A man has suffered a witnessed out-of-hospital arrest. Unfortunately it was 67 minutes before the ambulance arrived during which time no resuscitation was attempted.
The paramedics subsequently restored a spontaneous circulation. He is brought into hospital intubated with a pulse of 120 bpm and a good blood pressure. His Glasgow coma score is three.
Arterial blood gas analysis reveals:
Inspired oxygen 50% (FiO2 0.5) normal values
PaO2 8.4 kPa >10 kPa on air
pH 7.18 7.35 – 7.45
PaCO2 6.5 kPa 4.7 – 6.0 kPa
Bicarbonate 15 mmol l-1 22 – 26 mmol l-1
During his arrest the patient was not ventilated nor was he circulation intact. As a consequence his cells would have had to metabolise and anaerobically. This is metabolism without oxygen which causes accumulation of lactic acid. This lactic acid will cause an acidaemia.
In the acute phase much of this acid will combine with the bicarbonate causing the bicarbonate levels to also fall. His PaCO2 may have initially been raised but now he is ventilated this may have improved.
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. In this example the PaO2 is significantly lower than you would expect so there is severe hypoxia.
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 although this is very slight in this example.
The final part of the process is to look at the bicarbonate to determine whether there is a metabolic disorder. The bicarbonate has dropped quite markedly which would indicate that there is a metabolic acidosis, which I said before is probably because the bicarbonate is being used to mop up the excess hydrogen ions.
So to summarise this blood gas we have an acidaemia caused by a metabolic acidosis and little respiratory compensation
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