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Date May 2017 Marks available 2 Reference code 17M.3.HL.TZ2.19
Level Higher level Paper Paper 3 Time zone Time zone 2
Command term Distinguish Question number 19 Adapted from N/A

Question

In control subjects, blood potassium levels are maintained, through homeostasis,between 3.5 and 4.5 mmol litre–1. In patients with anorexia, blood potassium canfall below this level. This is known as hypokalemia. In patients with kidney failure, levels can rise above this range, causing hyperkalemia. The traces show the electrocardiograms (ECGs) of a patient with hypokalemia, a normal subject and a patient with hyperkalemia.

Distinguish between the ECG trace of the patient with hypokalemia and the patient with hyperkalemia.

[2]
b.i.

Outline the events that occur within the heart that correspond to the QRS complex.

[3]
b.ii.

Severe hypokalemia can lead to ventricular fibrillation. Describe the medical response to ventricular fibrillation.

[3]
b.iii.

Sometimes hyperkalemia occurs as a body tries to respond to low blood pH. State the normal range of blood pH in the human body.

[1]
b.iv.

Explain how low blood pH causes hyperventilation (rapid breathing).

[3]
b.v.

Markscheme

a. hypokalemia has a flat T-wave whereas hyperkalemia has a heightened T-wave OWTTE
OR
hypokalemia S-T interval longer Accept vice versa

b. hypokalemia has narrower/faster QRS complex compared to hyperkalemia  Accept vice versa

c. hypokalemia trace/baseline «overall» lower than hyperkalemia Accept vice versa

b.i.

a. arrival of signal at AV node 

b. transmission via conducting fibres/bundle of His/Purkinje fibres 

c. ventricles depolarize 

d. atrioventricular valves close
OR
semilunar valves open 

e. ventricular systole/contraction 

f. contraction begins at apex/base

b.ii.

a. use a defibrillator 

b. place electrodes on exposed chest of victim 

c. in a line with the heart in the middle of a diagonal line between the two paddles 

d. the device determines whether fibrillation is happening 

e. if it is, an electric discharge is given off to restore a normal heart rhythm

b.iii.

around 7.4 or 7.35 to 7.45

b.iv.

a. increased CO2 lowers blood pH 

b. chemoreceptors in carotid/aorta detect lower pH 

c. signal/impulses to medulla «oblongata»
OR
signal/impulses to respiratory centre 

d. «from medulla/respiratory centre» to intercostal muscles/diaphragm 

e. ventilation rate increase occurs to expel CO2

b.v.

Examiners report

[N/A]
b.i.
[N/A]
b.ii.
[N/A]
b.iii.
[N/A]
b.iv.
[N/A]
b.v.

Syllabus sections

Option D: Human physiology » Option D: Human physiology (Core topics) » D.4 The heart
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Option D: Human physiology » Option D: Human physiology (Core topics)
Option D: Human physiology

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