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BIOMEDICAL RECORDERS

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TYPES

•Electrocardiography(ECG)

• Vectorcardiography(VCG)

• Phonocardiography(PCG)

•

Electroencephalograph(EEG)• Electromyograph(EMG)

• Apex cardiography

• Ballistocardiograph(BCG)

• Electro-oculograph(EOG)

• Electroretinograph(ERG)

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Electrocardiography(ECG)

• It records the electrical avtivity of heart.

• It provides valuable information about widerange of cardiac disordersas presence of 

inactive ( infarction) or enlargement (cardiachyperophy)

• Electric field generated by heart can be

characteristed by voltage difference inmV.frequency range of 0.05 to 150 hz.

CMMR=100-120db

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Block diagram of ECG machine

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• Potiential picked up by the patient electrodes aretaken to the leads sector switch.

• The signal is connected symmetrically to the longtail pair differential amplifiers

• differential amplifiers has negative feedback.

•

The amplified o/p is given to power amplifiers• Feedback is obtained from selective feedback

network.

• The o/p is single ended and feed to pen motor

which deflects the writing arm on the paper.• Instead of preamplifier isolated preamplifier is

used.

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Isolated preamplifier block diagram

• An isolated preamplifier used in modernelectrocardiographs.

• Difference signal obtained from the right

arm(RA),left arm(LA),and right leg(RL) is givento a low pass filter .(cut off frequency 10khz)

• Filter ckt is followed by high voltage and over

voltage protection circuits so that amplifiercan withstand large voltage duringdefibrillation.

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• Lead selector switch is used to derive the

required lead configuratins and give it to a dccoupled amplifier (1mv).

• Isolation of patient ckt is obtained using a low

capacitance transformer whose primarywinding is driven from 100khz oscillator.

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Improvement in CMRR

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• The presence of stray capacitance at the inputof the preamplifier causes Commonmode

currents to flow in LA,RA resulting in a voltagedrop at the electrodes.

• To balance the stray capacitance the

potentials A,B,C are equalised through an in-phase component of the common modevoltage ,which the amplifier delivers via C1,c2to LA and RA.

• As a result the potentials at A,B,C are keptequal independent of the imbalance inelectrode resisters and stray capacitance.

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ECG Leads- bipolar limb leads

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• In all lead connections ,he difference of 

potential measured between two electrodes is

always with reference to a third point on the b

• Third point is conventionally taken as RL.

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Unipolar limb leads

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Unipolar chest leads

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• The ECG measured from any of the three basic limbleads is a time-varient single dimentional component

of the vector.• Electric field of the heart could be represented as

triangle called Einthoven triangle .

• The sides of triangle represent the lines along which

the three projections of ECG vector measured.

• The instantaneous voltage measured from any one

of the three limb lead position is approximately 

equal to the algebric sum of the other twoor that 

the vector sum of the projections on all three lines isequal to zero.

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Einthoven triangle 

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Types of unipolar leads• Limb leads :

• Two of the limb leads are tied together and recordedwith respect to third limb.

• In the lead identified AVR the right arm is recorded withrespect to reference by joining the left arm and left legelectrodes.

• In the AVL lead, the left arm is recorded w.r.t to common junction of the right arm and left leg.

• In AVF lead, the left leg is recorded w.r.t two armelectrodes tied together. Also called Augumented leads

or averaging leads.• The resistance inserted between electrode –machine

connections are called averaging resistance.

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Effects of Artefacts on ECG Recording

• Interference from power 

line :

• Interference may due to

stray effect of thealternating current on the

patient

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Effects of Artefacts on ECG Recording

• Muscle Tremor :

• Irregular trembling of 

the ECG trace.

• The patient must beadvised to get warm

and to relax to avoid

tension.

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• Shifting of the base line

• Movement of patient or

electrodes

•

It is observedimmediately after

application of 

electrodes.