Question 3.4

For each set of the following biochemical and arterial blood gas parameters:

  • Describe the abnormalities.
  • Give one example of an associated clinical scenario.

Any reasonable scenario accepted that was both biochemically correct AND clinically likely.



Normal Adult Range


145 mmol/L

135 – 145


4.0 mmol/L

3.2 – 4.5


101 mmol/L

100 – 110


34 mmol/L

24 – 32



7.35 – 7.45


90 mmHg (11.7 kPa)

35 – 45 (4.6 – 5.9)

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College Answer

Acute respiratory acidosis with metabolic alkalosis.

Clinical scenario – acute respiratory failure in COAD (Acute on chronic respiratory failure.


Let us dissect these results systematically.

  1. The A-a gradient cannot be calculated - the information is missing
  2. There is acidaemia
  3. The PaCO2 is massively elevated, suggesting a primary respiratory acidosis
  4. The SBE is not supplied, but the bicarbonate is 34, suggestive of a metabolic alkalosis
  5. The metabolic compensation may not have titrated the pH back to normality, but it appears to be excessive - the HCO3is expected to increase by 1mmol/L for every 10mmHg increase in CO2 in an acute respiratory acidosis, which would give us (10 × 5 + 24) = 29mmol/L. The fact that the HCO3- is 34mmol/L suggests that there is a metabolic alkalosis present.
  6. The anion gap is essentially normal: 
    (145) - (101 + 34) = 10, or 14 when calculated with potassium
  7. The delta ratio is of no use here.
  8. The urinary electrolytes and pH would not be helpful.

Thus, this is a respiratory acidosis with a coexisting  metabolic alkalosis. Anion gap is normal. Scenarios which might explain these findings may be any of the following:

  • Hypercapneic respiratory failure in chronic COPD
  • Respiratory failure due to acute pulmonary oedema in a patient on chronic loop diuretic therapy
  • Hypercapneic respiratory failure due to obtundation in a vomiting opium fiend


Bear, R., et al. "Effect of metabolic alkalosis on respiratory function in patients with chronic obstructive lung disease." Canadian Medical Association Journal117.8 (1977): 900.

Bruno, Cosimo Marcello, and Maria Valenti. "Acid-base disorders in patients with chronic obstructive pulmonary disease: a pathophysiological review."BioMed Research International 2012 (2012).