ECG Instrumentation Flashcards by Faith Taws

What is an electrocardiograph?

instrument for recording the electrical activity of the heart

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What is the role of an electrocardiogram?

produces tracing of the electrical activity of the heart

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what are the units of current?

Amps (I)

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what are the units of potential difference?

volts (V)

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What are the unit of resisitance?

ohms

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What is a biopotential system?

system that measues something biological

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What are the 3 core componenets that make up the biopotential system?

-tranducer
-processor
-display

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What is the role of a transducer?

converts one form of energy to another

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What is the role of a producer?

proccesses the signal
-amplify

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whats the role of a display?

to show the digitilised results of a test eg- ecg, temperature

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Whats the aim of a biopotential signal?

to get a signal from input to output without changing it

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What are the measurements of ECGs?

-amplitude (voltage)
-Time

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What is an ECG in regards to measurements?

a voltage time graph

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Name some equiptment specific requirements

-input impedance
-common mode
-rejection ratio
-linearity
-time constant
-patient isolation

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Name some operator adjustable features of an ECG

-sensitivity
-paper speed
-frequency response

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What is input impedance?

a measure of the impedance to current flow offered by an electrocardiograph

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What is ohms law?

V=IR

the voltage across a conductor is directly proportional to the current flowing through it

What is the equation for total resistance?

R total =R1 +R2 +R3 (series)

What does AHA state about input impendence?

input impendance should be greater than 0.5 M ohms

Describe the common mode rejection ratio

A measure of how well the machine is detecting noise

-electrodes pick up noise including common mode voltage

-the ability of the ECG amplifier to reduce common mode voltage noise

What does the AHA say the CMRR should be ?

1000:1
between 45-65 Hz when imbalanced at 5K ohms

What does AHA state about linearity?

maximum error must be less than 5% of the chart width

What happens if the linearity is not met?

-ECG waveforms will be distorted
-ECG should be as linear as possible
-Cant change it

What is the time constant?

the ability of the ECG system to process low frequency or slowly changing signals

-Time taken for the amplitude of a step change voltage to decay to 36.8% of its initial value

What does AHA state about time constant?

must be greater than 3.2 seconds

What happens if the time constant is incorrect?

-low frequency parts of an ECG waveform will be distorted -time constant should be long enough to display an undistorted T wave

Define patient isolation

the maximum amount of current that the system can pass to the patient

What does AHA state that patient isolation should be?

less than 10 microamps

What are the effects of patient isolation if its incorrect?

depends on the frequency and current but can vary from mild sesation to burns, ventricular fibrilation

What are some dangerous patient isolation values?

0.9-1.2mA – current perceptible 15-20mA – muscle contraction (unable to release grip) 50-100mA – ventricular fibrillation (death) 100-200mA – severe burns, constriction of muscle groups

How do you check the patient isolation is correct?

Check the Electrocardiograph has had an electrical safety test. Visually inspect the system before use.

What is sensitivity?

The sensitivity of the ECG may be altered to increase or decrease the amplitude of the ECG on the trace Common choices are 5 mm/mV 10 mm/mV 20 mm/mV

What does AHA state sensitivity should be?

1mV must equal 10mm on the paper

What are the effects if sensitivity is not met?

The ECG waveform amplitude will be reduced or increased in size. Possible misdiagnosis.

What does sensitivity effect?

waveform size

What should I do to check the sensitivity of the ECG?

Use a sensitivity of 10mm/mV Check machine setting before you start Use alternative in exceptional circumstances Clearly annotate ECG trace Additional ECG on 10mm/mV

Whats the effect if paper speed is not met?

The ECG waveform will be reduced or increase in duration Heart rate will appear higher or lower

How should I check ECG paper speed?

Paper speed affects the timing of the waveform Use 25 mm per second Check the setting before you start Use an alternative in exceptional circumstances Annotate trace Use 25mm/s as well (record both)

What is frequency response?

the range of frequencies over which the ECG works

What does AHA state about frequency response?

equal to 0.05 to 150 Hz

What happens if frequency response is not met?

waveform is distorted

What should I do to check frequency response?

switch the mains (50 Hz) filter on ONLY once all noise sources have been excluded Clearly annotate ‘Filter on’ -effects waveform shape