Clinical Details
An 8 month old female baby was admitted with a one day history of lethargy. She had vomited several times. Her mother said she appeared "intoxicated". Examination confirmed the obtunded mental state but she was easily rousable and muscle tone was normal. Resp rate was 60/min. Pupils were normal. There was no evidence of dehydration. Abdomen was soft and nontender. BP was 112/62. Peripheral perfusion was clinically assessed as normal. Heart and chest examination was normal. Plantar response was normal.

Investigations: Na⁺ 135, K⁺ 4.2, Cl^- 116, bicarbonate 5.7, glucose 5.9 (All in mmol/l). Other results: Urine: pH 5.0, negative for glucose and ketones. Numerous calcium oxalate crystals were seen on urine microscopy [This example is Case 2 reported by Saladino & Shannon]

The alerting information in the history is the comment about the baby appearing "intoxicated": such a CNS sign suggests a toxin such as ethylene glycol or methanol as the cause. However, an ill baby is typically listness and one should not read to much into this comment. The finding of numerous calcium oxalate crystals strongly suggests ethylene glycol toxicity and as this is such a serious diagnosis, it should be aggressively pursued.

There is no evidence of hypoperfusion, hyperglycaemia or ketosis. Also there is no history given of any inherited metabolic disorder which could result in a lactic acidosis or a renal tubular acidosis but these are possibilities. The amount of vomiting will not result in alkalosis and the baby is too old for infantile pyloric stenosis. Unfortunately, no urea or creatinine result is provided. 

Secondly, the acid-base diagnosis:  
1. pH:  The severe acidaemia indicates a severe acidosis is present.
2. Pattern: The pattern of a low bicarbonate & low pCO2 indicates that this is a metabolic acidosis. 
3. Clues: The anion gap is normal (ie 135-116-5.7 = 13.3mmol/l) and the chloride is raised. This indicates a normal anion gap acidosis. 
4. Compensation:  The expected pCO2 is (1.5 x 6.2 + 8) = 17.3mmHg. The actual pCO2 is close so respiratory compensation is maximal. It takes 12 to 24 hours for compensation to reach this maximal level & the history indicates sufficient time has passed.
5. Formulation: A normal anion gap metabolic acidosis with maximal respiratory compensation. This is typically due to either GIT causes (eg diarrhoea) or a renal cause (renal tubular acidosis). There is no history supporting a GIT cause. The urine pH is 5 which is appropriately low and does not support a diagnosis of type 1 (or distal) renal tubular acidosis. The absence of hypokalaemia & the severity of the acidosis are also against the diagnosis of a type 2 renal tubular acidosis.  
6. Confirmation: 

Finally, the Clinical Diagnosis:
A severe acidaemia is present. This in association with a low HCO3 with a low pCO2 confirm a metabolic acidosis.

Is the fall in pCO2 appropriate?

Expected pCO2 = (1.5 x 6.2) + 8 (+/- 2) = 17.3 (and range about 15 to 19).

The actual pCO2 falls within the expected range. There is no evidence of a co-existent respiratory acid-base disorder.

Delta ratio = Increase in AG / decrease in HCO3 = (13.3-12)/(24-6) = 0.07

A normal anion gap (or hyperchloraemic) metabolic acidosis is present. The D/D ratio close to zero suggests a ‘pure’ hyperchloraemic acidosis without any evidence of a co-existent high anion gap acidosis.

A hyperchloraemic acidosis results from loss of base from either the gut or the kidney (or rarely from gain of HCl from some infusions eg NH4Cl).