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Difficulty: HardNephrology

A 29-year-old woman is brought to the emergency department by her husband due to progressive lethargy and profound weakness. For the past 4 days, she has had severe, voluminous watery diarrhea and poor oral intake following a sick contact. Her medical history is significant for idiopathic intracranial hypertension, for which she takes acetazolamide. Her temperature is 37.2°C (99.0°F), blood pressure is 98/60 mmHg, pulse is 112/min, and respiratory rate is 16/min. Physical examination shows dry mucous membranes, decreased skin turgor, and generalized diminished muscle strength in all extremities.

Laboratory studies show the following:
Serum sodium: 136 mEq/L
Serum potassium: 2.2 mEq/L
Serum chloride: 116 mEq/L
Serum bicarbonate: 10 mEq/L
Blood urea nitrogen: 42 mg/dL
Serum creatinine: 1.6 mg/dL

Arterial blood gas (room air):
pH: 7.11
pCO2pCO_2: 32 mmHg
pO2pO_2: 88 mmHg

Which of the following best describes this patient's primary acid-base disturbances?

  1. Normal anion gap metabolic acidosis and concurrent respiratory acidosisAnswer
  2. B
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Normal anion gap metabolic acidosis and concurrent respiratory alkalosis
  4. D
    High anion gap metabolic acidosis and concurrent respiratory acidosis
  5. E
    High anion gap metabolic acidosis with appropriate respiratory compensation

Answer

Normal anion gap metabolic acidosis and concurrent respiratory acidosis
The correct answer accurately identifies a mixed acid-base disorder. The patient's severe diarrhea and acetazolamide use (a carbonic anhydrase inhibitor) both independently cause renal and gastrointestinal bicarbonate losses, resulting in a primary normal anion gap metabolic acidosis (AG=136−126=10AG = 136 - 126 = 10). To determine if respiratory compensation is adequate, Winter's formula is applied: Expected pCO2=1.5×[HCO3−]+8±2pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2. For a bicarbonate of 10 mEq/L, the expected pCO2pCO_2 is 23±223 \pm 2 mmHg. The patient's actual pCO2pCO_2 is 32 mmHg. Because the actual pCO2pCO_2 is higher than the calculated expected range, the patient is relatively hypoventilating, meaning a secondary respiratory acidosis is present. This is a classic presentation of profound hypokalemia (2.2 mEq/L) causing skeletal muscle and diaphragmatic weakness, leading to respiratory fatigue and failure to mount the necessary compensatory hyperventilation.

Step-by-Step Solution

1
Identify the primary disturbance by evaluating the pH and serum bicarbonate.
The pH is 7.11 (acidemia) and bicarbonate is markedly low (10 mEq/L), establishing a primary metabolic acidosis.
Initial categorization of the acid-base disorder requires assessing the direction of the pH and the primary driver (HCO3−HCO_3^- or pCO2pCO_2).
2
Calculate the serum anion gap to classify the metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)=136−(116+10)=10Na^+ - (Cl^- + HCO_3^-) = 136 - (116 + 10) = 10 mEq/L.
An anion gap of 10 mEq/L is within the normal limits (8-12 mEq/L), confirming a normal anion gap metabolic acidosis (NAGMA), consistent with bicarbonate losses from severe diarrhea and acetazolamide use.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×10)+8±2=23±2pCO_2 = (1.5 \times 10) + 8 \pm 2 = 23 \pm 2 mmHg.
Winter's formula determines if the respiratory response to metabolic acidosis is appropriate. The expected range is 21 to 25 mmHg.
4
Compare the expected pCO2pCO_2 with the patient's actual pCO2pCO_2.
The actual pCO2pCO_2 is 32 mmHg, which is greater than the maximum expected value of 25 mmHg.
An actual pCO2pCO_2 higher than the expected range indicates a concurrent, secondary respiratory acidosis. In this patient, the profound hypokalemia (2.2 mEq/L) likely caused diaphragmatic/respiratory muscle weakness, preventing adequate hyperventilation.

Key Concept

Diagnosis of mixed acid-base disorders requires sequential evaluation: identifying the primary disturbance, calculating the anion gap for metabolic acidosis, and utilizing Winter's formula to uncover superimposed respiratory disorders.
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Topics

Nephrology

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