Q8.

(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.

The candidate's answer will be assessed holistically.The answer will be assigned to one of three levels according to the following criteria.

High Level (Good to excellent): 5 or 6 marks

The information conveyed by the answer is clearly organised, logical and coherent, using appropriate specialist vocabulary correctly.

The form and style of writing is appropriate to answer the question.

The candidate accurately describes measures to ensure good contact between the electrodes and the skin including the use of conducting gel. The candidate will mention the need for more than one electrode and the need for the patient to remain relaxed and still. They will need at least one property of the amplifier.

Intermediate Level (Modest to adequate): 3 or 4 marks

The information conveyed by the answer may be less well organised and not fully coherent.

There is less use of specialist vocabulary, or specialist vocabulary may be used incorrectly.

The form and style of writing is less appropriate.

The candidate will include most measures to ensure good contact between electrodes and the skin. They might give a property of the amplifier or mention the need for the patient to remain relaxed and still.

Low Level (Poor to limited): 1 or 2 marks

The information conveyed by the answer is poorly organised and may not be relevant or coherent.

There is little correct use of specialist vocabulary.

The form and style of writing may be only partly appropriate.

The candidate will mention electrodes connected to the skin and might make another sensible comment on the arrangement.

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

(6 marks)

(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

(2 marks)

(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.

(1 mark)

(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

(3 marks)

(Total 12 marks)