Biomedical Instrumentation

Cardiac Output

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Cardiac output (Q or ) 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 dm³ equals 1000 cm³ or 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.

Q=Stroke Volume × Heart rate

Heart is a 'demand pump', that pumps out whatever blood comes back into it from the venous system, it is effectively the amount of blood returning to the heart that determines how much blood the heart pumps out (Q).

Stroke Volume (SV) = EDV – ESV

Ejection Fraction (EF) = (SV / EDV) × 100%

Cardiac Output (Q) = SV × HR

Measuring cardiac output

The Fick Principle

The Fick principle was first described by Adolf Eugen Fick in 1870 and assumes that the rate at which oxygen is consumed is a function of the rate of blood flows and the rate of oxygen picked up by the red blood cells. The Fick principle involves calculating the oxygen consumed over a given period of time from measurement of the oxygen concentration of the venous blood and the arterial blood. Q can be calculated from these measurements:

V_O₂ consumption per minute using a spirometer (with the subject re-breathing air) and a CO₂ absorber
the oxygen content of blood taken from the pulmonary artery (representing mixed venous blood)
the oxygen content of blood from a cannula in a peripheral artery (representing arterial blood)

From these values, we know that:

V_O₂ = (Q×C_A) - (Q×C_V)

where

C_A = Oxygen content of arterial blood
C_V = Oxygen content of venous blood.

This allows us to say

Q = (VO₂/[C_A - C_V])*100

and therefore calculate Q.

Dilution methods

This method was initially described using an indicator dye and assumes that the rate at which the indicator is diluted reflects the Q. The method measures the concentration of a dye at different points in the circulation, usually from an intravenous injection and then at a downstream sampling site, usually in a systemic artery. More specifically, the Q is equal to the quantity of indicator dye injected divided by the area under the dilution curve measured downstream (the Stewart (1897)-Hamilton (1932) equation):

The trapezoid rule is often used as an approximation of this integral.

Pulmonary Artery Thermodilution (Trans-right-heart Thermodilution)

The indicator method was further developed with replacement of the indicator dye by heated or cooled fluid and temperature change measured at different sites in the circulation rather than dye concentration; this method is known as thermodilution. The pulmonary artery catheter (PAC), also known as the Swan-Ganz catheter, was introduced to clinical practice in 1970 and provides direct access to the right heart for thermodilution measurements.

The PAC is balloon tipped and is inflated, which helps "sail" the catheter balloon through the right ventricle to occlude a smaller branch of the pulmonary artery system. The balloon is deflated. The PAC thermodilution method involves injection of a small amount (10ml) of cold glucose at a known temperature into the pulmonary artery and measuring the temperature a known distance away (6–10 cm) using the same catheter.

The Q can be calculated from the measured temperature curve (The “thermodilution curve”). High Q will change the temperature rapidly, and low Q will change the temperature slowly. Usually three or four repeated measures are averaged to improve accuracy.

Impedance cardiography

Impedance cardiography (often related as ICG or TEB) is a method that measures changes in impedance across the thoracic region over the cardiac cycle. Lower impedance indicates greater the intrathoracic fluid volume and blood flow. Therefore, by synchronizing fluid volume changes with heartbeat, the change in impedance can be used to calculate stroke volume, cardiac output, and systemic vascular resistance.Both invasive and non-invasive approaches are being used.The noninvasive approach has achieved some acceptance with respect to its reliability and validity. The clinical use of this approach in a variety of diseases continues

Links

http://en.wikipedia.org/wiki/Cardiac_output
http://www.skillstat.com/PDF/mceCO.pdf
http://www.cardiovascular.cx/video.htm

BLOOD PRESSURE

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The determination of an individual's blood pressure is one of the most useful clinical
measurements that can be taken. By "blood pressure" we mean the pressure exerted
by the blood against the vessel walls, the arterial blood pressure being the most useful,
and hence the most frequently measured pressure. One should become familiar with
the following pressures used in cardiovascular physiology.

· Systolic blood pressure. The highest pressure in the artery, produced in the
heart's contraction (systolic) phase. The normal value for a 20-year-old man is
120 mm Hg.

· Diastolic blood pressure. The lowest pressure in the artery, produced in the
heart's relaxation (diastolic) phase. The normal value for a 20-year-old man is
80 mm Hg.

· Pulse pressure. The difference between the systolic and diastolic pressures.
The normal value is 40 mm Hg.

· Mean blood pressure. Diastolic pressure plus one third of the pulse pressure.
This is the average effective pressure forcing blood through the circulatory
system. The normal value is 96 to 100 mm Hg.

IMPORTANCE OF BLOOD PRESSURE MEASUREMENT:

The mean blood pressure is a function of two factors - cardiac output (CO) and total
peripheral resistance (TPR). Peripheral resistance depends on the calibre (diameter) of
the blood vessels and the viscosity of the blood.

Mean BP = Cardiac output (ml/sec) x TPR

Cardiac output (ml/min) = Heart rate/min x Stroke volume (ml)

Thus, the measurement of blood pressure provides us with information on the heart's
pumping efficiency and the condition of the systemic blood vessels. In general, we
say that the systolic blood pressure indicates the force of contraction of the heart,
whereas the diastolic blood pressure indicates the condition of the systemic blood
vessels (for instance, an increase in the diastolic blood pressure indicates a decrease in
vessel elasticity).

MEASUREMENT OF BLOOD PRESSURE:

Blood pressure can be measured by several techniques. Basically they are categorized
into two methods,

1. Direct method
2. Indirect method

1. Direct method

The direct method involves directly inserting a tube or catheter into a blood vessel.
The catheter is connected to a blood pressure transducer, which generates an electrical
signal.

2. Indirect method

In this method, we measure the arterial blood pressure using two different methods:

1. The first method uses the sense of touch: it is thus called the palpatory
method.
2. The second method uses the sense of hearing: it is thus called the
auscultatory method.

In either of these indirect methods, pressure is applied to the artery using an
instrument called the sphygmomanometer.

