What creates the electrical signals measure by an ECG?
Ionic movement during depolarisation and depolarisation of cardiac muscle cells
What is a biopotential?
An electrical potential generated by biological activity due to ion movement
Why can ECG signals be measured on the skin?
Electrical fields created by ionic movement in the heart propagate through body tissues
Why do ions move slower than electrons?
Ions are larger and have more mass than electrons
When are surface ECG voltages largest?
When approximately half the ventricular mass is depolarised
Why does partial depolarisation create a voltage?
As depolarised and resting tissue have different electrical states
What is the function of an ECG electrode?
To act as a transducer concerting ionic current into electronic current
What happens when positive charge moves towards the positive electrode?
ECG trace defects upward
What happens when positive charge moved away from the positive electrode?
ECG trace deflects downwards
What determines ECG signal amplitude?
Angle between the depolarisation wave and the lead vector
What is a lead?
A specific viewpoints of cardiac electrical activity defined by electrode placement
What is a lead vector?
An imaginary line between 2 electrodes showing direction and magnitude of measurement
When is ECG deflection maximal?
When depolarisation travels parallel to the lead vector
When is ECG deflection minimal?
When depolarisation travels perpendicular to the lead vector
What is Eithoven’s triangle?
A geometric arrangement of leads I, II, III around the heart
What is Eithoven’s Law?
Lead I + Lead III = Lead II
In which plane do limb leads view the heart?
Frontal plane
Why are augmented leads needed?
To provide additional views of the heart
What are the augmented leads?
aVR, aVL, aVF
How are augmented leads created?
One active electrode compared to the average of the other two
Why are they called ‘augmented’?
Removing the resistor from the active electrode increases signal amplitude
What is the purpose of the right leg electrode?
Noise reduction and patient reference
What does a driven right leg circuit do?
Feeds back inverted common-mode voltage to cancel interference
Why are ECG signals amplified?
They are very small (microvolt to millivolt range)
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Echocardiography73
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Transducer and Dynamic Response23
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Basic Circuit Concepts Part 118
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Potential Drivers (basic circuit concepts part 2)25
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AC, Frequency Response and Interference38
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Amplification27
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Filtration37
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Electrical Safety and Medical Devices42
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Digitisation51
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Radiation Protection33
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Medical Imaging57
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ECG Electronics37
