A patient begins to complain of abdominal pain. The temperature has also risen and they have become hypotensive despite 1 L of fluid resuscitation.
His respiratory rate is about 35 breaths per minute and he is requiring 6 L of oxygen. He is tachycardic and his urine output is poor.
Arterial blood gas analysis reveals:
Inspired oxygen 40% (FiO2 0.4) normal values
PaO2 8.7 kPa >10 kPa on air
pH 7.24 7.35 – 7.45
PaCO2 4.2 kPa 4.7 – 6.0 kPa
Bicarbonate 11 mmol l-1 22 – 26 mmol l-1
One would probably suspect that this patient is becoming septic. The rising temperature, the tachycardia and the hypotension would all point towards this diagnosis.
During a septic spell, because of hypoperfusion, there will be an increase in the acid load causing an acidaemia. This will result in the acute phase in a fall in the bicarbonate.
The patient may also begin to breathe more quickly to try to blow off some of the acid as carbon dioxide.
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 below 4.7 indicating a respiratory alkalosis, which has probably been caused by the body’s attempt to compensate for the acidosis by breathing off the carbon dioxide.
It has not compensated fully, and it is an important point that the body very rarely completely compensates, so the acidaemia is still present.
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 some respiratory compensation in the form of a respiratory alkalosis.
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