A sphygmomanometer, an instrument that measures pressure, is needed in both
methods. Each sphygmomanometer consists of a cuff which is connected by lengths
of tubing to an inflating bulb with a needle valve and to a mercury manometer.

PALPATORY METHOD:

1. Have the subject seated, with his or her arm resting on a table. Wrap the
pressure cuff snugly around the bare upper arm, making certain that the
inflatable bag within the cuff is placed over the inside of the arm where it can
exert pressure on the brachial artery. Wrap the end of the cuff around the arm
and tuck it into the last turn, or press the fasteners together to secure the cuff
on the arm. Close the valve on the bulb by turning it clockwise.

2. With one hand, palpate (feel) the radial pulse in the wrist. Slowly inflate the
cuff by pumping the bulb with the other hand and note the pressure reading
when the radial pulse is first lost. Then increase the pressure to around 20 mm
Hg above this point. Slowly reduce the pressure in the cuff by turning the
valve counterclockwise slightly to let air out of the bag. Note the pressure
when the radial pulse first reappears. This is systolic blood pressure, the
highest pressure in the systemic artery.

3. Let all the air out of the cuff, allow the subject to rest, and then run a second
determination. Do not leave the cuff inflated for more than 2 minutes, because
it is uncomfortable and will cause a sustained increase in blood pressure.

4. The systolic pressure recorded with the palpatory method is usually around 5
mm Hg lower than that obtained using the auscultatory method. A major
disadvantage of the palpatory method is that it cannot be used to measure the
diastolic pressure.

AUSCULTATORY METHOD:

1. Place the bell of the stethoscope below the cuff and over the brachial artery
where it branches into the radial and ulnar arteries. Use your fingers, rather
than your thumb, to hold the stethoscope over the artery; otherwise you may
be measuring the thumb arterial pressure rather than the brachial artery
pressure. With no air in the cuff no sounds can be heard.

2. Inflate the cuff so the pressure is above diastolic (80-90 mm Hg), and you will
be able to hear the spurting of blood through the partially occluded artery.
Increase the cuff pressure to around 160 mm Hg; this pressure should be
above systolic pressure so that the artery is completely collapsed and no
sounds are heard.

3. Now, open the valve and begin to slowly lower the pressure in the cuff. As
the pressure decreases you will be able to hear four phases of sound changes;
these were first reported by Korotkoff in 1905 and are called Korotkoff
sounds.

· Phase 1. Appearance of a fairly sharp thudding sound that increases in
intensity during the next 10 mm Hg of drop in pressure. The pressure
when the sound first appears is the systolic pressure.
· Phase 2. The sounds become a softer murmur during the next 10 to 15
mm Hg of drop in pressure.
· Phase 3. The sounds become louder again and have a sharper thudding
quality during the next 10 to 15 mm Hg of drop in pressure.
· Phase 4. The sounds suddenly become muffled and reduced in
intensity. The pressure at this point is termed the diastolic pressure.
This muffled sound continues for another drop in pressure of 5 mm
Hg, after which all sound disappears. The point where the sound
ceases completely is called the end diastolic pressure. It is sometimes
recorded along with the systolic and diastolic pressures in this manner:
120/80/75.

ANALYSIS:

The auscultatory method has been found to be fairly close to the direct method in the
pressures recorded; usually the systolic pressure is about 3 to 4 mm Hg lower than
that obtained with the direct method.

Blood pressure varies with a person's age, weight, and sex. Below the age of 35, a
woman generally has a pressure 10 mm lower than that of a man. However, after 40 to
45 years of age, woman's blood pressure increases faster than does a man's. The old
rule of thumb of 100 plus your age is still a a good estimate of what your systolic
pressure should be at any given age. After the age of 50, however, the rule is invalid.
The increase in blood pressure with age is caused largely by the overall loss of vessel
elasticity with age, part of which is due to the increased deposit of cholesterol and
other lipids in the blood vessel walls.

EEG

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BACKGROUND:

Electroencephalography (EEG) is an electrophysiological investigation technique
used to record bioelectric activity of the brain at the scalp. It is a non-invasive
method that acquires measures of instantaneous activities within the cerebral
hemispheres (in particular in the cortex).

Brainwaves (EEGs) reflect the brain’s electrical activity. A neuron at rest is like a
little battery. Whenever a neuron is active, its voltage briefly changes. If millions of
neurons all fire at the same time, this produces electrical activity detectable to an
electrode placed on the head.

For example, if you hear a tone, many different groups of neurons activate to process
that tone. EEGs can tell us when and where these groups of neurons fire. Doctors
often use this technique to diagnose hearing disabilities, since EEGs can reveal which
groups of neurons are damaged.

ELECTRODES FOR EEG:

Macroelectrodes only measure the coordinated activity of many millions of neurons.

Microelectrodes only measure the activity of one or very few neurons.

The most common recording setup is a scalp macroelectrode. While it is possible to
get data from as few as two electrodes, most labs use an electrode cap. These caps are
specially designed so that each electrode is over a general region of the brain. This
makes it easier to estimate the source of any EEG activity detected at each electrode.

These are standardized electrode locations, called the International 10-20
system.

The International 10–20 System of Electrode Placement is the most widely used
method to describe the location of scalp electrodes. It is based on the relationship
between the location of an electrode and the underlying area of cerebral cortex. Each
site has a letter (to identify the lobe) and a number or another letter to identify the
hemisphere location.

THE BEHAVIOR OF THE EEG SIGNAL:

1. Event related potentials (ERPs): Brain’s response to a specific event, such
as a tone or flash.

2. Spontaneous or free-running EEG: Naturally produced, rhythmic
brainwaves; do not require outside activity.

Well known free running EEGs include:

1. The Alpha waves have the frequency spectrum of 8-13 Hz and can be
measured from the occipital region in an awake person when the eyes are
closed. Amplitude: 30 – 50 μV.

2. The frequency band of the Beta waves is 13-30 Hz; these are detectable over
the parietal and frontal lobes; indicate alertness. Amplitude: Less than 20 μV.

