'A' Level Medical Option Questions - The Heart - ECG
(a) An ECG trace is to be obtained for a patient. State and explain the procedure and some design features of the equipment needed to ensure a good trace is obtained.
Statements expected in a competent answer should include some of the following marking points.
To reduce contact resistance
• sandpaper skin/shave to remove hairs and some dead skin
• apply conducing gel
• securely attach more than one electrode
To remove unwanted signals
• electrodes should be non-reactive
• patient to remain relaxed and still
• shielded leads/reducing interference from ac sources
Properties of amplifier
• amplifier has large input impedance/high gain/low noise
(b) The diagram below shows an ECG trace for a healthy person.
(i) Add a suitable scale and unit to the potential axis.
See diagram - max amplitude is about 1.0 mV - mark in the 0 and 1
Unit of mV
(ii) Add a suitable scale to the time axis.
The average heart rate is about 70/minute 60/70 = 0.86s - therefore you should put 0.8 or 0.9 on the x-axis.
(iii) State the electrical events which give rise to the points P, R amd T.
- P depolarisation of atria
- R depolarisation of ventricles (and repolarisation of atria)
- T repolarisation of ventricles
(Total 12 marks)