A 30 year old male BIB police found at the shops acting erratically. The RMO accidentally canulates the brachial artery and a ABG is obtained:
pH 7.56 Na 142 mmol/l
pCO2 17mmHg K 4.2 mmol/l
pO2 126mmHg (FiO2 0.21) Cl 105 mmol/l
HCO3 15 mmol/l Glucose 5.5 mmol/l
Describe and Interpret the ABG
Alkalaemia pH 7.56
Respiratory alkalosis pCO2 17mmHg
Expected HCO3 = 24 – 2 x (40-pCO2)/10 = 19.4. Actual HCO3 15 mmol/l
AG = Na – (HCO3+CL) = 22
Delta Gap = (AG-12)/ (24-HCO3) = 1.1
Expected PAO2 = (FiO2x713) –(pCO2x1.25) =128.75
A-a gradient = PAO2 – PaO2 = 2.75
The ABG shows a primary respiratory alkalosis with an existing HAGMA. There is a delta ratio of 1.1 consistent with a HAGMA. There is a raised lactate and a normal A-a gradient.
The gas in the clinical context is consistent with a primary hyperventilation causing a respiratory alkalosis. The HAGMA is likely caused by a lactic acidosis that may be due to a toxilogical cause for example stimulants and salicylates, or sepsis with delirium. An intracranial pathology could also be the cause of the respiratory alkalosis resulting in increased ICP and respiratory drive.