3. The Delta waves have the frequency range of 0.5-4 Hz and are detectable in
infants and sleeping adults (deep sleep). Amplitude: Up to 100 – 200 μV

4. The Theta waves have the frequency range of 4-8 Hz and are obtained from
children and sleeping adults, during hypnosis and meditation. Amplitude: Less
than 30 μV.

5. The Mu waves have the frequency range of 8 – 13 Hz and are largest when
individual is not moving

NORMAL EEG

1. In adults who are awake, the EEG shows mostly alpha waves and beta waves.
2. The two sides of the brain show similar patterns of electrical activity.
3. There are no abnormal bursts of electrical activity and no consistently slow
brain waves detected on the EEG tracing.
4. If flashing lights (photic stimulation) are used during the test, one area of the
brain (the occipital region) may have a brief response after each flash of light,
but the brain waves remain normal.

EMG

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Electromyography (EMG) is an experimental technique concerned with the development, recording and analysis of myoelectric signals. Myoelectric signals are formed by physiological variations in the state of muscle fiber membranes

Typical benefits of EMG are:

· EMG allows to directly “look” into the muscle
· It allows measurement of muscular performance
· Helps in decision making both before/after surgery
· Documents treatment and training regimes
· Helps patients to “find” and train their muscles
· Allows analysis to improve sports activities
· Detects muscle response in ergonomic studies

FACTORS INFLUENCING THE EMG SIGNAL:

1) Tissue characteristics

The human body is a good electrical conductor, but unfortunately the electrical conductivity varies with tissue type, thickness, physiological changes and temperature. These conditions can greatly vary from subject to subject (and even within subject) and prohibit a direct quantitative comparison of EMG amplitude parameters calculated on the unprocessed EMG signal.

2) Physiological cross talk

Neighboring muscles may produce a significant amount of EMG that is detected by
the local electrode site. Typically this “Cross Talk” does not exceed 10%-15% of the
overall signal contents or isn’t available at all. However, care must been taken for
narrow arrangements within muscle groups. ECG spikes can interfere with the EMG
recording, especially when performed on the upper trunk / shoulder muscles. They are
easy to see and new algorithms are developed to eliminate them.

3) Changes in the geometry between muscle belly and electrode site

Any change of distance between signal origin and detection site will alter the EMG
reading. It is an inherent problem of all dynamic movement studies and can also be
caused by external pressure.

4) External noise

Special care must be taken in very noisy electrical environments. The most
demanding is the direct interference of power hum, typically produced by incorrect
grounding of other external devices.

5) Electrode and amplifiers

The selection/quality of electrodes and internal amplifier noise may add signal
contents to the EMG baseline. Internal amplifier noise should not exceed 5 Vrms.
Most of these factors can be minimized or controlled by accurate preparation and
checking the given room/laboratory conditions.

PROCEDURE OVERVIEW:

During the test, one or more small needles (also called electrodes) are inserted
through the skin into the muscle.

· Needle electrodes to study electrical activity of motor units
· Surface electrodes to study the electrical activity of muscles

EMG measures the electrical activity of muscle during rest, slight contraction, and
forceful contraction. Muscle tissue does not normally produce electrical signals
during rest. When an electrode is inserted, a brief period of activity can be seen on the
oscilloscope, but after that, no signal should be present.

After all of the electrodes have been inserted, you may be asked to contract the
muscle, for example, by lifting or bending your leg. The action potential (size and
shape of the wave) that this creates on the oscilloscope provides information about the
ability of the muscle to respond when the nerves are stimulated. As the muscle is
contracted more forcefully, more and more muscle fibers are activated, producing
action potentials.

ANALYSIS:

A healthy muscle will show no electrical activity (no signs of action potential) during
rest, only when it contracts. However, if the muscle is damaged or has lost input from
nerves, it may have electrical activity during rest. When it contracts its electrical
activity may produce abnormal patterns.

An abnormal EMG result may be a sign of a variety of muscle or nerve disorders,
including polymyositis (an inflammatory muscle disease that causes decreased muscle
power), muscular dystrophy (a chronic genetic disease that progressively affects
muscle function), myasthenia gravis (a genetic or immune disorder that occurs at the
point where the nerve connects with the muscle), and myotonic (stiff) muscles.

DETERMINATION OF CONDUCTION VELOCITIES IN MOTOR NERVES

THEORY:

Nerve conduction velocity (NCV) test is a measurement of the speed of conduction of
an electrical impulse through a nerve. NCV can determine nerve damage and
destruction.

During the test, the nerve is stimulated, usually with surface electrode patches
attached to the skin. Two electrodes are placed on the skin over the nerve. One
electrode stimulates the nerve with a very mild electrical impulse with pulse duration
of 0.2 to 0.5 m/s and the other electrode records it. The resulting electrical activity is
recorded by another electrode. This is repeated for each nerve being tested.

The nerve conduction velocity (speed) is then calculated by measuring the distance
between electrodes and the time it takes for electrical impulses to travel between
electrodes. This elapsed time is called latency.

The measurement of conduction velocity in motor nerves is used to indicate the
location and type of nerve lesion.

PROCEDURE:

The EMG electrode and the stimulating electrode are placed at two points on the skin, separated by a known distance (L1).
A brief electrical pulse is applied through the stimulating electrode.
The action potential picked up by the EMG electrode is displayed on the software screen along with the stimulating impulse.
The latency, between the stimulating impulse and muscle’s action potential is measured. (T1)
Now, the two electrodes are repositioned with the distance of separation as (L2) such that L2 <>
The latency is now measured (T2)
Record your findings on the data sheet
Calculate the conduction velocity.
Repeat the test for different nerves.

ANALYSIS:

The speed of nerve conduction is related to the diameter of the nerve and the degree
of myelination (a myelin sheath is a type of "insulation" around the nerve). A
normally functioning nerve will transmit a stronger and faster signal than a damaged
nerve.

In general, the range of normal conduction velocity will be approximately 50 to 60
meters per second. However, the normal conduction velocity may vary from one
individual to another and from one nerve to another.

