ECG Instrumentation Flashcards
What is an electrocardiograph?
instrument for recording the electrical activity of the heart
What is the role of an electrocardiogram?
produces tracing of the electrical activity of the heart
what are the units of current?
Amps (I)
what are the units of potential difference?
volts (V)
What are the unit of resisitance?
ohms
What is a biopotential system?
system that measues something biological
What are the 3 core componenets that make up the biopotential system?
-tranducer
-processor
-display
What is the role of a transducer?
converts one form of energy to another
What is the role of a producer?
proccesses the signal
-amplify
whats the role of a display?
to show the digitilised results of a test eg- ecg, temperature
Whats the aim of a biopotential signal?
to get a signal from input to output without changing it
What are the measurements of ECGs?
-amplitude (voltage)
-Time
What is an ECG in regards to measurements?
a voltage time graph
Name some equiptment specific requirements
-input impedance
-common mode
-rejection ratio
-linearity
-time constant
-patient isolation
Name some operator adjustable features of an ECG
-sensitivity
-paper speed
-frequency response
What is input impedance?
a measure of the impedance to current flow offered by an electrocardiograph
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
