Police find a man unconscious in his car near Marion Shopping Centre. The car en
ID: 143706 • Letter: P
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)
Explanation / Answer
a.As the man inside the car has inhaled large amounts of exhaust gases which mainly consists of carbon monoxide gas. he got unconscious. It is easily absorbed through the lungs. In comparison to oxygen, CO joins readily to the hemoglobin and stays there for a longer time devoiding oxygen to bind to the hemoglobin. The chemical reactions that stop happening when there's no oxygen are the ones that make ATP, the form of energy that all of our cells use. The CO has such a high affinity for hemoglobin, that oxygen cannot displace it. CO also shifts O2 curve to the left making O2 that did bind to Hb unable to dissociate.
b.Blood pH is below the normal range due to the increase of acidity because of the low level of HCO3- IONS showing acidity. Probably the increasing acidity of the blood from whatever cause hastens the elimination of carbon monoxide from the blood independently of ventilatory rate. It is the natural defense mechanism of the body. Administration of carbon dioxide helps and provides a dual function -firstly it increases hyperventilation and secondly, It increases blood hydrogen ion concentration which in itself favours CO elimination.
c.In this patient metabolic acidosis is occurring which is a consequence of inadequate oxygen delivery to the peripheral tissues, resulting from an anaerobic metabolism and lactic acid production.