Abnormal results may be caused by some sort of neuropathy (damage to the nerve)
that can result from a contusion or traumatic injury to a nerve. Various diseases can
also cause the impulses to slow down.

Nerve conduction velocity is often used along with an EMG to differentiate a nerve
disorder from a muscle disorder. NCV detects a problem with the nerve whereas an
EMG detects whether the muscle is functioning properly in response to the nerve's
stimulus.

Diseases or conditions that may be evaluated with NCV include, but are not limited
to, the following:

· Guillain-Barré syndrome - a condition in which the body's immune system
attacks part of the peripheral nervous system. The first symptoms may include
weakness or tingling sensations in the legs.

· carpal tunnel syndrome - a condition in which the median nerve, which runs
from the forearm into the hand, becomes pressed or squeezed at the wrist by
enlarged tendons or ligaments. This results in pain and numbness in the
fingers.

· Charcot-Marie-Tooth disease - a hereditary neurological condition that affects
both the motor and sensory nerves. One characteristic is weakness of the foot
and lower leg muscles.

· herniated disc disease

· chronic inflammatory polyneuropathy and neuropathy - conditions resulting
from diabetes or alcoholism

· sciatic nerve problems

· pinched nerves

· peripheral nerve injury

Nerve conduction studies may also be performed to identify the cause of symptoms
such as numbness, tingling, and continuous pain.

Heart Sounds

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Auscultation of the heart means to listen to and study the various sounds arising from the heart as it pumps blood. These sounds are the result of vibrations produced when the heart valves close and blood rebounds against the ventricular walls or blood vessels. The heart sounds may be heard by placing the ear against the chest or by using a stethoscope. The vibrations producing the sounds can be visually displayed through the use of a heart sound microphone and physiological recorder to produce a phonocardiogram. There are four major heart sounds, but only the first two can be heard without use of special amplification.

First heart sound. Produced at the beginning of systole when the atrioventricular (AV) valves close and the semilunar (SL; the aortic and pulmonary) valves open. This sound has a low-pitched tone commonly termed the lub sound of the heartbeat.
Second heart sound. Occurs during the end of systole and is produced by the closure of the SL valves, the opening of the AV valves, and the resulting vibrations in the arteries and ventricles. Owing to the higher blood pressures in the arteries, the sound produced is higher pitched than the first heart sound. It is commonly referred to as the dub sound.
Third heart sound. Occurs during the rapid filling of the ventricles after the AV valves open and is probably produced by vibrations of the ventricular walls.
Fourth heart sound. Occurs at the time of atrial contraction and is probably due to the accelerated rush of blood into the ventricles.

RELATIONSHIP BETWEEN HEART SOUNDS AND ECG:

FIRST HEART SOUND Coincide with R wave of ECG
SECOND HEART SOUND Coincide with the ending part of T wave of ECG

HEART VALVE FAILURE DISEASES

Aortic Stenosis:

Here the blood is ejected from the left ventricle through a small opening of the aortic
valve. Because of the resistance to ejection, the pressure in the left ventricle rises.
This causes turbulent blood flow. This turbulent blood impinging the aortic valve
causes intense vibration; it produces loud murmur (sounds related to non laminar flow
of blood in the heart).

Aortic Regurgitation:

No sound is heard during systole, but during diastole blood flows backward from the
aorta into the left ventricles, causing a blowing murmur. This is produced due to the
valves are damaged.

Mitral Regurgitation:
Here blood flows backward through the Mitral valve during systole. This produces
sound during systole.

Mitral Stenosis:

Here the blood passes with difficulty from the left atrium into the left ventricle due to
pressure difference. It produces murmur which is very weak.

ECG

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BACKGROUND:

Sum of the electrical signals from the cardiac muscle as recorded on the surface of the body.
Pattern of the electrical activity depends on the orientation of the electrodes and the electrical activity of the cardiac cells.

EINTHOVEN TRIANGLE:

Willem Einthoven (1860-1927) attempted to explain the principles of the ECG in
scientific terms. In Einthoven's triangle, the heart may be considered to lie at the
centre of an equilateral triangle and the corners of the triangles are the effective
sensing points - the right arm, left arm and left leg electrodes.

CONVENTIONAL ECG ELECTRODE DERIVATIONS:

· 12 standard leads

· ECG is recorded:

Bipolar recording: between two points of the body (= bipolar recording)
Unipolar recording: between one point of the body (different electrode) and ground (indifferent electrode)
6 limb leads: I, II, III (bipolar) and aVR, aVL, aVF (unipolar)
6 precordial leads: V1 - V6 (unipolar)

LEADS CONNECTION:

Standard Limb Leads: I, II, III; bipolar, form a set of axes 60° apart

Lead I: Composed of negative electrode on the right arm and positive electrode on the left arm.
Lead II: Composed of negative electrode on the right arm and positive electrode on the left leg.
Lead III: Composed of negative electrode on the left arm and positive electrode on the left leg.

Augmented Voltage Leads:

aVR, aVL aVF; unipolar ; form a set of axes 60° apart but are rotated 30° from the axes of the standard limb leads.

· aVR: Exploring electrode located at the right shoulder.
· aVL: Exploring electrode located at the left shoulder.
· aVF: Exploring electrode located at the left foot.

Reference Point for Augmented Leads: The opposing standard limb lead; i.e., that
standard limb lead whose axis is perpendicular to the particular augmented lead.

Chest Leads: Vl, V2, V3, V4, V5, V6, explore the electrical activity of the heart in
the horizontal plane; i.e., as if looking down on a cross section of the body at the level
of the heart. These are exploring leads.

ECG curve:

P wave: Atrial depolarization (Small, rounded and upright)
QRS complex: Ventricular depolarization (Spiked with one or more deflections from the baseline)
T wave: Ventricular repolarization (Broad, rounded. if .QRS. then must be a .T. wave)
PR segment: AV nodal delay
ST segment: Ventricles are contracting and emptying the action potential of ventricular muscle cells in plateau phase
TP interval: Ventricle cells at rest, ventricular filling.

