Sun10October0740PM 15

This set of arterial blood gases has been taken from a 24-year-old male:

pH 7.6 (7.36-7.44)

PaO2 28.7 mmHg (80-100)

3.78 kPa (10.5-13.1)

PaCO2 10.3 mmHg (35-45)

1.36 kPa (4.6-6)

BE -5.7 -

Standard bicarbonate 9.87 mmol/L (20-28)

Lactate 1.8 mmol/L (1-2)

SaO2 78% -

Hb 194 g/L (130-180)

What is the single most likely cause for this arterial blood gas picture?

(Please select 1 option)

Massive pulmonary embolism

Brainstem tumour

Cor pulmonale

Carbon monoxide poisoning Incorrect answer selected

Effect of altitude This is the correct answer

Explanation

This patient had his arterial gases taken on Everest at high altitude. The primary abnormality is a chronic respiratory alkalosis secondary to hypoxia. Renal compensation has resulted in excretion of bicarbonate. Adaptation to chronic hypoxia has resulted in polycythaemia in order to improve oxygen carrying capacity.

Carbon monoxide has 230 times greater affinity for haemoglobin than oxygen. The partial pressure of oxygen in blood would in theory be unaffected. The SaO2 as measured with a laboratory co-oximeter would be low and carboxyhaemoglobin levels high. Standard two-wavelength pulse oximetry will over read.

Massive pulmonary embolism with such severe hypoxia would be an acute event and not lead to chronic compensatory change.

Brainstem tumours can lead to acute and chronic respiratory alkalosis but the hypoxia and polycythaemia do not tie in.

Whilst cor pulmonale is a cause of chronic hypoxia and polycythaemia it is unlikely to have caused such degree of hyperventilation.

Answer Statistics

1

11%

2

2%

3

3%

4

14%

5

72%

Times answered: 266