Biomedical Instrumentation B18/BME2

Biomedical

Instrumentation Revision Session

B18/BME2

Dr Gari Clifford

Biomedical Instrumentation B18/BME2

The Electrocardiogram

If two surface electrodes are attached to

the upper body (thorax), the following

electrical signal will be observed:

This is the electrocardiogram or ECG

Biomedical Instrumentation B18/BME2

1st Problem: Electric Field Interference

Capacitance between power lines and

system couples current into the patient

This capacitance varies but it is of the order

of 50pF (this corresponds to 64MΩ at 50Hz

... recall Xc=1/C )

If the right leg is connected to the common

ground of the amplifier with a contact

impedance of 5kΩ, the mains potential will

appear as a ~20mV noise input.

RA

LL RL

LA

Electrical power system

50 pF

5kΩ the 50 Hz interference is common to

both measuring electrodes!

(common mode signals)

Biomedical Instrumentation B18/BME2

The solution The ECG is measured as a differential signal.

The 50Hz noise, however, is common to all the

electrodes.

It appears equally at the Right Arm and Left Arm

terminals.

Rejection therefore depends on the use of a

differential amplifier in the input stage of the

ECG machine.

The amount of rejection depends on the ability

of the amplifier to reject common-mode voltages.

Biomedical Instrumentation B18/BME2

Common Mode Rejection Ratio

(CMRR)

CMRR = Ad / Acm

vin= vcm+ vd vout= Acmvcm + Advd Ad & Acm

(ratio of differential gain to common mode gain)

Often expressed in power: CMMR = 12log10(Ad/Acm)2

Biomedical Instrumentation B18/BME2

Three Op-Amp Differential Amplifier

Biomedical Instrumentation B18/BME2

Three Op-Amp Differential Amplifier

Ad1 =

)2

1)((

)1(

)1(

1

212

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1

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2

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1

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2

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1

221

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.

Ad1 =

Biomedical Instrumentation B18/BME2

Ad1 =

)2

1)((

)1(

)1(

1

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1

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2

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When v1 = v2 = vcm, Acm = 1

Three Op-Amp Differential Amplifier

Biomedical Instrumentation B18/BME2

Ad1 =

)2

1)((

)1(

)1(

1

212

'

1

'

2

1

1

22

1

2'

2

2

1

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cmcm

dd

A.A

A.ACMRR =

Three Op-Amp Differential Amplifier

CMRR is product of CMRR

for each input amplifier

Biomedical Instrumentation B18/BME2

2nd problem: Magnetic Induction

Current in magnetic fields

induces voltage in the loop

formed by patient leads

RA

LL RL

LA

The solution is to minimise

the coil area (e.g. by twisting

the lead wires together)

The noise is now common to

both inputs and is cancelled

at differential amplifier

Twisted pair cabling

minimises cross talk in

communication lines

Biomedical Instrumentation B18/BME2

3rd problem:

Source impedance unbalance

If the contact impedances are not balanced (i.e. the

same), then the body’s common-mode voltage will be

higher at one input to the amplifier than the other.

Biomedical Instrumentation B18/BME2

3rd problem:

Source impedance unbalance

If the contact impedances are not balanced (i.e. the

same), then the body’s common-mode voltage will be

higher at one input to the amplifier than the other.

Hence, a fraction of the common-mode voltage will be

seen as a differential signal.

Biomedical Instrumentation B18/BME2

Summary

Output from the differential amplifier consists of

three components:

The desired output (ECG)

Unwanted common-mode signal because the

common-mode rejection is not infinite

Unwanted component of common-mode signal

(appearing as pseudo-differential signal at the input)

due to contact impedance imbalance

Biomedical Instrumentation B18/BME2

ECG - BP – PPG - RESP

Biomedical Instrumentation B18/BME2

Blood pressure measurements

Blood pressure is generally recorded using two

measurements (in mmHg):

Systolic Pressure

Diastolic Pressure

Blood pressure is usually reported as "Systolic over

Diastolic“; e.g. 120/70 is a systolic pressure of 120

mmHg and a diastolic pressure of 70 mmHg.

Biomedical Instrumentation B18/BME2

Systolic blood pressure

Systolic blood pressure (SBP) is the arterial

pressure when the heart is beating (during systole).

It is, broadly speaking, the highest pressure present

in the arterial (and vascular) system.

It is a reflection of how hard the heart is pumping.