NORMAL VALUES FOR AMPLITUDES AND DURATIONS OF
IMPORTANT ECG PARAMETERS:

Amplitude:

P – wave 0.25 mV
R – wave 1.60 mV
Q – wave 25% of R wave
T – wave 0.1 to 0.5 mV

Duration:

P – R interval - 0.12 to 0.22 s
Q – T interval - 0.35 to 0.44 s
S – T interval - 0.05 to 0.15 s
P – wave interval - 0.11 s
QRS interval - 0.09 to 0.10 s

Links:

http://library.med.utah.edu/kw/ecg/
http://www.anaesthetist.com/icu/organs/heart/ecg/Findex.htm
http://nobelprize.org/educational/medicine/ecg/

Blood Gas analysis (part 2)

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Po2 electrode

History

Leland Clark (Professor of Chemistry, Antioch College, Yellow Springs, Ohio, and Fels Research Institute, Yellow Springs, Ohio) had developed the first bubble oxygenator for use in cardiac surgery. However, when he came to publish his results, his article was refused by the editor since the oxygen tension in the blood coming out from the device could not be measured. This instigated Clark to develop the oxygen electrode

Principle

Based on Redox reaction
Cathode (Pt) is reduced
Anode (Ag/Agcl) is oxidised
The resulting current linearly proportional to oxygen concentration
Operating voltage 0.68V (Since in V I characteristics of Po2 electrode around 0.6 to 0.7V current is constant)

Construction

Membrane - Polystyrene

Complete Blood Gas Analyzer

Separate amplifier for each electrode output
Sample size : 25uL
Response time: 1 - 5 mins

Accurate measurement of following parameters
a) pH
b) pCO2
c) pO2
d) Haematocrit and Hemoglobin
e) Electrolytes Sodium,Potassium andChloride Ca++ and Magnesium
f) Lactate
g)The equipment should possess electrodes with long life at least 2 years

Electrodes placed in temperature controlled chamber

Students activity

Design a temp control circuit to be utilized for Blood gas analyzer. The aim is to reduce / increase temperature if it changes from set-point.

Blood Gas analysis (part 1)

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Blood gas analysis, also called arterial blood gas (ABG) analysis, is a procedure to measure the partial pressure of oxygen (O₂) and carbon dioxide (CO₂) gases and the pH (hydrogen ion concentration) in arterial blood. Oxygen content (O2CT), oxygen saturation (SaO2) and bicarbonate (RCO3 -) values are also measured.

Blood is most commonly drawn from the radial artery because it is easily accessible, can be compressed to control bleeding, and has less risk for occlusion. The femoral artery (or less often, the brachial artery) is also used, especially during emergency situations or with children. Blood can also be taken from an arterial catheter already placed in one of these arteries.

The syringe is pre-packaged and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again. Once the sample is obtained, care is taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results.

The sealed syringe is taken to a blood gas analyzer. If the sample cannot be immediately analyzed, it is chilled in an ice bath in a glass syringe to slow metabolic processes which can cause inaccuracy. Samples drawn in plastic syringes are not iced and are analyzed within 30 minutes

Purpose

To evaluate gas exchange in the lungs.
To assess integrity of the ventilatory control system.
To determine the acid-base level of the blood.
To monitor respiratory therapy

Normal ABG values fall within the following ranges:

PaO2: 75 to 100 mm Hg
PacO2: 35 to 45 mm Hg
pH: 7.35 to 7.45
O2CT: 15% to 22%
SaO2: 95% to 100%
HCO3 -: 24 to 28 mEq/L.

pH electrode

Conventional Glass electrode
Sensitivity 59mV/pH
Use of Syringe electrodes

pCo2 electrode

Principle:

Conventional pH electrode covered with rubber membrane
water is kept in between membrane and electrode
Diffused Co2 mixes with water to form H2CO3
H2CO3 dissociates into H+ and HcO3- ions
This H+ ions are sensed by pH electrode

pH = log HCO3 - log k - log a - log PCO2

In commercial electrode rubber is replaced by Teflon membrane and water replaced by sodium bicarbonate solution.

References:

Medical Instrumentation and design, John Webster, Page No: 442
http://en.wikipedia.org/wiki/Arterial_blood_gas

Generalized Medical Instrumentation System

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The major difference between this system and a conventional instrumentation system is:

The source of the signals (measurand) is a living tissue or energy is applied to living tissue.

Measurand

Physical quantity, property, or condition that is being measured by the
system.

* most important issue : accessibility
- internal (blood pressure), on body surface (ECG, EEG)
- emanate from the body (infra-red radiation)
- derived from a tissue sample (blood or biopsy)

Medically important measurands

• Biopotentials (ECG, EEG, EMG, EOG, etc.)
• Pressure, flow, dimensions (imaging)
• Displacement (velocity dx/dt, acceleration d2x/d2t, and force =
md2x/d2t)
• Impedance, temperature and chemical concentration

The measurand may be localized to a specific organ or anatomical

Sensor

* The transducer or sensor should only respond to the form of energy present in the measurand to the exclusion of all others!
* The sensor should interface with the living system to minimize the energy extracted and being minimally invasive!

Signal conditioning

Usually the sensor output can not directly drive the display, therefore
signal processing or conditioning is required

Examples of signal processing:
1. Impedance matching
2. Amplification
3. Filtering
4. Mathematical mapping
5. Linearizing
6. Analog-to-digital conversion (ADC)
7. Digital-to-analog conversion (DAC)
8. Signal averaging to reduce noise (i.e. evoked response)
9. Transformation (time domain
frequency domain)
10. Compensation for undesirable sensor characteristics
11. Etc.

