Medical Instrumentation- body parameters measurement

UEEG002
Medical Instrumentation
Measurement of Body Parameters
D.Poornima, AP(Sr.Gr)/EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore

Blood Pressure
• Blood pressure is the force of blood pushing against
the walls of the arteries.
• Each time heart beats, it pumps blood into the
arteries.
• Blood pressure is highest when the heart beats,
pumping the blood - called systolic pressure.
• When the heart is at rest between beats, blood
pressure falls - called diastolic pressure.
• Usually the systolic number comes before or above
the diastolic number.
• For example, 120/80 means a systolic of 120 and a
diastolic of 80.
• All blood pressure measurements are made with
reference to the atmospheric pressure.

Blood Pressure Category
Systolic
Blood
Pressure
Diastolic
Blood
Pressure
Normal Less than 120 and Less than 80
High Blood Pressure (no other heart risk factors) 140 or higher or 90 or higher
High Blood Pressure (with other heart risk
factors, according to some providers)
130 or higher or 80 or higher
Dangerously high blood pressure - seek medical
care right away
180 or higher and 120 or hig

Measurement of Blood Pressure
• The nominal values of pressure in the basic circulatory system are:
 Arterial system 30–300 mmHg
 Venous system 5–15 mmHg
 Pulmonary system 6–25 mmHg
• The most frequently monitored pressures, in medium and long
term patient monitoring, are the arterial pressure and the venous
pressure
• There are two basic methods for measuring blood pressure
 Direct
 Indirect

Direct methods Indirect methods
Provide continuous and much more
reliable information about the absolute
vascular pressure
Based on the adjustment of a known
external pressure equal to the vascular
pressure so that the vessel just collapses.
Uses probes or transducers inserted
directly into the blood stream
Consist of simple equipment
Accurate information is obtained Data is Intermittent and less informative
Increased disturbance to the patient and
complexity of the equipment.
Cause very little discomfort to the subject

Direct Methods of
Monitoring Blood Pressure
• Used when the highest degree of absolute accuracy, dynamic
response and continuous monitoring is required
• Very essential in the management of critically ill patients in
intensive cardiac care or patients undergoing cardiac
catheterization.
• To measure the pressure in deep regions inaccessible by indirect
means
• A catheter or a needle type probe is inserted through a vein or
artery to the area of interest
• Two types of probes can be used.
– catheter tip
– fluid-filled catheter type

• Catheter tip probe
 the sensor is mounted on the tip of the probe and the pressures
exerted on it are converted to the proportional electrical signals.
 provide the maximum dynamic response and avoid acceleration
artefacts
• Fluid-filled catheter type
 transmits the pressure exerted on its fluid-filled column to an
external transducer.
 requires careful adjustment of the catheter dimensions to obtain
an optimum dynamic response

• A typical set-up of a fluid-filled system for
measuring blood pressure is shown
• Before inserting the catheters into the blood
vessel it is important that the fluid-filled system
should be thoroughly flushed.
• In practice a steady flow of sterile saline is
passed through the catheter to prevent blood
clotting in it.
• As air bubbles dampen the pulse response of
the system and degrades it - system should be
free from them

Pressure Transducer Circuit
• Circuit diagram for processing the electrical signals received from the
pressure transducer
• The transducer is excited with a 5 V dc excitation.
• The electrical signals corresponding to the arterial pressure are amplified
in an operational amplifier or a carrier amplifier.
• The modern preamplifier for processing pressure signals are of the
isolated type and therefore comprise of floating and grounded circuits
similar to ECG amplifiers.
• The excitation for the transducer comes from an amplitude controlled
bridge oscillator through an isolating transformer, which provides an
interconnection between the floating and grounded circuits.
• The input stage is a differential circuit, which amplifies pressure change,
which is sensed in the patient connected circuit.
• After RF filtering, the signal is transformer-coupled to a synchronized
demodulator for removing the carrier frequency from the pressure signal

• For the measurement of systolic pressure, a conventional
peak reading type voltmeter is used.
• When a positive going pressure pulse appears at A, diode D3
conducts and charges C3 to the peak value of the input signal,
which corresponds to the systolic value.
• Time constant R3C3 is chosen in such a way that it gives a
steady output to the indicating meter.
• The value of diastolic pressure is derived in an indirect way.
• A clamping circuit consisting of C1 and D1 is used to develop
a voltage equal to the peak-to-peak value of the pulse
pressure.
• This voltage appears across R1. Diode D2 would then conduct
and charge capacitor C2 to the peak value of the pulse signal.
• The diastolic pressure is indicated by a second meter M2
which shows the difference between the peak systolic minus
the peak-to-peak pulse pressure signal.

