gas analyzers are used to measure the pH, partial pressure of CO2 (pCO2), partial pressure

of O2(O2) of the body fluids with special reference to human blood. The measurements of these

parameters are essential to determine the acid base balance.




conditions widely encountered in clinical medicine. Any tendency of the pH of the blood to deviate

towards these conditions is dealt with the following three physiological mechanisms: (i) buffering

by chemical means, (ii) respiration and (iii) excretion, into the urine by kidneys.

The blood & tissue fluids contain chemical buffers, which react with added acids & bases &

minimize the resultant changes in hydrogen ions & they respond to changes in carbon dioxide

concentration in seconds. Respiration can adjust sudden changes in carbon dioxide back to

normal in few seconds. Kidney requires many hours to readjust hydrogen ion concentration by

excreting highly acidic or alkaline urine to enable body conditions to return to normal. In order to

maintain pO2, pCO2 & pH within normal limits, the rate & depth of respiration vary automatically

with changes in metabolism.

Blood pH measurement:

The acidity or alkalinity of a solution depends on the concentration of hydrogen ions. The

increased concentration of hydrogen ions makes the solution more acidic & decreased

concentration makes it more alkaline. pH is the measure of hydrogen ion concentration

expressed logarithmically. It is the negative exponent (log) of the hydrogen ion concentration.

pH= -log(H*)

if the number 10-7

represents the concentration of hydrogen ions in a certain solution, then its pH

would be 7. As hydrogen ion increases, pH falls because logarithm gets smaller & vice versa. pH

of +7 is considered as neutral solution, pH of +6 represents acid & pH of +8 represents alkali.

Concentration of ions is expressed in mols/ litre.

Electrochemical pH determination uses the difference in potential occurring between solutions of

different pH separated by special glass membrane. The device used to this effect is called glass

electrode.

Glass Electrode:


The potential of the glass electrode can be written by means of Nernst equation:

Where, Eo=standard potential, R=gas constant, T=absolute temperature, F=faraday constant,

∆pH= pH value deviation from 7.

The above equation shows that the emf developed in the electro chemical pH cell is a linear

function of ∆pH. Change of pH of one unit=58.2mV @20oC=62.2mV @40

oC

The factor -2.3036 RT/F is called the slope factor & it depends on the solution temperature. With

one degree change in temperature, emf changes by 0.2mV. the figure shows the relation

between pH & emf at different temperatures. The reference electrode provides a constant

potential against which the potential af the indicator or the glass electrode is measured. Almost

universally employed reference electrode is the saturated calomel electrode.

pH Measurement:

For making pH measurements, the solution is taken in a beaker. A pair of electrodes, one glass

or indicating electrode & the other reference calomel electrode is immersed in the solution. The

voltage developed across the electrodes is applied to an electronic amplifier, which transmits the

amplified signal to the display. The pH meter has controls for calibration & temperature

compensation. The glass electrode exhibits a high degree of electrical resisitance on the orser of

100 – 1000Mohms. The emf measurement necessitates the use of measuring circuits with high

input impedance.

Electrodes for blood pH measurement:

The common type used is the syringe electrode, preferred for taking small samples of blood an

aerobically. The small dead space between the electrode bulb & the inner surface of the syringe

barrel is usually filled with dilute heparin solution to prevent blood coagulation.

Microcapillary glass electrodes are preferred when it is required to monitor pH continuously, for

example during surgery

Microelectrode for clinical applications requires only 20 – 25 microlitre of capillary blood for the

determination on pH. The electrode is enclosed in a water jacket with circulating water at a

constant temperature of 38 degrees. The water contains 1% NaCl for shielding against static

interference. The capillary is protected with polyethylene tubing. The internal reference electrode

is silver/ silver chloride & the calomel reference electrode is connected to a small pool of

saturated KCl, through a porous pin. An accuracy of 0.001pH can be obtained with this electrode

against a constant buffer. The figure shows the constructional details of a typical blood pH

electrode & the measurement set up used in practice.

