Consultant Pediatric Intensivist Apollo Hospitals, Chennai. *
Metabolic acidosis is characterized by decreased pH, HCO3, and PaCO2. By far, the commonest cause of metabolic acidosis in a sick child is poor circulatory status associated with anaerobic respiration due to tissue hypoxia, dehydration, catabolism and sepsis. Metabolic acidosis due to errors of metabolism is usually associated with an increase in anion gap.

Anion gap = [Na+ + K+] - [Cl- + HCO3 -], normal upper limit: 15-20 mmol/L
  1. Differential Diagnosis:
    1. Renal loss of bicarbonate
      • Normal anion gap, increased Cl-, urinary pH>5, may be accompanied by other signs of renal tubular dysfunction
    2. Intestinal loss of bicarbonate
      • Normal anion gap, increased Cl-, urinary pH may be elevated because of hypokalemia and secondary increase of urinary ammonium

    3. Organic aciduria - Increased anion gap, lactate or ketones

    Ketosis is a physiological response to fasting, catabolic state or ketogenic diet. In some children ketosis is associated with nausea and vomiting. "Ketonemic" vomiting of infants with normal blood sugar is rarely caused by a primary IEM. Permanent ketosis may in rare cases indicate a ketolytic defect. Ketosis in addition to other metabolic abnormalities is frequently found in organic aciduria or mitochondrial respiratory chain disorders.
    Ketonuria in neonate is often indicative of a primary IEM but may be physiological in late infancy or older children. But ketosis is abnormal if it causes acidosis.
  2. Investigations
  3. Lactate
    Plasma ketone, urine ketones
    Plasma organic acids
    Acylcarnitine (dry blood spots)

    Normal value: Blood <2.1 mmol/L, CSF <1.8 mmol/L Blood sample should be taken from an uncuffed vein or artery and in a relaxed child. Lactic acidosis will not usually result in acidosis unless the level is more then 5 mmol/L. Pyruvate sample is usually not indicated. Measurement may be considered when lactate is elevated to determine the lactate / pyruvate ratio (redox state, normal <20). Alanine reflects the concentration of pyruvate and indirectly lactate, but is not affected by cuffing. Normal <450 umol/L.

    Lactic acidosis is most commonly secondary to:
    • the use of a tourniquet or difficulty in drawing the blood
    • muscular activity, assisted ventilation, seizures
    • severe systemic disease: central or peripheral hypoxia, ischemia, shock, cardiac failure,
            liver failure, renal failure, septicemia, diabetes mellitus

    IEM associated with lactic acidosis:
    • respiratory chain disorders or mitochondrial cytopathy
    • fatty acid oxidation defects
    • organic aciduria
    • glycogen storage disease, gluconeogenesis defects
  4. Treatment
  5. Treatment of metabolic acidosis or lactic acidosis secondary to IEM is treatment of the primary disorders. Sodium bicarbonate may be used to treat severe acidosis uncontrolled by the treatment of underlying disorder. Its use is discouraged if there is hyperammonemia as it may increase cellular influx of ammonia in the brain.
    Stop all protein intake if organic aciduria or amino acidopathies are suspected.
    If mitochondrial respiratory chain defects are suspected, vitamins or cofactors may be useful as antioxidants, and in an effort to improve energy production:
      li>Coenzyme Q 4 - 10 mg/kg/dayli>Vitamin C 100 mg/day
    • Riboflavin 100 mg/day
    • Thiamine 100 mg/day
    If organic aciduria is suspected, addition of vitamin cocktail may be useful while awaiting for confirmation of diagnosis:
    • Oral Thiamin 100 mg daily
    • Oral Riboflavin 100 mg daily
    • Oral Biotin 10 mg daily

Algorithm for metabolic acidosis

MMA = Methylmalonic Acidemia, PA = Propionic Acidemia, IVA = Isovaleric Acidemia
MSUD = Maple Syrup Urine Disease, PDH = Pyruvate Dehydrogenase
FAOD = Fatty Acid Oxidation, GSD1 = glycogen Storage Disease type 1
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