'A' Level Medical Option Questions - The Heart - ECG
Q5. Electrodes are placed on the surface of a body to record an ECG trace for a healthy person. The trace obtained for one heartbeat is shown below.
(a)
(i) Label approximate scales on each axis.
y-scale: +1 to –0.2 V 
x-scale: 0 to 1.0 s
(ii) State what electrical event happens at points A and B and the physical change that results.
position A:
event - sino atrial node fires
result - atria contracts
position B:
event - ventricular node fires
result - ventricles contract
6 marks
(b) State, giving a reason, one precaution you would take when attaching the electrodes to the surface of the skin to ensure a good signal is obtained.
precaution: remove dead skin [or use conducting/electrode gel]
reason: to give best possible contact between person and electrode
2 marks
(c) The amplifier used must have a high gain. State two other properties of the amplifier.
low noise
high input impedance
2 marks
Total 10 marks
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