Indirect Methods of Monitoring Blood Pressure
• Classical method of making an indirect measurement of
blood pressure is by the use of a cuff over the limb
containing the artery – called SPHYGMOMANOMETER.
• Introduced by Riva-Rocci for the determination of
systolic and diastolic pressures.
• Pressure in the cuff is raised to a level well above the
systolic pressure so that the flow of blood is completely
terminated.
• Pressure in the cuff is then released at a particular rate.
• When it reaches a level, which is below the systolic
pressure, a brief flow occurs and operator can hear
some crashing snapping sounds- KOROTKOFF SOUNDS
• If the cuff pressure is allowed to fall further, just below
the diastolic pressure value, the flow becomes normal
and uninterrupted – Korotkoff sounds disappear or
become muffled
• Systolic pressure is the one at the starting of Korotkoff
sound
• Diastolic is the one when the sounds disappear

• The exact instant at which the artery just opens and
when it is fully opened gives the systolic and
diastolic pressures.
• The method given by Korotkoff, based on the
sounds produced by flow changes - normally used
in the conventional sphygmomanometers.
• The sounds first appear when the cuff pressure falls
to just below the systolic pressure - produced by
the brief turbulent flow .
• The sounds disappear or change in character at just
below 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 cuff.

Oscillometric Blood Pressure Measurement
• Similar to sphygmomanometer
• But small fluctuations (oscillations) in cuff pressure is
measured rather than direct pressure
• When the blood breaks through after occlusion, the
walls of artery begins to vibrate slightly
• The fluctuating walls slightly alter the pressure, giving
rise to oscillations in blood pressure
• The onset of the pressure oscillations correlates well
with the systolic pressure
• The diastolic pressure corresponds to the point where
the rate of amplitude decrease suddenly

Ultrasonic Pressure
Measurements
• Ultrasonic waves are used to to measure the blood pressure.
• They are subject to Doppler shift (a slight alteration of frequency)
when reflected from a moving object.
• When the waves encounter a fluctuating vessel wall some of its
energy is reflected back
• If ΔF is the Doppler shift, F± ΔF describes the frequency of the
reflected wave
• The presence of ΔF corresponds to the Korotkoff sounds, and it
diminishes when near laminar flow resumes

Measurement of Body
Temperature
• The most popular method -- a mercury-in-glass thermometer.
• They are
– slow
– difficult to read
– susceptible to contamination
– reliable accuracy cannot be attained over the wide range
• Electronic thermometers are
– convenient
– reliable
– generally more accurate
• They mostly use probes incorporating a thermistor or thermocouple sensor which have rapid response
characteristics.
• The probes are generally reusable and their covers are disposable.
• Useful where continuous or frequent sampling of temperature is desirable, as in the operating theatre, post-
operative recovery room and intensive care unit, and during forced diuresis, massive blood transfusion, and
accidental hypothermia

Thermistor
• The transducer normally used for temperature measurement
in a patient monitoring system is a thermistor.
• Thermistors are resistors whose resistance drops significantly
as temperature increases.
• They are composed of a compressed and sintered mixture of
metallic oxides of manganese, nickel, cobalt, copper,
magnesium, titanium and other metals.
• Changes in resistance of the thermistor with changes in
temperature are measured in a bridge circuit and indicated on
a calibrated meter.
• The measuring range is 30–42°C. In a patient monitoring
system, provision for two channel temperature measurements
are usually made.
• Similar to ECG monitoring, the output circuits are isolated
through opto-couplers.
• Provision for inoperate conditions are also included in such
type of monitoring equipment

