Metabolic syndromes: metabolic acidosis
The emergency care of acid–base problems.
Metabolic acidosis may occur as a result of:
• Abnormal loss of bicarbonate.
• Abnormal accumulation of hydrogen ions in association with a non-volatile organic anion.
These two states can be differentiated by calculating the anion gap (i.e. the difference between plasma [Na+] and the sum of plasma [Cl–] and [HCO3– ]). The normal anion gap is 10–15mmol/L.
When metabolic acidosis is due to bicarbonate loss from either the gut or kidney:
• The anion gap is normal.
• Hyperchloraemia is usually present.
To distinguish bicarbonate loss from the gut or from the kidney:
• A history of diarrhoea will distinguish between hyperchloraemia due to GI losses from that due to renal tubular losses.
• Urine net charge (UNC), calculated as [Na++ K+] – [Cl–], is used to estimate urine ammonium (NH4+) when there is no accumulated organic acid. A negative UNC implies the presence of adequate or increased urinary ammonium; therefore the acidosis results from abnormal GI loss of bicarbonate. (Note: Urine ammonium is low in renal tubular disorders.)
IMDs associated with RTA include—galactosaemia; hereditary fructose intolerance; hepatorenal tyrosinaemia; cystinosis; glycogen storage disease I; Fanconi–Bickel syndrome; congenital lactic acidosis; Wilson disease; vita-min D dependency; osteopetrosis with RTA; Lowe syndrome.
When metabolic acidosis is due to accumulated organic anion:
• It is associated with failure to thrive.
• Tachypnoea may be present.
• s hypoglycaemia leads to a neurological syndrome.
• Organic anion may lead to distinct smell of sweat or urine.
• The anion gap is raised.
The causes include the following:
• Lactic acidosis: pyruvate accumulation (e.g. pyruvate dehydrogenase deficiency, pyruvate carboxylase deficiency, multiple carboxylase deficiency); NADH accumulation (e.g. defect of mitochondrial electron chain).
• Ketoacidosis: s to IMD (e.g. maple syrup urine disease, organic acido-pathies, glycogen storage disease, disorders of gluconeogenesis; rare p disorders of ketone utilization, e.g. B-ketothiolase deficiency, succinyl-CoA: 3-ketoacid transferase deficiency.
Organic aciduria: a large spectrum of disorders.