ECG instrumentation Flashcards
What is an electrocardiograph?
An machine that measures the electrical activity of the heart
What is an electrocardiogram?
The trace produced by an electrocardiograph
Name the components of a biopotential system and explain their role
Transducer, converts one form of energy into another
Processor- processes the signal, digitalizes or amplifies it
Display- the output, e.g., chart recorder, aim it to get the input and output signal to be as similar as possible
What is a biopotential system?
Biopotentials are electrical signals produced by the cells in our bodies, creating ionic flows measured in a system
X axis & y Axis on ECG
Y axis is voltage (mV)
X axis is time (m/s)
If the pen moves up on an ECG then…..
The current is moving towards the electrode & vice versa if the pen moves down
State the equipment specifications
- input impedance
- CMRR
- Time constant
- Patient isolation
- Linearity
These are all unable to be changed, set during manufacturing, influence can occur during patient isolation
Name operator adjustable components of ECG machine
- Sensitivity
- Paper speed
- Frequency
State Ohms law
The law states that the current is proportional to the voltage across it and is inversely proportional to the resistance
V=IR
AHA states that Input impedance
should be greater than 0.5 ohms
If not, ECG will show reduced amplitude, input impedance should be high, but cannot be changed
What is CMRR
Stands for common mode rejection ratio
Electrodes pick up background noise, it is the ability of the amplifier to reduce this common mode noise
AHA states that CMRR
The ratio should be 1000:1, between 45-65 Hz, when imbalanced ECG will be noisy, affecting interpretation of results
Define linearity
If points on a graph occur in a straight line, it is said to be linear
AHA states that linearity
The maximum error must be less than 5% of chart width, if not met ECG waves will be distorted, leading to a possible misdiagnosis. ECG should be as linear as possible, but this cannot be changed
Define time constant
The ability of an ECG to process slow or low-frequency signals, it is the time taken for the amplitude to decay due to a change in voltage
AHA states that time constant
It must be greater than 3.2 seconds, if not met low frequency areas of ECG will be distorted. This cannot be changed set by manufacturers
Define Patient isolation
The maximum amount of current that can be passed to a patient, needs to be checked, prevents shocking the patient and can also influence the quality of impedance
current should be as low as possible
AHA states that patient isolation
should be less than 10 microamps, if not met mild sensations or burns could occur
Importance of sensitivity
Amplitude could decrease/ increase based on sensitivity
common choices are 5mmV, 10mmV & 15mmV
AHA states that sensitivity
Must be 1mV for every 10mms, if not met amplitude may be distorted, leading to a possible misdiagnosis
If changes to sensitivity made, make a note of it to ensure correct interpretation
Importance of paper speed
To prevent overlapping of waves,
AHA states that paper speed
25mm is equal to one second, if not met HR may appear higher or lower, leading to misdiagnosis
Define frequency response
The range of frequencies over which an ECG works
AHA states that frequency response
AHA states that frequency should equal to 0.5-150Hz, if not met wave will be distorted, as frequency affects wave shape, clearly annotate if filter is on. Switch on filter when all external noise has been excluded