Medical Instrumentation- body parameters measurement

Thermocouple
• When two wires of different materials are joined together at either end, forming
two junctions which are maintained at different temperatures, a thermo-
electromotive force (emf) is generated causing a current to flow around the circuit -
called a thermocouple.
• The junction at the higher temperature is termed the hot or measuring junction
and that at the lower temperature the cold or reference junction.
• The cold junction is usually kept at 0°C.
• Over a limited range of temperature, the current produced is proportional to the
• temperature difference existing between the junctions.
• By inserting one junction in or on the surface of the medium whose
• temperature is to be measured and keeping the other at a lower and
• constant temperature (usually O°C), a measurable emf is produced
• proportional to the temperature difference between the two junctions.
• The reference junction is normally held at O°C inside a vacuum flask
• containing melting ice

Measurement of Respiration
Rate
• Functions of the respiratory system ----- supply oxygen
and remove carbon dioxide from the tissues.
• The respiration rate is the number of breaths a
person takes per minute.
• Usually measured when a person is at rest
• Involves counting the number of breaths for one
minute by counting how many times the chest
rises.
• Respiration rates may increase with fever, illness,
and other medical conditions.
• Many methods for measurement – use of a
particular method depends on
– The ease of application of the transducer
– Their acceptance by the subject under test.

• Commonly used methods for the measurement
of respiration rate are
– Displacement method
– Thermistor method
– Impedance Pneumography
– CO2 method
– Apnoea detectors

Displacement Method
• The respiratory cycle is accompanied by changes in the
thoracic volume.
• Volume changes can be sensed by means of a displacement
transducer
• Transducer has a strain gauge or a variable resistance
element.
• The transducer is held by an elastic band, which goes
around the chest.
• The respiratory movements result in resistance changes of
the strain gauge element connected as one arm of a
Wheatstone bridge circuit.
• Bridge output varies with chest expansion and yields signals
corresponding to respiratory activity

• Changes in the chest circumference can also be
detected by a rubber tube filled with mercury.
• The tube is fastened firmly around the chest.
• With the expansion of the chest during an
inspiratory phase, the rubber tube increases in
length
• Resistance of the mercury from one end of this
tube to the other changes.
• Resistance changes can be measured by sending a
constant current through it and measuring the
changes in voltage developed with the respiratory
cycle.

Thermistor Method
• Air is warmed during its passage through the lungs and
the respiratory tract
• There is a detectable difference of temperature between
inspired and expired air
• Difference of temperature can be sensed by using a
thermistor placed in front of the nostrils by means of a
suitable holding device
• If difference in temperature is small, the thermistor can
even be initially heated to an appropriate temperature
and the variation of its resistance in synchronism with the
respiration rate, as a result of the cooling effect of the air
stream, can be detected.

• The thermistor is placed as part of a voltage dividing
circuit or in a bridge circuit whose unbalance signal
can be amplified to obtain the respiratory activity.
• The method is
– simple
– works well
– satisfies the majority of clinical needs including for
operative and post-operative subjects.
• Unconscious patients display a tendency for the
uncontrolled tongue to block the breathing system.
• Under such systems, not even a single millilitre of air is
inhaled but the patient’s thorax is carrying out large,
even though frustral breathing motions.
• In this condition, the impedance pneumograph and
switch methods will show the correct state.

Impedance Pneumography
• Indirect technique for the measurement of
respiration rate
• Using externally applied electrodes on the
thorax, it measures respiratory rate through
the relationship between respiratory depth
and thoracic impedance change.
• The technique avoids encumbering the
subject with masks, tubes, flowmeters or
spirometers, does not impede respiration and
has minimal effect on the psychological state
of the subject.
• Impedance method for measuring respiration
rate consists in passing a high frequency
current through the appropriately placed
electrodes on the surface of the body and
detecting the modulated signal.
• The signal is modulated by changes in the
body impedance, accompanying the
respiratory cycle.

