Lab case 440 interpretation


PH = 7.396, that is within normal range, very slightly to the acidaemic side/

pCO2 = 52 mmHg, that is high. It is suggestive of the presence of respiratory acidosis

HCO3 = 31 mmol/L, that is high and suggest the presence of metabolic alkalosis.

Since PH is normal, most probably this patient has combined acidosis and alkalosis. Since the calculation is easier when we start with the metabolic process.. So we will start this way.

We will assume that the primary process here is the metabolic alkalosis. Then we will look for the expected CO2 from the respiratory compensation for this process.

Expected pCO2 is calculated as: Expected CO2 = 0.7 x HCO3 + 20 (+/-5). Accordingly expected pCO2 should be 0.7 x 31 + 20 = 41.7 (Range 0f 36.7 to 46.7). pCO2 for this patient is 51. So, we have additional respiratory acidosis.

It is a good practice to calculate anion gap for every blood gas case. Anion Gap is calculated as AG = Na – (Cl + HCO3) = 12. So, we don’t have additional HAGMA.

Other Abnormal Findings:

K = 3.1 mmol/L, that is mild hypokalaemia

Na = 141 mmol/L, Cl = 98 mmol/L. Both these levels are within normal ranges. However, Na and Cl levels should go in the same directions. Here we have Na towards the high side while Cl level is toward the low side. That is suggestive of chronically  elevated HCO3 level.

Next we will look at the causes of metabolic alkalosis. For the differential diagnosis of metabolic alkalosis we use the mnemonic CLEVER PD.

  • C – contraction (dehydration) – Patient wasn’t dehydrated
  •  L – liquorice (diuretic), laxative abuse – excluded by taking history
  •  E – endocrine (Conn’s, Cushing’s) – possible
  •  V – vomiting, GI loss (villous adenoma) – excluded by taking history
  •  E – excess alkali (antacids) – excluded by taking history
  •  R – renal (Bartter’s), severe K depletion – unlikely, normal bloods previously
  •  P – post hypercapnia – possible
  •  D – diuretics  – excluded by taking history.

That patient was not hypertensive and she had respiratory rate of about 30/min.

Accordingly, the team looked for possibility of respiratory alkalosis as that might be the reason for post hypercapnia.

CXR revealed pneumonia which led to sepsis and that caused tachypnoea.

For the causes of acute respiratory alkalosis, we use the mnemonic CHAMPS

  • C = CNS diseases
  • H = Hypoxia
  • A = Anxiety
  • M = Mechanical ventilation/ over ventilation
  • P = Progesterone
  • S = Salicylates / sepsis

After the treatment of the patient it turned out that the patient wasa chronic CO2 restainer with CO2 around 60 mmHg was the usual for the patient.