Severe metabolic acidosis - an unusual case history

A recently published paper describes the sad case history of a 17-year-old man who was brought, unconscious and totally unresponsive (Glasgow Coma Score 3), by ambulance to the emergency department of his local hospital. Blood gas analysis revealed he was suffering severe metabolic acidosis (pH 7.25, bicarbonate 9 mmol/L) and marked increase in blood lactate (20.3 mmol/L). 

At the time of admission the cause of this young man’s perilous state of health was unknown, but it eventually transpired that he was the unfortunate victim of deliberate cyanide poisoning, a rare cause of lactic acidosis.

The case history details the non-specific but devastating clinical effect of cyanide ingestion and highlights the value of determining the arterio-venous oxygen saturation difference for diagnosis of cyanide poisoning. In this case it was the abnormal finding that there was no difference in the oxygen saturation of arterial and venous blood that first alerted clinicians to the possibility that the cause of the metabolic (lactic) acidosis in this case might be cyanide poisoning. 

Confirmation of cyanide poisoning was made by measurement of plasma thiocyanate. The case history includes full discussion of the mechanism of cyanide toxicity and antidote treatment. The perpetrator eventually confessed to administering 1.5 g cyanide dissolved in a drink; this dose is 10 times the lethal limit (140-250 mg).

The authors conclude that the high cyanide dose and inevitable delay in delivering antidote treatment conspired in the death of this patient 56 hours after admission. The authors have identified three key clinical features that were exhibited by this patient, which if present together in any patient should raise strong suspicion of cyanide poisoning. They are: coma, unexplained lactic acidosis in combination with normal arterial pO2 and bradycardia.