• A 50-50kHz a.c signal is produced by oscillator circuit and is given to the
chest of the patient through electrodes.
• The signal voltage applied to the amplifier (Differential amplifier) block is
the voltage drop across the resistance .
• V = I(R+ R)
• Where V= Output voltage (V)
• I= Current through the chest (A)
• R= chest impedance without respiration (R)
• R= change of chest impedance due to respiration (Q)
– The output of the amplifier is given to demodulator and filter block.
– Hence low pass filter is used to remove the
– residual carrier signal.
– The output of the impedance pneumograph
– contains respirating rate data.

CO2 Method
• Respiration rate can also be derived by continuously monitoring
the CO2 contained in the subject’s alveolar air.
• Useful in supervising patients suffering from respiratory paralysis,
and other cases where there is respiratory
involvement.
• Based on the absorption property of infrared rays by certain
gases
• When infrared rays are passed through the expired air which contains certain
amount of CO2, some of radiations are absorbed by it.
• There is loss of heat energy associated with the rays .
• The detector changes the loss in heating effect of the rays into an electrical signal.
• It is used to get the average respiration rate.

• Two infrared sources are available in this set up.
• The beam from one infrared source falls on the test cuvette side.
• The beam from another infrared source falls on the reference cuvette
side.
• The detector has two identical portions.
• These portions are separated by a thin, flexible metal diaphragm.
• Because of the absorption of CO2 in the test cuvette, beam falling on
the test side of the detector is weaker that falling on the reference side.
• The gas in reference side is heated more than that on the test side. So
diaphragm is pushed slightly to the test side of the detector.
• This diaphragm forms one plate of a capacitor.
• The a.c signal appears across the detector is amplified and recorded
using recorder.
• The amplified output is integrated for continuous monitoring of the
respiration rate.

Apnoea Detectors
• Apnoea is the cessation of breathing
which may precede the arrest of the
heart and circulation
• Causes may be head injury, drug
overdose, anaesthetic complications and
obstructive respiratory diseases.
• If apnoea persists for a prolonged period,
brain function can be severely damaged.
• Apnoeic patients require close and
constant observation of their respiratory
activity.
• Apnoea monitor is used to watch the
apnoea patients respiration rate.

• Apnoea monitor gives audio signals and visual signals, when no inspiration occurs for a particular period of
time.
• Input from the sensor is connected with the amplifier circuit having high input impedance.
• The output of the amplifier circuit is connected with motion and respiration channel blocks.
• Motion channel block differentiates motion and the
respiration based on the frequency.
• The frequency below 1.5 Hz is identified as respiration.
• The frequency above 1.5 Hz is identified as motion.
• High frequency signal above the threshold is sensed by
positive detector.
• The frequency below the threshold is sensed by
negative detector.

• The output of the motion channel is connected with comparator circuit.
• It compares the amplitude of motion and respiration.
• Based on the output corresponding lamp will glow.
• In the respiration channel, low pass filter is used to remove high frequency signal.
• If there is no respiration, then schmitt trigger circuit gives
the output to switch on the alarm.
• Apnoea period selector circuits contain low frequency
alarm oscillator and tone oscillator, and audio amplifier.
• Apnoea period selector circuit drives the alarm circuit.
• The output of alarm circuit is connected with the speaker.
• So, where there is no respiration for a period of 10 or 20 sec,
then audio signal through the speaker and visual signal
through the flash light is delivered.

Heart Rate Measurement
• For most adults, a normal resting heart rate—the
number of heartbeats per minute while at rest—
ranges from 60 to 100 beats per minute.
• A normal heart rate can vary from person to
person.
• A heart rate that is slower than 60 beats per
minute is considered bradycardia ("slow heart")
• A heart rate that is faster than 100 beats per
minutes is termed tachycardia ("fast heart").
• A healthy heart rate will vary depending on the
situation.
• Taking a pulse can also indicate:
– Heart rhythm
– Strength of the pulse
– An unusually high or low resting heart rate can be a
sign of trouble.