Output displays

Examples of output displays:
1. Numerical
2. Graphical
3. Discrete
4. Continuous
5. Permanent or temporary

• Most displays rely on our vision, but auditory sense is also sometimes used (for example, Doppler ultrasonic signals)

• User controls and output displays should conform to human factors engineering guidelines for the design of medical devices

Auxiliary Elements

*Calibration signal with the properties of the measurand should be applied to the sensor input or as early in the signal processing chain as possible

**Many forms of feedback (automatic or manual) may be required to elicit the measurand, to adjust the sensor and signal conditioner and to direct the flow of output (display, storage, transmission)

***Data storage for signal conditioning or examination of alarm conditions or implementation of different processing algorithms

**** Data communication transmission of patient data to remote display at nurse’s station and medical center

Operation Modes

1. Direct and Indirect Modes
• Direct: Measurand directly to sensor
- readily accessible or
- acceptable invasive procedure
For example: direct blood pressure measurement

• Indirect: measurand not accessible
- Use another measurand with known relation to the desired one
- Use some form of energy or material that interacts with the
desired measurand to generate a new accessible one

For example:
Cardiac output (volume of blood pumped/min by the heart)
- Measurements of respiration & blood gas concentration
- Dye dilution
- Morphology of internal organs determined from X-rays

2. Sampling or Continuous Modes
• Sampling: Parameters that change slowly do not require continuous measurements

For example: body temperature, ionic concentrations, etc.

• Continuous: Parameters that change fast enough to require continuous measurements

For example: ECG, EEG, EMG, respiratory gas flow, etc.
Note: Frequency content of the measurand, the objective of the measurement, the condition of the patient and the potential liability of the physician influence how often data should be acquired

3. Generating and Modulating Sensors

• Generating: Produce output from energy taken directly from measurand

For example: photovoltaic cell (output voltage related to irradiation)

• Modulating: Measurand changes flow of energy from an external source that affects the output of a sensor

For example: photoconductive cell (apply external power to the sensor to measure changes in resistance with irradiation)

4. Analog and Digital Modes

• Analog: Continuous (parameter takes on any value within the dynamic range)

For example: Parameters that change fast enough to require continuous measurements: ECG, EEG, EMG, respiratory gas flow, etc.

• Digital: Discrete (parameter takes on a finite number of different values)

* Most sensors are analog (i.e., strain gages, thermistors, etc.)
* Very few sensors are digital in nature (i.e., shaft encoders)

5. Real-time and delayed-time Modes

Real-time: Sensors must acquire signals as they actually occur

• Output is not always displayed immediately, because some types of signal processing (i.e. averaging, transformations, etc) require considerable amount of date before production of final results

Delayed-time Often acceptable (short delays) unless urgent feedback & control depend on output

• Cell cultures provide an example where several days of delay may be required before an output is obtained!

VIVA QUESTIONS FOR DESIGN PROJECT LAB (CYCLE I)

2011-01-06T01:19:00.000-08:00

1. What is a counter?

2. What are the types of counter?

3. Difference between BCD and Binary counter?

4. Difference between positive and negative edge triggering?

5. What is the use of batch counter?

6. What is the maximum count of a BCD counter?

7. What is a decoder?

8. Construct a BCD to seven segment decoder (using logical gates)?

9. What is the need for BCD to seven segment decoder?

10. What are the types of displays?

11. Compare common anode and cathode LED?

12. Mention the steps in designing a counter?

13. Identify the internal components of IC7490 and IC7447?

14. What are TTL IC’s?

15. What is ADC?

16. What is DAC?

17. Mention the types of ADC?

18. Mention the types of DAC?

19. What is the resolution of DAC?

20. What is the advantage of using R-2R over weighted resistor method?

21. Define conversion time, settling time, monotonocity.

22. Which is the fastest ADC? Why?

23. Give some examples of monolithic ADC and DAC?

24. Write the output expression for a N bit DAC?

25. What is sampling?

26. Define Sampling theorem.

27. What is quantization?

28. What is aliasing?

29. What is the significance of 3½ digits?

30. Explain briefly the operation of dual slope DVM?

31. Mention the value of integrating current?

32. How will you choose the integrating resistor and capacitor?

33. What is the operating frequency?

34. What are the ways of connecting oscillator to ICL 7107?

35. What is the use of test pin in ICL 7107?

36. How will you choose auto zero capacitor?

37. What is auto zero phase?

38. Mention the guide lines for choosing reference voltage in ICL 7107?

39. How will you generate a negative supply from +5V for ICL 7107?

40. What is the calibrator?

41. Mention the components of ICL 7107?

42. Identify the operating voltage, temperature condition for ICL 7107?

43. What is meant by regulation?

44. Define load and line regulation.

45. Mention the IC fixed positive voltage regulation?

46. Mention the IC fixed negative voltage regulation?

47. What is the need for variable regulators?

48. What is the need for step down transformer?

49. Compare iron and air core transformers.

50. Define Transformer Utilization Factor.

51. Define turns ratio.

52. What are the types of rectifier?

53. What is meant by ripple factor?

54. How will you choose a diode based on current rating?

55. What is the need for filter?

56. What are the types of filter?

57. What is the maximum input voltage range for IC 78xx and IC 79xx?

58. What is meant by dropout voltage?

59. What is telemetry?

60. What are the types of telemetry?

61. Mention the standard ranges for current, voltage and pressure telemetry.

62. Mention the transducer available for measurement of temperature?

63. Which temperature transducer has highest sensitivity?

64. Which temperature transducer has highest stability?

65. What is maximum possible temperature range of PT100?

66. Write the calendar equation?

67. Write the equation of resistance for RTD and Thermister?

68. Mention the need and advantage of Instrumentational amplifier?

69. What are the types of V/I converter?

70. What is the gain of instrumentational amplifier?

71. How will you test IC741 for proper working?

72. Mention monolithic instumentaional amplifier?

73. What is servo mechanisms?

74. What is linear regulator?

75. What is switching regulator?

76. What is the difference between buck and boost converter?

77. What are the factors to be considered when designing a regulated voltage supply?

78. Define PIV

79. What is the use of zero comparator in 3½ digit voltmeter?

80. Derive the output voltage for the following digital inputs

a) 1000

b) 0100

c) 0010

d) 0001 in R-2R ladder network.

