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.
(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:
Pneumonia(although afebrile- elderly may be)
Toxins such as aspirin OD