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Difficulty: HardNephrology

A 54-year-old woman is brought to the emergency department due to 3 days of fever, worsening abdominal pain, and profound lethargy. She has a history of Crohn's disease and underwent an extensive ileal resection two years ago. For the past week, she has had severe, voluminous watery diarrhea and has been unable to tolerate oral fluids. Her temperature is 38.9°C (102.0°F), blood pressure is 86/50 mm Hg, pulse is 122/min, and respirations are 24/min. Physical examination shows dry mucous membranes, decreased skin turgor, and diffuse abdominal tenderness with guarding.

TestResult
Arterial blood gas (room air)
pH7.26
pCO2pCO_223 mm Hg
pO2pO_288 mm Hg
Serum chemistries
Sodium135 mEq/L
Potassium3.2 mEq/L
Chloride105 mEq/L
Bicarbonate10 mEq/L
Blood urea nitrogen42 mg/dL
Creatinine2.3 mg/dL
Lactate5.5 mmol/L

Which of the following best characterizes this patient's acid-base status?

  1. A
    Elevated anion gap metabolic acidosis with appropriate respiratory compensation
  2. B
    Mixed elevated anion gap and normal anion gap metabolic acidosis with concurrent respiratory alkalosis
  3. C
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  4. Mixed elevated anion gap and normal anion gap metabolic acidosis with appropriate respiratory compensationAnswer
  5. E
    Mixed elevated anion gap metabolic acidosis and metabolic alkalosis with appropriate respiratory compensation

Answer

Mixed elevated anion gap and normal anion gap metabolic acidosis with appropriate respiratory compensation
This patient presents in hypovolemic shock with a severe mixed acid-base derangement. First, calculate the anion gap (AG): Na+−(Cl−+HCO3−)=135−(105+10)=20 mEq/LNa^+ - (Cl^- + HCO_3^-) = 135 - (105 + 10) = 20 \text{ mEq/L}. An AG > 12 confirms a primary elevated anion gap metabolic acidosis (AGMA), which is driven here by lactic acidosis (lactate of 5.5 mmol/L) secondary to tissue hypoperfusion. Second, perform a delta gap calculation to evaluate for additional metabolic disorders. The excess AG is 20−12=820 - 12 = 8. In a pure AGMA, the bicarbonate level should drop by 1 mEq/L for every 1 mEq/L increase in the AG. Therefore, the expected HCO3−HCO_3^- is 24−8=16 mEq/L24 - 8 = 16 \text{ mEq/L}. Because the patient's measured HCO3−HCO_3^- (10 mEq/L) is significantly lower than the expected 16 mEq/L, there must be a concurrent process further depleting bicarbonate. This indicates an underlying normal anion gap metabolic acidosis (NAGMA), which aligns clinically with her severe, voluminous diarrhea and history of extensive ileal resection. Finally, calculate the expected respiratory compensation using Winter's formula: Expected pCO2=(1.5×measured HCO3−)+8±2=(1.5×10)+8±2=23±2 mm HgExpected\ pCO_2 = (1.5 \times measured\ HCO_3^-) + 8 \pm 2 = (1.5 \times 10) + 8 \pm 2 = 23 \pm 2 \text{ mm Hg}. The measured pCO2pCO_2 is exactly 23 mm Hg, indicating that her respiratory compensation is perfectly appropriate for her degree of acidemia.

Step-by-Step Solution

1
Calculate the Anion Gap (AG).
AG=Na+−(Cl−+HCO3−)=135−(105+10)=20 mEq/LAG = Na^+ - (Cl^- + HCO_3^-) = 135 - (105 + 10) = 20 \text{ mEq/L}.
An AG > 12 establishes the presence of a primary elevated anion gap metabolic acidosis (AGMA).
2
Calculate the Delta Gap to identify mixed metabolic disorders.
Excess AG = 20−12=8 mEq/L20 - 12 = 8 \text{ mEq/L}. Expected HCO3−=24−8=16 mEq/LHCO_3^- = 24 - 8 = 16 \text{ mEq/L}. Measured HCO3−HCO_3^- (10 mEq/L) is less than the expected HCO3−HCO_3^-.
Because the measured bicarbonate is significantly lower than the expected value for a pure AGMA, a concurrent normal anion gap metabolic acidosis (NAGMA) is also present.
3
Calculate expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×10)+8±2=23±2 mm HgpCO_2 = (1.5 \times 10) + 8 \pm 2 = 23 \pm 2 \text{ mm Hg}. The measured pCO2pCO_2 is exactly 23 mm Hg.
Since the measured pCO2pCO_2 falls exactly within the expected range, the respiratory response is appropriate, ruling out an independent respiratory disorder.

Key Concept

Interpretation of complex mixed acid-base disorders using the anion gap, delta-delta gap, and Winter's formula.
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Topics

Nephrology

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