Modelling of Electrode Electrolytic Interface

2010-12-29T22:58:00.000-08:00

Bio-potentials: Ionic voltages produced as a result of the electrochemical activity of excitable cells.

Electrodes: Transducers to convert ionic potentials into electrical potentials

Electrode Electrolytic Interface

Charge separation occurs which leads to electrical double layer.
Half cell potential is generated
Electrolyte offers some resistance
DC offset of the electrode presents some resistance.

Human body electrode Interface:

Resistance offered by epidermis layer needs to be considered

Hence to reduce the effect of body resistance

use of Gel between electrode and skin
Preparing the skin surface by removal of stratum corneum

Important aspects:

Electrodes – Basics

High-quality biopotential measurements require

Good amplifier design
Use of good electrodes and their proper placement on the patient
Good laboratory and clinical practices

Electrodes should be chosen according to the application
Basic electrode structure includes:

The body and casing
Electrode made of high-conductivity material
Wire connector
Cavity or similar for electrolytic gel
Adhesive rim

The complexity of electrode design often neglected

Electrodes - Basics

Skin preparation by abrasion or cleansing
Placement close to the source being measured
Placement above bony structures where there is less muscle mass
Distinguishing features of different electrodes:

How secure? The structure and the use of strong but less irritant adhesives
How conductive? Use of noble metals vs. cheaper materials
How prone to artifact? Use of low-junction-potential materials such as Ag-AgCl
If electrolytic gel is used, how is it applied? High conductivity gels can help reduce the junction potentials and resistance but tend to be more allergenic or irritating

Ag-AgCl, Silver-Silver Chloride Electrodes

The most commonly used electrode type
Silver is interfaced with its salt silver-chloride
Choice of materials helps to reduce junction potentials

Junction potentials are the result of the dissimilar electrolytic interfaces

Electrolytic gel enhances conductivity and also reduces junction potentials

Typically based on sodium or potassium chloride, concentration in the order of 0.1 M weak enough to not irritate the skin

The gel is typically soaked into a foam pad or applied directly in a pocket produced by electrode housing
Relatively low-cost and general purpose electrode
Particularly suited for ambulatory or long term use

References:

Action Potential

2010-12-28T02:12:00.000-08:00

Steps in Action Potential:

Cell in its normal state (polarized cell) called Resting state block Na+ ions.
So outside of Cell is positive wrt to inside. This potential is -70 mV
When cell is stimulated the cell membrane momentarily allows Na+ ions.
So inside of cell becomes positive (Depolarized cell). This is called action potential (20mV)
Immediately after effect of stimulation ceases Na+ ions are transported back outside (Repolarized cell).

Conduction of action potential is rapid in mylinated axons.

References:

1. http://outreach.mcb.harvard.edu/animations/actionpotential.swf
2. http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html

Human Physiology & Problems encountered in measuring living systems

2010-12-28T01:53:00.000-08:00

Human physiology is the science of the mechanical, physical, and biochemical functions of normal humans or human tissues or organs.

Physiology focuses principally at the level of organs and systems.

Major systems of Human body:

1. Cardiovascular system - can be compared to closed loop hydraulic system with a 2 synchronized isolated functioning two stage pump.

It consists of the heart and blood vessels (arteries, veins, capillaries). The heart propels the circulation of the blood, which serves as a "transportation system" to transfer oxygen, fuel, nutrients, waste products, immune cells, and signalling molecules (i.e., hormones) from one part of the body to another. The blood consists of fluid that carries cells in the circulation, including some that move from tissue to blood vessels and back, as well as the spleen and bone marrow.

2. Respiratory system - can be compared to a closed loop pneumatic system with two elastic bag and air pump to create alternatively positive and negative pressures.

The respiratory system consists of the nose, nasopharynx, trachea, and lungs. It brings oxygen from the air and excretes carbon dioxide and water back into the air.

3. Nervous System - self adapting information processor with high speed communication network and a myraid of input and output channels.

The nervous system consists of the central nervous system (which is the brain and spinal cord) and peripheral nervous system. The brain is the organ of thought, emotion, and sensory processing, and serves many aspects of communication and control of various other systems and functions. The special senses consist of vision, hearing, taste, and smell. The eyes, ears, tongue, and nose gather information about the body's environment.

Problems encountered in measuring living systems (shortcut: VIT LEAPS)

1. Inaccessibility of Variables
2. Variability of data
3. Lack of knowledge about interrelationships
4. Interaction among Physiological systems
5. Effect of Transducer on measurement
6. Artifacts
7. Energy limitations
8. Safety considerations

References:

1. Biomedical Instrumentation and mesurements by Leslie cromwell, Second Edition. pp 16 to 24.
2. http://www.innerbody.com/htm/body.html
3. http://free-ed.net/free-ed/HealthCare/Physiology/default.asp
4. http://www.stcms.si.edu/hbs/hbs_student.htm

Introduction to Biomedical Engineering

2010-12-17T19:38:00.000-08:00

Who is a Biomedical Engineer?

A Biomedical Engineer uses traditional engineering expertise to analyze and solve problems
in biology and medicine, providing an overall enhancement of health care. Students choose
the biomedical engineering field to be of service to people, to partake of the excitement of
working with living systems, and to apply advanced technology to the complex problems of
medical care. The biomedical engineer works with other health care professionals including
physicians, nurses, therapists and technicians. Biomedical engineers may be called upon in
a wide range of capacities: to design instruments, devices, and software, to bring together
knowledge from many technical sources to develop new procedures, or to conduct research
needed to solve clinical problems.

What are Some of the Specialty Areas?

In this field there is continual change and creation of new areas due to rapid advancement
in technology; however, some of the well established specialty areas within the field of
biomedical engineering are: bioinstrumentation; biomaterials; biomechanics; cellular,
tissue and genetic engineering; clinical engineering; medical imaging; orthopaedic surgery;
rehabilitation engineering; and systems physiology.

