History: A 24-year-old man with a severe intellectual disability is brought in by his family who report that he is lethargic and drowsy and has had a cough for 1 week. Below is his arterial blood gas on arrival:
|pH 7.31||Na 174 mmol/L|
|pCO2 52 mmHg||K 3.1 mmol/L|
|pO2 120 mmHg (FiO2 0.4)||Cl 137 mmol/L|
|HCO3 25 mmol/L||Glucose 33 mmol/L|
|Osmolality 402 mmol/kg||Urea 25.7 mmol/L|
What’s the pH?
7.31 = acidaemia
What’s the primary process?
pCO2 52 = primary respiratory acidosis
Is there any compensation?
Expected HCO3 = 24 + ((Measured pCO2 – 40)/10)
= 24 + ((52 – 40)/10)
Therefore there is full metabolic compensation for an acute respiratory acidosis.
Are there any other clues?
Expected PAO2 = (713 × FiO2) – (pCO2 × 1.25)
= (713 × 0.4) – (52 × 1.25)
A-a gradient = PAO2 – PaO2
= 220.2 – 120
Therefore there is a significantly elevated A-a gradient.
Corrected sodium = Na + (glucose – 5)/3
= 174 + (33 – 5)/3
There is severe hypernatraemia.
Calc osmolality = (2 × Na) + urea + glucose
= (2 × 174) + 25.7 + 33
Osmolar gap = Measured osmolality – Calculated osmolality
= 402 – 406
Therefore there is no osmolar gap.
Serum osmolality is markedly elevated. There is hyperglycaemia, severe hypernatraemia and severe hyperchloraemia. Urea is markedly elevated. Potassium is slightly low.
What’s the differential diagnosis?
Description: This arterial blood gas reveals a maximally compensated primary acute respiratory acidosis with an elevated A-a gradient. There is severe hyperglycaemia and evidence of significant dehydration with severe hypernatraemia, hyperchloraemia and elevated urea. There is significant hyperosmolality, but no osmolar gap. Mild hypokalaemia is also present.
Interpretation: In this clinical context, this gas would be consistent with severe hypernatraemic dehydration and type 2 diabetes complicated by hyperglycaemic hyperosmolar state (HHS). It is likely that there is some alteration in conscious state resulting in failing ventilation and acute respiratory acidosis. Considering the history of cough and elevated A-a gradient, pneumonia should be suspected as a precipitating factor. Treatment will include gradual rehydration, correction of potassium, insulin, and management of sepsis.
Additional information: Strong ion difference Na – Cl = 37, suggesting a normal anion gap. Urine osmolality 609 mmol/kg.
Patient has a rare congenital abnormality, limited mobility and severe intellectual delay (2y old), eats a mostly soft diet and has a well-established preference for sugary foods and drinks, in particular strawberry milk. This was a first presentation of type 2 diabetes with hyperglycaemic hyperosmolar state secondary to community acquired pneumonia with sepsis. HbA1C 11%.