Metabolic Acidosis: Causes and Metabolites

Acid-Base Disorders

• Acid-base disorders are pathologic changes in carbon dioxide partial pressure (Pco2) or serum bicarbonate (HCO3−) that typically produce abnormal arterial pH values.

Acidemia and Alkalemia

• Acidemia: serum pH < 7.35
• Alkalemia: serum pH > 7.45

Acidosis and Alkalosis

• Acidosis: physiologic processes that cause acid accumulation or alkali loss
• Alkalosis: physiologic processes that cause alkali accumulation or acid loss

Bicarbonate

• Bicarbonate is the main buffer

Definitions and Main Causes of Acid-Base Disorders

• Metabolic acidosis: process that primarily reduces bicarbonate
  • Causes: excessive H+ formation, reduced H+ excretion, excessive HCO3- loss
• Metabolic alkalosis: process that primarily raises bicarbonate
  • Causes: extracellular fluid volume loss, excessive potassium loss with subsequent hyperaldosteronism
• Respiratory acidosis: process that primarily causes elevation in PaCO2
  • Causes: reduced effective ventilation

Common Causes of Metabolic Acidosis

• Diarrhea: excessive HCO3- loss
• Uremia: phosphoric, sulfuric, and lactic acids
• Diabetic ketoacidosis: increased ketones
• Strenuous exercise: lactic acid
• Methanol: formic acid
• Paraldehyde: β-Hydroxybutyric acid
• Isopropanol: propionic acid
• Ethylene glycol: glycolic acid, and some oxalic and formic acids
• Salicylate/aspirin: sulfasalicylic acid (SSA)

Anion Gap

• Anion gap is the difference between primary measured cations (sodium Na+ and potassium K+) and the primary measured anions (chloride Cl- and bicarbonate HCO3-) in serum
• Reference range: 16 ± 4 mEq/L (if the calculation employs potassium)
• Causes of anion gap acidosis:
  • Normal anion gap: gastrointestinal losses of bicarbonate, renal tubular acidosis, treatment with carbonic anhydrase inhibitors, urinary diversion procedures
  • High anion gap: renal failure, ketoacidosis, lactic acidosis, salicylate poisoning, glycol ingestion, excessive administration of 0.9% saline or propylene glycol, methanol ingestion

Respiratory Alkalosis

• Reduced carbon dioxide due to increased alveolar ventilation
• Buffering processes lower plasma bicarbonate concentration (rapid but limited response, ~1-2 meq/l)
• Kidney response: reduce net acid excretion (eliminate bicarbonate into the urine or decrease ammonium excretion)
• Compensation: kidneys control HCO3-
• Lab values: pH 7.35–7.45, increased CO2 35–45 mm Hg, decreased HCO3- 22–26 mmol/L

Respiratory Acidosis

• Respiratory acidosis is an accumulation of carbon dioxide in the body due to decreased excretion by the lungs
• Decreased carbon dioxide levels lead to pH imbalance
• Most common causes:
  • Pulmonary disease (COPD, asthma, pneumonia, pulmonary edema)
  • Respiratory failure Type 2
• Lab values: pH 7.35–7.45, decreased CO2 35–45 mm Hg, increased HCO3- 22–26 mmol/L

Metabolic Alkalosis

• Metabolic alkalosis is an accumulation of bicarbonate in the body caused by loss of stomach acid
• Excess bicarbonate leads to pH imbalance
• Usually accompanied by hypokalemia