Glass electrodes deteriorate if allowed to remain in contact with the bl;ood for a long time. This

results in the change of emf – pH slope. Therefore , as a precautionary measure, in an apparatus

where blood necessarily remains in contact around the electrode for long time, the response must

be checked frequently against the buffer solution. The poisoning effect can be reduced by putting

the electrode in pepsin & 0.1 N Hcl followed by careful wiping with the tissue paper.

The pH of blood is found to change linearly with temperature in the range of 18 to 38 degree C.

the temperature co efficient for the pH of blood is 0.0147 pH unit per degree C.


Buffer solutions are primarily used for (i) creation & maintenance of a desired, stabilized pH in a

solution & (ii) standardization of electrode chains for pH measurements. The presence of a buffer

in a solution is to counteract pH changes in the solution caused by the addition or removal of

hydrogen ions.

12. Explain the working of spirometer. (NOV 2012)

SPIROMETER

The instrument used to measure lung capacity & volume is called spirometer. The record

obtained from this is called spirogram. Spirometers are calibrated containers that collect gas &

make measurements of lung volume or capacity that can be expired. The gas analyzer combined

with a spirometer makes a complete pulmonary testing system.

Basic Spirometer:

Respiratory measurements are carried out by using classic water sealed spirometer. This

consists of an upright water filled cylinder containing an inverted counter weighted bell. Breathing

into the bell changes the volume of gases trapped inside, and the change in volume is translated

into vertical motion, which is recorded on the moving drum of a kymograph. The excursion of the

bell will be proportional to the tidal volume. For most purpose, the bell has a capacity of the order

of 6 – 81. The spirometer cannot respond to rapid breathing unless a light weight bell is

provided.frequency response of the spirometer must be adequate for the measurement of the

forced expiratory volume. The instrument should have no hysteresis. The same volume should be

reached whether the spirometer is being filled or being emptied to that volume.

As the water filled spirometer includes moving masses in the form of the bell 7 conterweights, this

leads to the usual problems of inertia & possible oscillation of the bell. This can lead to

overestimation of expiratory volume. So a spirometer having a bell of large diameter & which

closely fits into the central core is used. So the area of water covered by the bell is small in

relation to that of water tank.

The spirometer is a mechanical integrator since the input is airflow & the output is volume

displacement. An electrical signal proportional to volume displacement can be obtained by using

a linear potentiometer connected to the pulley portion of the spirometer. The spirometer is

aheavily damped device so that small changes in inspired & expired air volumes are not

recorded. The spirometer can be fitted with a linear motion potentiometer, which directly converts


spirometer volume changes into an electrical signal. The signal may be used to feed a flow

volume differentiator for the evaluation & recording of data. The response is usually ±-1% to 2Hz

±10% to 10Hz..

Tests made in spirometer are not analytical & not completely objective becoz the results are

dependent on the cooperation of the patient & the coaching efforts of good respiratory technician.

Transducers have been designed to transform the movement of the bell, bellows or piston of

volume spirometers into electrical signals. These are then used to compute the numerical value

electronically.

Wedge Spirometer:

A wedge spirometer consists of two square pane. Parallel to each other & hinged along one

edge. The first pan is permanently attached to the wedge casting stand & contains a pair of 5cm

inlet tubes. The other pan swings freely along its hinge with respect to the fixed pan. A space

existing between the two pans is sealed airtight with vinyl bellows. The bellow is flexibile in

direction of pan motion but offers high resistance to ballooning or inward & outward expansion

from the spirometer. So when a pressure gradient exists between the interior of the wedge & the

atmosphere, there will only be a negligible distortion of bellows.

As gas enters or leaves the wedge, the moving pan will change position in compensation for this

change in volume. The construction of the wedge is such that the moving pan will respond to very

slight changes in volume. Under normal conditions the pressure gradient that exists between the

wedge & the atmosphere amounts to a fraction of millimeter of water. Volume & floew signals of

the wedge are obtained independently from two linear transducers. The transducer are attached

to the fixed frame & are coupled to the edge of the moving pan. One transducer produces a dc

signal proportional to displacement ( volume), while the other has the dc output proportional to

the velocity (flow).

