College Answer

The ECG device detects and amplifies the small electrical changes on the skin that are caused 
when the heart muscle depolarizes (0.5 – 2 mV). This is reflected as rises and falls in the 
voltage between two electrodes placed either side of the heart which is displayed either on a 
screen or on paper. Usually more than 2 electrodes are used and they can be combined into a 
number of pairs (For example: Left arm (LA), right arm (RA) and left leg (LL) electrodes form 
the three pairs LA+RA, LA+LL, and RA+LL). The output from each pair is known as a lead. 
Each lead is said to look at the heart from a different angle.
Electrodes are commonly made of silver or silver chloride components that are attached to the 
main unit of the machine. Most ECG machines use 12 electrodes. Better answers made 
mention of the two lead types: unipolar and bipolar. 
Methods to reduce artefact include improving signal detection (conductive paste, skin 
preparation (dry, no hair, etc.)) and minimizing external electrostatic forces (common earthed 
environment, diathermy, etc.,) or patient environment (avoid shivering). The amplifier has three essential functions: High input impedance so as to minimize signal loss 
and reject interference (50 – 60 Hz), differential amplification, (to amplify the potential 
difference detected by the skin electrodes), and high common mode rejection (e.g. > 50Hz) to 
aid eliminating muscle artefact or electrical interference from the power grid.). 
Vector analysis is not a component of the ECG machine and so was not required to answer the question.