Lab Case 53 – Interpretation

Elderly patient with collapse presents critically unwell.

Her ABG shows:


Normal pH with severe respiratory acidosis and metabolic alkalosis

A-a gradient: 40, high indicating VQ mismatch or shunt


Assuming that the primary disorder is Respiratory and acute (a CO2 of 89 is very uncommonly chronic even in patients with Emphysema) –

if compensated (1:4 rule) the HCO3 should be max of 44

Assuming that the primary disorder is metabolic, you can never compensate to a CO2 of 89

calculation for expected CO2 = 0.7XHCO3 + 20 = 58, which is probably maximal CO2 compensation

2. Causes include:

Respiratory acidosis

– central (stroke, haemorrhage, drugs)

– peripheral (lung, chest wall – less likely)

Metabolic alkalosis

– severe dehydration

– severe vomiting

– drug – diuretics

VQ mismatch/ shunt

– pneumonia

– PE

– aspiration

3. Management:

– establish limits of care

– secure airway

– hyperventilate to blow off CO2, reverse ??opioid toxicity

– look for underlying cause and treat appropriately