A young man has suffered a head injury during a rugby match. Initially he was not knocked unconscious however he has become increasingly drowsy.
On the way to hospital the paramedics have inserted an oropharyngeal airway and given him high flow oxygen.
Arterial blood gas analysis reveals:
Inspired oxygen 40% (FiO2 0.4) normal values
PaO2 15.6 kPa >10 kPa on air
pH 7.22 7.35 – 7.45
PaCO2 9.2 kPa 4.7 – 6.0 kPa
Bicarbonate 25.4 mmol l-1 22 – 26 mmol l-1
This patient has a reduced level of consciousness and an impaired airway which will probably both result in him not breathing out his carbon dioxide effectively. It may also mean that is oxygenation is poor. Because it is an acute situation the bicarbonate level is unlikely to have changed very much as the kidneys only respond relatively slowly.
As a rule of thumb the patients PaO2 should be no less than 10 points lower than the inspired oxygen that is being delivered. So in this case if there was no problem with the oxygenation you would expect the PaO2 to be around 30kPa. It isn’t, so there is some hypoxia occurring.
We next need to ask ourselves whether there is an acidaemia or an alkaleamia. In this case the pH is less than 7.35 so there is an acidaemia.
We then look at the PaCO2, remembering that values above 6 would indicate a respiratory acidosis, and values below 4.7 would indicate a respiratory alkalosis. In this example the PaCO2 is above 6 so the patient has a respiratory acidosis.
The final part of the process is to look at the bicarbonate to determine whether there is a metabolic disorder. The bicarbonate in this example falls within the normal range so there is no metabolic disorder at present.
So to summarise this blood gas we have an acidaemia caused by a respiratory acidosis and no metabolic compensation.
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