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Updated: Jan 7 2021

Acid-Base Differential Diagnosis

  • Introduction
    • Acid-base disorders are commonly seen throughout medicine
      • acidosis results in an abnormally elevated H+ concentration in the blood (thus called acidemia)
      • alkalosis results in an abnormally decreased H+ concentration in the blood (thus called alkalemia)
    • These acid-base disorders may be caused by
      • a primary disturbance in HCO3- resulting in a metabolic acidosis or alkalosis
      • a primary disturbance in PCO2 resulting in a respiratory acidosis or alkalosis
    • Simple acid-base disorders
      • blood pH abnormalities caused by one acid base disorder
      • metabolic acidosis
        • due to a decrease in blood HCO3-
          • secondary to impaired H+ excretion, fixed H+ ingestion, or increased H+ production
            • look at "anion gap" below
      • metabolic alkalosis
        • due to an increase in blood HCO3-
          • secondary to increased HCO3- intake or loss of fixed H+
            • loop diuretics (e.g., furosemide)
            • vomiting and nasogastric tube suction
            • antacid use
            • hyperaldosteronism
            • laxative abuse
              • mild abuse results in alkalosis due to hypokalemia and the resulting shift of H+ into the cells
              • if abuse is severe, metabolic acidosis results due to the excessive diarrhea and loss of HCO3-
      • respiratory acidosis
        • due to an increase in PCO2
          • due to hypoventilation
            • airway obstruction (e.g., epiglottitis)
            • acute lung disease
            • chronic lung disease
            • opioids and sedatives
            • respiratory muscle weakness
      • respiratory alkalosis
        • due to a decrease in PCO2
          • due to hyperventilation
            • normal pregnancy
            • hysteria
            • hypoxemia (e.g., high altitude)
            • salicylates (early)
            • pulmonary embolism
            • pneumonia
    • Anion gap
      • under normal conditions, anions and cations are equal to each other within any body fluid compartment (e.g., plasma)
        • the major cation that is measured is Na+
        • the major anions that are measured is HCO3- and Cl-
        • because there is more Na+ than HCO3- and Cl- there are unmeasured anions that allow us to achieve electroneutrality
          • unmeasured plasma anions include plasma proteins, citrate, phosphate, and sulfate
        • anion gap = Na+ - (Cl- + HCO3-)
          • normal = 8-16 mEq/L
      • anion gap is very useful for narrowing the differential diagnosis of metabolic acidosis
        • normal anion gap metabolic acidosis (also called hyperchloremic metabolic acidosis with a normal anion gap)
          • Hyperalimentation
          • Addison's disease
          • Renal tubular acidosis
          • Diarrhea
          • Acetazolamide
          • Spironolactone
          • Saline infusion
          • mnemonic: HARDASS
        • elevated anion gap metabolic acidosis
          • Methanol (formic acid)
          • Uremia
          • Diabetic ketoacidosis
          • Propylene glycol
          • Iron tablets or isoniazid
          • Lactic acidosis (such as by metformin toxicity)
          • Ethylene glycol
          • Salicylates (late)
          • mnemonic: MUDPILES
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