• Heart rate is derived by the amplification of the ECG signal and by
measuring either the average or instantaneous time intervals
between two successive R peaks.
• Techniques used to calculate heart rate:
– Average calculation
• oldest and most popular technique
• Average rate (beats/ min) is calculated by counting the number of pulses in a given
time.
• does not show changes in the time between beats
• does not represent the true picture of the heart’s response to exercise, stress and
environment.
– Beat-to-beat calculation
• done by measuring the time (T), in seconds, between two consecutive pulses, and
converting this time into beats/min ( 60/T)
• accurately represents the true picture of the heart rate
– Combination of beat-to-beat calculation with averaging
• based on a four or six beats average.
• Similar to the beat-to-beat monitoring system

Instruments for measuring heart rate
• Average Heart Rate Meters
– a part of the patient monitoring systems
– usually of the average reading type
– work on the basis of converting each R wave
of the ECG into a pulse of fixed amplitude and
duration and then determining the average
current from these pulses.
– incorporate specially designed frequency to a
voltage converter circuit to display the average
heart rate in terms of beats per minute.

Instantaneous Heart Rate Meters
• facilitates detection of arrhythmias and permits the timely observation
of incipient cardiac emergencies
• Calculation of heart rate from a patient’s ECG is based upon the reliable
detection of the QRS complex
• but instruments are sensitive to the muscle noise (artefact) generated
by patient movement.
• This noise often causes a false high rate that may exceed the high rate
alarm.
• A method to reduce false alarm is by using a QRS matched filter.
• This filter is a fifteen sample finite impulse-response-filter whose
impulse response shape approximates the shape of a normal QRS
complex.
• The filter, therefore, would have maximum absolute output when
similarly shaped waveforms are input.
• The output from other parts of the ECG waveform, like a T wave, will
produce reduced output.

Block Diagram
• The ECG is sampled every 2 ms.
• Fast transition and high amplitude components are attenuated by a slew rate
limiter
• Two adjacent 2 ms samples are averaged and the result is a train of 4 ms samples.
• In order to remove unnecessary high frequency components of the signal, a 30 Hz,
infinite-impulse-response
butterworth filter is employed.
• This produces 8 ms samples
in the process.
• Any dc offset with the signal
is removed by a 1.25 Hz high
-pass filter

• The clamped and filtered ECG waveform is finally passed
through a QRS matched filter.
• The beat detector recognizes QRS complexes in the processed
ECG waveform value that has occurred since the last heart
beat.
• If this value exceeds a threshold value, a heart beat is counted.
• The beat interval averaged over several beats is used to
calculate the heart rate for display, alarm limit comparison,
trending and recorder annotation.
• The threshold in this arrangement gets automatically adjusted
depending upon the value of the QRS wave amplitude and the
interval between the QRS complexes.
• Following each beat, an inhibitory period of 200 ms is
introduced during which no heart beat is detected.
• This reduces the possibility of the T wave from getting counted

OXIMETRY
• Oximetry refers to the determination of the percentage of oxygen saturation of the circulating arterial
blood.
• Oxygen saturation =
[𝐻𝑏𝑂2]
𝐻𝑏𝑂2 +[𝐻𝑏]
where
[HbO2] is the concentration of oxygenated haemoglobin
[Hb] is the concentration of deoxygenated haemoglobin
• Percentage of oxygen saturation in the blood is of great importance as it is a bio-constant.
• Main application areas of oximetry are the diagnosis of cardiac and vascular anomalies; the treatment of
post-operative anoxia and the treatment of anoxia resulting from pulmonary infections.
• A major concern during anaesthesia is the prevention of tissue hypoxia, necessitating immediate and
direct information about the level of tissue oxygenation.
• Oximetry is now considered a standard of care in anaesthesiology and has significantly reduced
anaesthesia-related cardiac deaths

• The plasma (liquid part of the blood) is a very poor carrier of oxygen.
• At the pressures available, only 0.3 ml of oxygen can dissolve in 100 ml of plasma, which is
quite insufficient for the needs of the body.
• The red blood cells contain haemoglobin which can combine with a large volume of oxygen
very quickly
• By the time the red blood cells reach the lungs it may become 97% saturated forming a
compound called oxyhaemoglobin.
• The actual amount of oxygen with which the haemoglobin combines depends upon the
partial pressure of oxygen.
• Thus, under normal conditions, during each cycle through the tissues, about 5 ml of oxygen
is consumed by the tissues from each 100 ml of blood which passes through the tissue
capillaries.
• Then, when the blood returns to the lungs, about 5 ml of oxygen diffuses from the alveoli
into each 100 ml of blood and the haemoglobin is again 97% saturated.

