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 P_CO2 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 P_CO2
      • 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 P_CO2
      • 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