Biomedical Instrumentation B18/BME2

Diastolic pressure

Diastolic blood pressure (DBP) is the arterial pressure

when the heart is not beating (during diastole).

It is, broadly speaking, the lowest pressure present in the

arterial (but not vascular) system.

It is a reflection of the resistance against which the heart

is pumping.

Biomedical Instrumentation B18/BME2

Oscillometry

Marey (1885) noted that the pressure in a vessel

containing an arm fluctuated with the beating of

the heart.

The magnitude of these pressure fluctuations

varied with the applied pressure

Modern version involved an inflatable cuff

around the arm

Biomedical Instrumentation B18/BME2

Pressure fluctuations

First assumption:

The start of the pressure

fluctuations occurs at

systolic pressure.

The end of the pressure

fluctuations occurs at

diastolic pressure.

It turns out, however,

that this is not the case.

Biomedical Instrumentation B18/BME2

Pressure fluctuations

Empirical studies have

shown that:

The onset of the

oscillations occurs well

above systolic pressure.

The oscillations do not

disappear until well

below diastolic pressure.

Biomedical Instrumentation B18/BME2

Pressure fluctuations

The pressure at which the oscillations have their maximum

amplitude is the Mean Arterial Pressure (MAP).

Biomedical Instrumentation B18/BME2

Empirical determination of

systolic and diastolic pressures

time

Oscillation

amplitude

time

Maximum oscillation

Systolic pressure

diastolic pressure

Systolic pressure = cuff pressure when the oscillation amplitude is 55% of the maximum amplitude Diastolic pressure = cuff pressure when the oscillation amplitude is 85% of the maximum amplitude

Biomedical Instrumentation B18/BME2

Oscillometry – system overview

Biomedical Instrumentation B18/BME2

NIBP Device specs

Dynamic range = 0-250 mmHg

Maximum output from pressure sensor = 50 mV

Differential amplifier typical gain = 20 ...

Therefore max output of differential amp = 1V

Quantization: 0.2 mV per mmHg

LPF with cut-off of 0.05Hz

DC supply 5-9V

Max output for amplifier & LPF ~8V.

DC gain = 80-100

Biomedical Instrumentation B18/BME2

Oscillometry – system overview

The signal from the

pressure transducer

has two components:

the underlying pressure

of the cuff

the cardiac-synchronous

oscillations

Biomedical Instrumentation B18/BME2

Pressure measurement system

The pressure measurement system

consists of the following:

A pressure transducer to sense the cuff

pressure (including the cardiac-synchronous

oscillations)

Amplification and filtering

Analogue-to-digital conversion

Biomedical Instrumentation B18/BME2

Amplification and filtering

Instrumentation amplifier

(as with ECG or EEG

measurement) is used to

amplify the differential

pressure transducer output.

Amplifier output is low-pass

filtered to derive the cuff

pressure signal and high-pass

filtered to extract the cardiac-

synchronous fluctuations.

Biomedical Instrumentation B18/BME2

Analogue-to-digital conversion

Sampling frequency:

Accurate peak detection requires 10 samples per cycle of

cardiac-synchronous fluctuations → choose a sampling

frequency of 50 Hz or above for that channel.

Amplitude resolution:

8-bit accuracy should be sufficient for A-D conversion:

Low-pass filtered cuff pressure signal needs to be resolved to

1 or 2 mmHg in a range of 0 to 300 mmHg

High-pass filtered cardiac-synchronous fluctuations will be

digitised with 0.5% accuracy.

Biomedical Instrumentation B18/BME2

Signal processing

Software will perform the following functions:

Initiating the measurement cycle and driving the cuff controller (or

telling the user to pump’)

Reading in the digitised data

Recording the amplitude of the cardiac-synchronous fluctuations at

the different cuff pressures

Computing mean, systolic and diastolic pressures

Computing pulse rate, if required

(It is of course possible for the low-pass and high-pass

filtering of the overall cuff pressure signal also to be

performed in software after A-D conversion of the amplifier

output.)

Biomedical Instrumentation B18/BME2

Revision / Exam Tips

Read past papers – practice answering them in

the allotted time

Recap the connection between signals – ECG,

BP, PPG, resp

Revise simple electronics – Ohm’s law &

Kirchoff’s first law!!!

Just trace around the circuit and remember the

current is conserved at any junction

Biomedical Instrumentation B18/BME2

ECG - BP – PPG - RESP

Biomedical Instrumentation B18/BME2