Bioinstrumentation is the application of electronics and measurement techniques to
develop devices used in diagnosis and treatment of disease. Computers are an essential
part of bioinstrumentation, from the microprocessor in a single-purpose instrument used to
do a variety of small tasks to the microcomputer needed to process the large amount of
information in a medical imaging system.

Biomaterials include both living tissue and artificial materials used for implantation.
Understanding the properties and behavior of living material is vital in the design of
implant materials. The selection of an appropriate material to place in the human body
may be one of the most difficult tasks faced by the biomedical engineer. Certain metal
alloys, ceramics, polymers, and composites have been used as implantable materials.
Biomaterials must be nontoxic, non-carcinogenic, chemically inert, stable, and
mechanically strong enough to withstand the repeated forces of a lifetime. Newer
biomaterials even incorporate living cells in order to provide a true biological and
mechanical match for the living tissue.

Biomechanics applies classical mechanics (statics, dynamics, fluids, solids,
thermodynamics, and continuum mechanics) to biological or medical problems. It includes
the study of motion, material deformation, flow within the body and in devices, and
transport of chemical constituents across biological and synthetic media and membranes.
Progress in biomechanics has led to the development of the artificial heart and heart
valves, artificial joint replacements, as well as a better understanding of the function of the
heart and lung, blood vessels and capillaries, and bone, cartilage, intervertebral discs,
ligaments and tendons of the musculoskeletal systems.

Cellular, Tissue and Genetic Engineering involve more recent attempts to attack
biomedical problems at the microscopic level. These areas utilize the anatomy,
biochemistry and mechanics of cellular and sub-cellular structures in order to understand
disease processes and to be able to intervene at very specific sites. With these capabilities,
miniature devices deliver compounds that can stimulate or inhibit cellular processes at
precise target locations to promote healing or inhibit disease formation and progression.

Clinical Engineering is the application of technology to health care in hospitals. The
clinical engineer is a member of the health care team along with physicians, nurses and
other hospital staff. Clinical engineers are responsible for developing and maintaining
computer databases of medical instrumentation and equipment records and for the
purchase and use of sophisticated medical instruments. They may also work with
physicians to adapt instrumentation to the specific needs of the physician and the hospital.
This often involves the interface of instruments with computer systems and customized
software for instrument control and data acquisition and analysis. Clinical engineers are
involved with the application of the latest technology to health care.

Medical Imaging combines knowledge of a unique physical phenomenon (sound,
radiation, magnetism, etc.) with high speed electronic data processing, analysis and
display to generate an image. Often, these images can be obtained with minimal or
completely noninvasive procedures, making them less painful and more readily repeatable
than invasive techniques.

Orthopaedic Bioengineering is the specialty where methods of engineering and
computational mechanics have been applied for the understanding of the function of bones,
joints and muscles, and for the design of artificial joint replacements. Orthopaedic
bioengineers analyze the friction, lubrication and wear characteristics of natural and
artificial joints; they perform stress analysis of the musculoskeletal system; and they
develop artificial biomaterials (biologic and synthetic) for replacement of bones, cartilages,
ligaments, tendons, meniscus and intervertebral discs. They often perform gait and motion
analyses for sports performance and patient outcome following surgical procedures.
Orthopaedic bioengineers also pursue fundamental studies on cellular function, and
mechano-signal transduction.

Rehabilitation Engineering is a growing specialty area of biomedical engineering.
Rehabilitation engineers enhance the capabilities and improve the quality of life for
individuals with physical and cognitive impairments. They are involved in prosthetics, the
development of home, workplace and transportation modifications and the design of
assistive technology that enhance seating and positioning, mobility, and communication.
Rehabilitation engineers are also developing hardware and software computer adaptations
and cognitive aids to assist people with cognitive difficulties.

Systems Physiology is the term used to describe that aspect of biomedical engineering in
which engineering strategies, techniques and tools are used to gain a comprehensive and
integrated understanding of the function of living organisms ranging from bacteria to
humans. Computer modeling is used in the analysis of experimental data and in

formulating mathematical descriptions of physiological events. In research, predictor
models are used in designing new experiments to refine our knowledge. Living systems
have highly regulated feedback control systems that can be examined with state-of-the-art
techniques. Examples are the biochemistry of metabolism and the control of limb
movements.

These specialty areas frequently depend on each other. Often, the biomedical engineer who
works in an applied field will use knowledge gathered by biomedical engineers working in
other areas. For example, the design of an artificial hip is greatly aided by studies on
anatomy, bone biomechanics, gait analysis, and biomaterial compatibility. The forces that
are applied to the hip can be considered in the design and material selection for the
prosthesis. Similarly, the design of systems to electrically stimulate paralyzed muscle to
move in a controlled way uses knowledge of the behavior of the human musculoskeletal
system. The selection of appropriate materials used in these devices falls within the realm
of the biomaterials engineer.

Examples of Specific Activities

Work done by biomedical engineers may include a wide range of activities such as:

Artificial organs (hearing aids, cardiac pacemakers, artificial kidneys and hearts, blood oxygenators, synthetic blood vessels, joints, arms, and legs).
Automated patient monitoring (during surgery or in intensive care, healthy persons in unusual environments, such as astronauts in space or underwater divers at great depth).
Blood chemistry sensors (potassium, sodium, O2, CO2, and pH).
Advanced therapeutic and surgical devices (laser system for eye surgery, automated delivery of insulin, etc.).
Application of expert systems and artificial intelligence to clinical decision making (computer-based systems for diagnosing diseases).
Design of optimal clinical laboratories (computerized analyzer for blood samples, cardiac catheterization laboratory, etc.).
Medical imaging systems (ultrasound, computer assisted tomography, magnetic resonance imaging, positron emission tomography, etc.).
Computer modeling of physiologic systems (blood pressure control, renal function, visual and auditory nervous circuits, etc.).
Biomaterials design (mechanical, transport and biocompatibility properties of implantable artificial materials).
Biomechanics of injury and wound healing (gait analysis, application of growth factors, etc.).
Sports medicine (rehabilitation, external support devices, etc.).