The following arterial blood gas report was obtained from a 75-year-old female admitted to hospital with gastric outlet obstruction. She has had a rapid response team call for tachypnoea with a diagnosis of aspiration pneumonia.

Parameter Patient Value Normal Adult Range
FiO2 0.3  
pH 7.53* 7.35 – 7.45
PCO2 31 mmHg (4 kPa)* 35 – 45 (4.6 – 6.0)
PO2 83.7 mmHg (11 kPa)  
Bicarbonate 25 mmol/L 22 – 28
Standard Base Excess 3.3 mmol/L* -2.0 – +2.0

a) Comment on the acid-base status.
b) Give an explanation for these results.

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College Answer


Mixed respiratory and metabolic alkalosis


Respiratory alkalosis from the hyperventilation due to the pneumonia
Metabolic alkalosis from vomiting (or diuretic use).



Let us dissect these results systematically.

  1. The A-a gradient is high; ~91mmHg
  2. There is alkalaemia
  3. The PaCO2 is contributing
  4. The SBE is 3.3, suggesting a mild metabolic alkalosis
  5. The respiratory compensation is inadequate - the expected PaCO2 (25 × 0.7) + 20 = 37.5mmHg, and so there is also a respiratory alkalosis according to the Boston rules.
  6. The anion gap and delta ratio canot be calculated.