PULSE OXIMETER
• Noninvasive method for monitoring a
person’s oxygen saturation.
• Peripheral oxygen saturation (SpO2)
readings are typically within 2% accuracy
(within 4% accuracy in the worst 5% of cases)
• More desirable than invasive reading of arterial oxygen
saturation (SaO2) from arterial blood gas analysis.

• Pulse oximetry is based on the concept that arterial oxygen saturation determinations can be made
using two wavelengths, provided the measurements are made on the pulsatile part of the
waveform.
• The two wavelengths assume that only two absorbers are present; namely oxyhaemoglobin (HbO2)
and reduced haemoglobin (Hb).
• These observations are based on the following:
– Light passing through the ear or finger will be absorbed by skin pigments, tissue, cartilage,
bone, arterial blood, venous blood.
– The absorbances are additive and obey the Beer-Lambert law: A = –log T = log lo/I = ε D C
• where Io and I are incident and transmitted light intensities,
• ε is the extinction coefficient,
• D is the depth of the absorbing layer
• C is concentration.
– Most of the absorbances are fixed and do not change with time.
– Even blood in the capillaries and veins under steady state metabolic circumstances is constant
in composition and flow, at least over short periods of time.
– Only the blood flow in the arteries and arterioles is pulsatile.
– Therefore, only measuring the changing signal, measures only the absorbance due to arterial
blood and makes possible the determination of arterial oxygen saturation (SaO2)
– This is uninfluenced by all the other absorbers which are simply part of the constant
background signal.

Construction of a Typical Pulse
Oximeter Probe
• Construction of a typical pulse oximeter probe.
• This has two LEDs (light emitting diodes), one that transmits
infrared light at a wavelength of approximately 940 nm and
the other transmitting light at approximately 660 nm.
• The absorption of these select wavelengths of light through
living tissues is significantly different for oxygenated
haemoglobin (HbO2) and reduced haemoglobin (Hb).
• The absorption of these selected wavelengths of light passing
through living tissue is measured with a photosensor
• The red and infrared LEDs within the probe are driven in
different ways, depending on the manufacturer.
• Most probes have a single photodetector (PIN-diode), so the
light sources are generally sequenced on and off.

• The output of the photodiode has a raw signal
• There will be one signal that represents the
absorption of red light (660 nm) and one that
represents infrared light (940 nm).
• The ac signal is due to the pulsing of arterial
blood while the dc signal is due to all the non-
pulsing absorbers in the tissue.
• Oxygen saturation is estimated from the ratio (R)
of pulse-added red absorbance at 660 nm to the
pulse-added infrared absorbances at 940 nm.
• 𝑅 =
𝑎𝑐 660 𝑑𝑐 660
𝑎𝑐940 𝑑𝑐940

• the analog signal processing technique used in pulse oximeters.
• To simplify the diagram, the circuitry required to drive the LEDs in the sensor are omitted, and only the
analog signal processing blocks between the sensor and the digital processing circuitry are shown.
• The signal from the sensor is a current. The first amplifier stage is a current to voltage converter.
• The voltage signal then goes through the amplifiers to further amplify the signal; noise filters to remove
different kinds of interference, a demultiplexer to separate the interleaved red and infrared signals;
bandpass filters to separate the low frequency (dc) component from the pulsatile, higher frequency (ac)
component; and an analog–digital converter to convert the continuously varying signal to a digital
representation.

References
• https://www.ifsc.usp.br/~reynaldo/eletronica_instru
mentacao/axon-guide/GuideCh07.pdf - thermistors
• https://www.hopkinsmedicine.org/health/conditions
-and-diseases/vital-signs-body-temperature-pulse-
rate-respiration-rate-blood-pressure
• https://www.brainkart.com/article/Respiratory-Rate-
Measurement_11866/

Medical Instrumentation- body parameters measurement

More Related Content

Similar to Medical Instrumentation- body parameters measurement (20)

More from Poornima D (19)

Recently uploaded (20)

Medical Instrumentation- body parameters measurement