Updated: 1/7/2021

Acid-Base Differential Diagnosis

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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

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80%	4	E
100%	5	M
Toss	0	T

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