Police find a man unconscious in his car near Marion Shopping Centre. The car en
ID: 3504800 • 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:
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 explanations for the low PCO2 and HCO3- levels observed, outlining any relevant
physiological mechanism(s). (5 marks)
RESULTS 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%) Carboxyhaemoglobin 40% (<2%, non smoker)Explanation / Answer
a. The reason for man's unconsciousness is lack of oxygen into the body. Exhaust gases of car's engine contains carbonmonoxide gas which have much higher affinity towards heamoglobin in comparison to oxygen. Carbonmonoxide forms carboxyhemoglobin upon binding with hemoglobin. Here level of carboxyhemoglobin is very much higher than refernce range.
b. Binding of carbonmonoxide with hemoglobin increases the level of carbonic acid (H2CO3) which disturb the physiological buffer system and decreases the pH of the blood.
c. PCO2: PCO2 indicates the partial pressure of carbondioxide gas in blood or indicates the amount of dissolved carbondioxide in blood. The low PCO2 occurs due to high ventilation rate. Ventilation volume increased during hypoxia to offset the decreased oxygen content of the body fluid which results in low PCO2. The decrease in PCO2 increases the ratio of bicarbonate concentration to PCO2 and, to increases the pH level.