30 year old male presents to ED with altered level of consciousness. According to his family he had an asthma attack in the car and then went unconscious after using his ventolin.

Respiratory alkalosis -pH 7.68, pCO2 14mmHg

Compensation – every decrease in CO2 of 10 causes a decrease of HCO3 of 2 – Expected HCO3 = 19mmol/l. Actual pCO2 14mmHg

AG = Na –(Cl+HCO3) = 20

Delta Ratio = Change in AG/ Change in HCO3 = AG- 12/24-HCO3 = 0.8

The above VBG shows a mixed respiratory alkalaemia and metabolic acidaemia. The metabolic acidosis is a HAGMA with a delta ratio of 0.8.  The patient has a mildly low potassium, a normal BSL and creatinine, with a moderately elevated lactate of 4.6 mmol/l.

The differential diagnosis for the above VBG and clinical context is broad.  A salicylate OD would be give the typical VBG with a mixed respiratory alkalosis and HAGMA.  Further collateral and salicylate levels should be considered. Tachypnoea secondary to asthma attack can cause the respiratory alkalosis, and the excessive use of Ventolin would cause the lactic acidosis and low potassium. In the context of an altered GCS and a lactic acidosis seizures could be the cause. The seizure could be caused by a stroke which can lead to tachypnoea and the respiratory alkalosis.

Further collateral was obtained from the wife and the patient had a previous similar presentation back home which was treated with ‘calming medications’ and the patient was discharged the same day. After a period of observation the patients GCS improved and his VBG returned to normal. The patients VBG was likely a result of hyperventilation in the context of  an asthma attack and panic attack, with the HAGMA due to a lactic acidosis from excess Ventolin use.