Lab case 403 interpretation

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PH = 7.745, that is severe alkalaemia.

pCO2  = 36 mmHg is low so we have respiratory alkalosis.  However this value is very close to normal value of 40. This won’t explain the severely elevated PH.

HCO3 = 52 mmol/L, this is very high (normal value is 24), this explains the very high PH. So the primary process is metabolic alkalosis.

Next we need to calculate the compensation for metabolic alkalosis. The formula we use:

Expected pCO2 = 0.7 x HCO3 + 20 (+/- 5). Accordingly, expected pCO2 for this metabolic alkalosis should be: 0.7 x 52 + 20 = 56.4 (51.4 to 61.4). Since the value we have is lower than that (36 mmHg), that confirms the presence of additional respiratory alkalosis.

Next we will calculate the anion gap to exclude the presence of additional HAGMA.

AG = Na – (Cl + HCO3) = 5 (This excludes the presence of HAGMA).

This patient has combined Metabolic and respiratory alkalosis.

Other abnormal findings

Na = 127 mmol/L, that is moderate hyponatraemia.

K = 2.7 mmol/L, that is moderate hypokalaemia.

Cl = 70, mmol/L, that is hypochloraemia.

Lactate = 3.7 mmol/L, that is moderate hyperlactataemia.

Creatinine = 363 mol/L that is elevated. That was an acute elevation as the base line creatinine for this patient was 100 umol/L.

Next we need to look at the causes:

For the differential diagnosis of Metabolic alkalosis we use the mnemonic CLEVER PD.

  •  C – contraction (dehydration) – Possible
  •  L – liquorice (diuretic), laxative abuse – excluded by taking history
  •  E – endocrine (Conn’s, Cushing’s) – unlikely with low Na level.
  •  V – vomiting, GI loss (villous adenoma) – Possible
  •  E – excess alkali (antacids) – excluded by taking history
  •  R – renal (Bartter’s), severe K depletion – unlikely, normal bloods previously
  •  P – post hypercapnia – No
  •  D – diuretics  – excluded by taking history.

Most probably, both vomiting and dehydration caused this patient’s severe metabolic acidosis.

Respiratory acidosis was due to anxiety and hyperventilation.

The presence of acute renal failure also is suggestive of dehydration.

This patient was managed by fluids and electrolytes replacement including Mg replacement. With is 2 days the condition and blood gases improved.