Acidosis, Metabolic, Increased Anion Gap

For further information, see CMDT Part 21-17: Metabolic Acidosis

For further information, see CMDT Part 21-18: Increased Anion Gap Acidosis (Increased Unmeasured Anions)

Key Features

Essentials of Diagnosis

• Hallmark of this disorder is that metabolic acidosis (thus low HCO₃^–) is associated with normal serum Cl^–, so that the anion gap increases

• Decreased HCO₃^– is also seen also in respiratory alkalosis, but pH distinguishes between the two disorders

General Considerations

• Calculation of the anion gap is useful in determining the cause of the metabolic acidosis

• Normochloremic (increased anion gap) metabolic acidosis

  • Generally results from addition to the blood of organic acids such as lactate, acetoacetate, β-hydroxybutyrate, and exogenous toxins

  • Uremia produces an increased anion gap metabolic acidosis via unexcreted organic acids and anions

Etiology

• Lactic acidosis

  • Type A: cardiogenic, septic, or hemorrhagic shock; carbon monoxide or cyanide poisoning

  • Type B: metabolic causes (eg, diabetes mellitus, ketoacidosis, liver disease, kidney disease, infection, leukemia, or lymphoma); toxins (eg, ethanol, methanol, salicylates, isoniazid, or metformin); nucleoside analog reverse transcriptase inhibitors

• Diabetic ketoacidosis

• Alcoholic ketoacidosis

  • Acid-base disorders in alcoholism are frequently mixed (10% have triple acid-base disorder)

  • Three types of metabolic acidoses: ketoacidosis, lactic acidosis, and hyperchloremic acidosis from bicarbonate loss in urine from ketonuria

  • Metabolic alkalosis from volume contraction and vomiting

  • Respiratory alkalosis from alcohol withdrawal, pain, sepsis, or liver disease

• Uremic acidosis (usually at glomerular filtration rate < 15–30 mL/min)

• Ethylene glycol toxicity

• Methanol toxicity

• Salicylate toxicity (mixed metabolic acidosis with respiratory alkalosis)

Clinical Findings

Symptoms and Signs

• Symptoms are mainly those of the underlying disorder

• Compensatory hyperventilation may be misinterpreted as a primary respiratory disorder

• When severe, Kussmaul respirations (deep, regular, sighing respirations indicating intense stimulation of the respiratory center) occur

Diagnosis

Laboratory Tests

• See Table 21–13

• Blood pH, serum HCO₃^–, and PCO₂ are decreased

• Anion gap is increased (normochloremic)

• Hyperkalemia may be seen

• In lactic acidosis, lactate levels are at least 4–5 mEq/L but commonly 10–30 mEq/L

• The diagnosis of alcoholic ketoacidosis is supported by the absence of a diabetic history and no evidence of glucose intolerance after initial therapy