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Department of Electrical and Computer Engineering UTEPEE 4385 Biomedical Instrumentation 1

BI1 Lecture Series Prepared by Homer Nazeran PhD

Please read carefully and check for typos!

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Biopotential Amplifiers

These are very important part of modern medicalinstrumentation

We need to amplify biopotentials which are generated in the

body at low levels with a high source impedance

Biopotentials amplifiers are required to increase signal strengthwhile maintaining fidelity

Basic Requirements of Biopotential AmplifiersEssential functions of a bioamplifier are:

To take a weak biopotential and increase its amplitude so that it

can be processed, recorded or displayed

To amplify voltage, but it could be considered as a poweramplifier as well

To amplify current since in some cases a biopotential amplifier

is used to isolate the load from the sourcecurrent gain only

Input Impedance (Zin)

All biopotential amplifiers must have high input impedance

minimize loading (remember the characteristics of biopotential

electrodes resulting into loading and distortion if input

impedance of the amplifier is not high enough) typical values

of Zin over the frequency range of the measurand = 10 M(remember the loading rule)

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Bandwidth (BW)

Frequency responserequirements

The biopotential amplifier must be sensitive to important

frequency components of the biosignal

Since biopotentials are low level signals, it is important to limitbandwidthoptimize signal-to-noise ratio

Gain (G)

Biopotential amplifiers have a gain of 1000 or greater

Mode of Operation

Very frequently biosignals are obtained from bipolar electrodes

Electrodes symmetrically located with respect to groundneed

differential amplification

High CMRRrequired because:

1.

Common mode signals much greater than the biosignal appear

on bipolar electrodes

2.

Symmetry with respect to ground is not perfect (mismatch

between electrode impedances) more on this later

Calibration Signal

Medical and clinical equipment require quick calibration

The gain of the biopotential amplifier must be calibrated to

provide us with an accurate indication of the signals amplitude

Push button to apply standard signal to the input of the

biopotential amplifier

Adjustable gain switch carefully selects calibrated fixed gains

(in microprocessorbased systems, gain adjustment can be

achieved by software setting)

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Electrocardiography

A very widely used medical instrument, which is utilized todiagnose and monitor cardiac beat abnormalities is the

electrocardiograph

It measures the electrical activity of the heart (more preciselybiopotential differences arising from the electrical activity ofmyocardium). Weve already talked about the genesis of the

ECG signal.

The ECG machine uses surface electrodes and high input

impedance

Differential amplifiers with good common mode rejection ratio

to record the electrocardiogram

Normal ECG amplitude ranges between 0.5-4 mV. Normalfrequency content of ECG (for diagnostic purposes) is 0.05-100Hz. A typical ECG waveform is shown below:

Obviously all human hearts are not the same and this leads into

variability in different parts of the ECG signalSignificant diagnostic features of the ECG signal are:

Duration of component parts of the signal

Polarities and magnitudes

The details of the ECG signal and the degree of variability in differentparts of the ECG signal is shown below:

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Some ECG waveform abnormalities that may indicate illness are:

An extended PR interval may be diagnosed as AV node block

A widening of the QRS complex may indicate conduction

problems in the bundle of His

An elevated ST segment may indicate occurrence of myocardial

Infarction (MI)

A negative polarity in the T wave may be due to coronaryinsufficiency

ECG Leads

Normal ECG recordings for the standard lead connections

leads I, II and III(Lead IIprovides the strongest signal)

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Obviously, all human hearts are not the same and this resultsinto a high degree of variability

Note the degree of variability of different parts of the ECG

signal

Some abnormalities that may indicate illness:

An extended P-R interval may be diagnosed as AV node

block

Widening of the QRS complex conduction problems in the

bundle of His

Elevated ST segment may indicate occurrence of MI

Negative polarity T wave may be due to coronary insufficiency

QRS amplitude, polarity, time domain, PR interval (indicator of

heat beat per min. & T-wave amplitude are some very important

distinctive features.

1.

Loss of P-wave (un-coordinated twitching)

2.

Atria undergoing very rapid regular flapping movement

3.

An irritable focus discharge in the ventricle (Extrasystole)

4.Loss of QRS blood circulation, severely impaired lots of ectopic

foci

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Origin of the ECG signal

We have already covered this concept extensively in theprevious lectures (The Dipole filedof the heart, the Eindhovens

Triangle , the electrical circuit modelfor the electrocardiographic

problem, etc.)

Standard Limb Leads (I, II, III)

The lead wires are color-coded according to some conventions.One example is: White RA(Right Arm), Black LA (Left Arm),

Green RL (Right Leg), Red LL (Left Leg), and Brown C

(Chest)Note:There is a CM(common mode) amplifier connected to the

right leg. We will discuss this in detail later.

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Augmented Limb LeadsThese leads offer a free 50% increase over leads VR, VL, and VF

connections (unipolar leads) with respect to Wilson terminal

AVR= -I III/2, AVL= I II/2, aVF= II I/2

Note:Each measurement is made from the reflected limb and the

average of the other two limbs.

