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Interpretation on ABG’s- Click above to watch a 12 minute video on interpretation of ABG’s.

Example 1


This is an elderly man with vomiting for 3 days, who presents with tachycardia. It would be expected that there be a metabolic alkalosis with loss of gastric contents.

He is on 50% oxygen.

His pH shows an alkalosis and he has raised bicarb.

He is hypokalaemic and hypocloraemic, with a raised BSL. The Na is low, when corrected for increased BSL it is 134.

With this metabolic alkalosis the expected pCO2 is (0.9 x HCO3) + 16 = 43. The actual pCO2 is 28.5. Therefore this is a mixed picture of Metabolic and Respiratory Alkalosis i.e., he has his metabolic alkalosis but is also breathing up more than he should.

A-a gradient

(FIO2(Patm-PH2O) – PaCO2/0.8) -PaO2

=(0.5 (760-47) – 28.5/0.8) – 234

= 320-234= 86

Expected Aa gradient is age/4 +4 = 22.5 – so a very high Aa gradient indicating a V/Q mismatch, or diffusion defect.

So when we think of causes, take both things into account- the vomiting and the Aa.

Causes to think about here include:

CNS tumour

Pneumonia(although afebrile- elderly may be)


Toxins such as aspirin OD

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