Lab case 425 interpretation

Question 1:

PH = 7.51, that is mild alkalaemia

PCO2 = 28, that is less than 40. So we have respiratory alkalosis.

Compensation for acute respiratory alkalosis: HCO3 drops by 2 mmol/L for every 10 mmHg decrease in pCO2 from 40mmHg. Accordingly, HCO3 should drop by (12 x 0.2) = 2.4, SO expected HCO3 is 21.6 (that is close to 23) so we have pure respiratory alkalosis.

PO2 = 56, this patient is hypoxic. PO2 = 5 x FiO2. expected PO2 = 21 x 5 = 85 mmHg.

It is a good practice to look at the A-a gradient ( we have arterial blood gases here).

A-a gradient = PAO2 – PaO2.

PAO2 = (760 – PH2O)x FiO2 – (PCO2/ 0.8) = 115. So A-a gradient = 59

Expected  A-a gradient for this man is less than [age in years/4] + 4 = 17, So we have high A-a gradient.

Other abnormal findings:

Cl = 107 mmol/L , that is mild hyperchloremia

Lactate = 2.5 mmol/L, that is moderate hyperlactataemia (2-4).

The final conclusion is: Acute respiratory alkalosis with Hypoxia and high A-a gradient.


Question 2:

For the differential diagnosis of respiratory alkalosis, we follow the mnemonic (CHAMPS).
C = CNS causes
H = Hypoxia (pulmonary causes).
A = Anxiety or Pain
M = Mechanical ventilation
S = Salicylate or sepsis.

This patient is hypoxic, so the cause is pulmonary related. (Pneumonia, Asthma or PE).

Pneumonia is usually associated with abnormal CXR and other features of infection.

Severe asthma can present with blood gases like these. (Hypoxia and elevated A-a gradient), The A-a gradient may reflect distal airway inflammation in severe asthma.

PE is the most probable diagnosis in this case.

Question 3: 

The first aim of the management is to stabilise the patient. Look at ABC and correct them if they are impaired. This patient is hypoxic therefore we need to apply O2, target SPO2 more than 92%. We don’t have further information about the rest of the vital signs.

We need to correct the physiology and fix the pathology.

Remember, mild PE is a different disease from moderate and severe PE.

Further management and choice of anticoagulation/ thrombolysis or clot retrieval depends on the further clinical findings, lab results (TROP/BNP) and CT findings.