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Chest Leads (Precordial)

Chest Lead Anatomical Positions

V1- 4th

intercostal space Right sternal margin

V2- 4th

intercostal space Left sternal margin

V3 Midway between V2& V4V4 5

thintercostal space on mid-clavicular line

V5 Same as V4, on the anterior axillary line

V6 Same as V5, on the mid-axillary line

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12 Lead Clinical Electrocardiography

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The ECG MachineMost representative Specs:

Zin= 10 M

Frequency response = 0.05 100 Hz

Strip Chart Recorder Speed = 25 mm/sec.

Fast Speed = 100 mm/sec.

For detailed Specs. Refer to the Table in your text Summary of

performance requirements for electrocardiographs

Block Diagram of a modern electrocardiograph

SensingElectrodes

Lead-failDetector

Amplifier

ProtectionCircuit

ALead

Selector

DrivenRight-legCircuit

Right legElectrode

IsolationCircuit

Driver

Amplifier

Recorder

Printer

Auto-

Calibration

Baseline

Restoration

Isolated

PowerSu l

ADC

Memory

Microcomputer

Control

Program

ECG AnalysisProgram

Operator

Display

Keyboard

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Problems frequently encountered in electrocardiography

There are many factors that must be considered in the design &application of the ECG machine

These considerations should address the common problems inelectrocardiography

The more common problems are:

Frequency distortion

Saturation or cut-off distortion

Ground loops

Open lead wires

Artifact from large electric transients

Interference from electric devices

Other sources of electric interferenceLets go through these problems carefully and focus on some of

their causes.

Frequency distortion

If the ECG machine does not meet the frequency response

requirements for amplification of the ECG signal, frequencydistortion occurs

a. True ECG

b. High frequency distortion due to inadequate gain of ECG amplifier at highfrequencies

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c. Low frequency distortion due to inadequate gain of ECG amplifier at lowfrequencies

Note:

In high frequency distortion, the corners are rounded off and the

amplitude of the QRS is diminished In low frequency distortion, the baseline gets distorted andmonophasic waves appear to be more biphasic

Saturation or cutoff distortion

This type of distortion occurs if there are high offset voltages at

the electrodes or the amplifiers in the ECG machine are improperly

adjusted.

a. Undistorted ECG recordingb. Clipping of peak of the ECG due to positive saturation effects in

amplifier

c. Clipping of lower voltages in the ECG due to negative saturation

or cutoff effects in amplifier

Note:

In saturation distortion, the combination of the ECG signal and

offset voltages drives the amplifier into saturation. As a result theQRS peaks are cutoff because the output of the amplifier can not

exceed the saturation voltage

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In cutoff distortion, the lower portions of the ECG are cutoff(clipped).

Ground loops

Ground loops can arise when patients having their ECGs takenon either a clinical ECG machine or on a cardiac monitor are alsoconnected to other pieces of apparatus

Each electric device has its own ground connection either

through the power line or in some cases through a heavy groundwire attached to some ground point in the room

Ground loop currents may present a safetyproblem

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They elevate the patients body potential to some voltageabove the lowest ground to which the instrumentation is attached

(ground A in the figure)

This potential produces common mode voltages that mayincrease the amount of interference seen if the CMRR of thedevice is not high enough

In electrocardiographs the ground leads usually run alongside

the ECG signal leads, magnetic fields caused by the groundingcircuit can induce small voltages in the signal lead wires

interferenceon the tracing

Ground loops represent closed current paths. They subtend some

geometrical areas within the loop. If this area is large and is in a

strong time varying magnetic filed, a current can be induced in the

ground loop, the current flows through the patientand commonmode voltages are generated

Open lead wires

A frequently occurring problem in electrocardiography is one ofthe lead wires becoming disconnected as a result of rough handling

This results in relatively high potentials being induced on the

open lead wire due to electric fields emanating from the power

lines or other electrical devices in the vicinity of the ECG machine

This in turn leads to a big constant-amplitude deflection of the

ECG pen at 50 or (60) Hz. Such an electrode is not making goodcontact with the patient. Therefore, this phenomena translates into

having a circuit for detecting poor electrode contact

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Artifacts from large electric transients

These transients occur during the following situations:

Defibrillating the patient with a high-current high-voltageelectric pulse called the defibrillating pulse (more on this in BI)

Motion artifacts

The patient encountering a build-up of electric charges that canbe partially discharged through the body

Switching leads manually in older ECG machines (not aproblem in new machines)

Note: These effects can be alleviated by reducing the source of the

artifact.

Include an electronic protection circuitry to limit max. Input

voltage across ECG amp. To minimize saturation duringdefibrillation

Reduce the build up of charge on personnel by having them use

conductive clothing, shoes, etc and touch the bed frame beforetouching the patient

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