Updated: 11/10/2020

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 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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(M1.RL.14.8) A 66-year-old man is brought to the emergency department by his daughter because of 3 days of fever, chills, cough, and shortness of breath. The cough is productive of yellow sputum. His symptoms have not improved with rest and guaifenesin. His past medical history is significant for hypertension, for which he takes hydrochlorothiazide. He has a 30-pack-year history of smoking. His temperature is 38.9 C (102.0 F), blood pressure 88/56 mm Hg, and heart rate 105/min. Following resuscitation with normal saline, his blood pressure improves to 110/70 mm Hg. His arterial blood gas is as follows:

Blood pH 7.52, PaO2 74 mm Hg, PaCO2 28 mm Hg, and HCO3- 21 mEq/L.

Which of the following acid-base disturbances best characterizes this patient's condition?
Tested Concept

QID: 104282
1

Normal acid-base status

2%

(3/142)

2

Metabolic acidosis

1%

(2/142)

3

Metabolic alkalosis

15%

(21/142)

4

Respiratory acidosis

2%

(3/142)

5

Respiratory alkalosis

77%

(110/142)

M 1 E

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(M1.RL.13.21) A 35-year-old patient with a history of diabetes presents to the ED with a myriad of systemic complaints. An arterial blood gas shows serum pH = 7.3, HCO3- = 13 mEq/L, PCO2 = 27 mmHg. Which of the following would you LEAST expect to observe in this patient? Tested Concept

QID: 100975
1

Increased anion gap

10%

(16/160)

2

Increased serum ketones

4%

(7/160)

3

Increased urine output

11%

(17/160)

4

Decreased respiratory rate

59%

(95/160)

5

Increased serum potassium

14%

(23/160)

M 2 D

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