The transducer out[ut are connected to an electronic circuit, which contains the power supply, an

amplifier & built in calibration networks.

A pointer attached to the moving pan & a scale affixed to the frame, combine to provide a

mechanical read out for determining the approximate volume position of the spirometer. When

one open to the atmosphere standing upright, the wedge will empty itself due to the force of

gravity acting on a moving member. An adjustsble tilt mechanism provides the means for

changing the resting point of the moving pan to any desired volume poin. An adjustable magnetic

stop insures a more highly defined resting position.


Thus the relatively small forces due to gravity & magnetic stop are overcome by a negligible rise

in pressure in patient’s lungs. When the gravitational return of the moving pan to the resting

position is undesirable, the wedge may be turned on its side so that at any point the pan will be in

state of equilibrium. The wedge may be calibrated with a selector switch, which determines the

magnitude of the calibration signal. The flow calibration signals for each particular wedge are

adjusted using special fixtures.

Ultrasonic Spirometer:

Ultrasonic transducers depend for their action on transmitting ultrasound between a pair of

transducers & measuring changes in transit time caused velocity of intervening fluid medium.

They employ piezoelectric transducers & are operated at their characteristic resonant frequency

for high efficiency. Gas floe meters generally operate in the range from about 40 – 200KHz. At

frequencies higher than 200KHz, the absorption losses in the gas are very high.

Ultrasonic spirometers utilize a pair of ultrasonic transducers mounted on opposite sides of a flow

tube. The transducers are capable of both transmitting & receiving ultrasonic pulses. In

conventional ultrasonic flow meters, pulses are transmitted through the liquid or gas in the flow

tube, against & then with the direction of flow. The pulse transit time upstream t1 and downstream

t2 can be expressed,

and


Where D is the distance between the transducers, C is the velocity of sound propagation in the

fluid & v’ is the fluid velocity vector along the path of the pulses. The average gas vel;ocity v

through the flow tube is a vector v’ so that,

V=v cosθ

The velocity of sound C does not appear in the final equation. Thus, the output accuracy is

unaffected by fluid density, temperature or viscosity. In gas flow measurements, pulmonary

function tubes larger than 3cm in diameter must be used. The single frequency systems that

measure time delay directly must be able to resolve nanoseconds since the total transit delay, t is

usually measured in microseconds. This technique is not easily implemented because of the

difficulty in measuring these small time differences.

13. Discuss a finger tip oxymeter. (NOV 2012)

OXIMETERS:

Oximeters are used to measure the percentage of oxygen saturation of the circulating arterial

blood.

Where [HbO2] is the concentration of oxygenated hemoglobin & [Hb] is the concentration of

deoxygenated hemoglobin. The measurement of oxygen saturation is mainly used in the

diagnosis of cardio respiratory functions. It is known that blood consists of red blood cells(RBC)

with density 4.62 – 6.2 millions/ microlitre & size 6.8 – 7.5 micrometer, white blood cells(WBC)

with density 0.004 – 0.011 millions/ microlitre & size 6 – 18 micrometer, platel;ets with density

0.15 – 0.40 millions/ microlitre & size 2 – 4 micrometer in liquid plasma. In plasma the oxygen is

dissolved about 0.3% & therefore the plasma is a very poor carrier of oxygen. Actually the oxygen

is carried by the red blood cells through the hemoglobin. Hemoglobin in the RBC combines with

oxygen & forms a compound called oxyhemoglobin which contains more oxygen. The amount of

oxygen combining with blood depends on the partial pressure of oxygen.


Oximeter is divided into two types, vitro oximeter & vivo oximeter. In vitro oximetry, the blood is

taken out & measurement for oxygen saturation is made at later time in the laboratory. In vivo

oximetry the oxygen saturation is measured while the blood is flowing through the circulatory

system.

14. Explain the working of spirometer with the help of functional diagram. (MAY 2013)

Same as Q12

15. Explain the rheographic method of blood pressure measurement. (MAY 2013)

Same as Q 6

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