Question

Police find a man unconscious in his car near Marion Shopping Centre. The car engine was running
with a hose taped to the exhaust carrying exhaust gases to the car's interior. The police take the
man to the FMC Emergency department where an arterial blood sample is taken and the man given
oxygen. Lack of oxygen may lead to changes in arterial blood gas results. In this case, carbon
monoxide, which is found in car exhaust, to various extents prevents oxygen from binding to
haemoglobin resulting in tissue hypoxia. Blood gas and co-oximetry measurements (the values of
oxyhemoglobin and carboxyhaemoglobin which show how much oxygen and carbon monoxide is bound to
haemoglobin, respectively) on the pre-treatment arterial blood give the following values:

RESULT UNITS REFERENCE RANGE
PO2 85 mmHg (80-100)
PCO2 30 mmHg (35-45)
HCO3- 18 mmol/L (21-28)
pH 7.31
(7.35-7.45)
Total
Haemoglobin 151 g/L
(130-175)
Oxyhaemoglobin 57%
(94-100%)
Carboxy-
haemoglobin 40%
(<2%, non-smoker)

a) Briefly explain the likely reason for this man’s unconsciousness. (1 mark)
b) Explain why his blood pH is below the normal range. Outline any relevant metabolic events and
indicate why these occur in this situation. (4 marks)
c) suggest explanation for the low PCO2 and HCO3- levels observed,
outlining any relevant mechanism(s). (5 marks)