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Difficulty: EasyNephrology

A 50-year-old male presents to the outpatient clinic reporting progressive weakness and a metallic taste in his mouth over the last several weeks. He has a 15-year history of poorly controlled hypertension. His blood pressure is 165/95 mm Hg165/95 \text{ mm Hg}, pulse is 84/min84/\text{min}, and respiratory rate is 22/min22/\text{min}. Physical examination reveals trace bilateral lower extremity edema and a pale complexion. Laboratory studies are obtained and reveal the following:

Serum chemistry:
Na+Na^+: 140 mEq/L140 \text{ mEq/L}
K+K^+: 5.2 mEq/L5.2 \text{ mEq/L}
Cl−Cl^-: 102 mEq/L102 \text{ mEq/L}
HCO3−HCO_3^-: 12 mEq/L12 \text{ mEq/L}
Blood urea nitrogen: 85 mg/dL85 \text{ mg/dL}
Serum creatinine: 5.5 mg/dL5.5 \text{ mg/dL}

Arterial blood gas (room air):
pH: 7.257.25
pCO2pCO_2: 26 mm Hg26 \text{ mm Hg}
pO2pO_2: 95 mm Hg95 \text{ mm Hg}

Based on these findings, which of the following best describes this patient's current acid-base status?

  1. High anion gap metabolic acidosis with appropriate respiratory compensationAnswer
  2. B
    High anion gap metabolic acidosis with superimposed respiratory acidosis
  3. C
    High anion gap metabolic acidosis with superimposed respiratory alkalosis
  4. D
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Primary respiratory alkalosis with appropriate metabolic compensation

Answer

High anion gap metabolic acidosis with appropriate respiratory compensation
This patient has a high anion gap metabolic acidosis secondary to advanced chronic kidney disease (uremia). The diagnosis requires a systematic approach to the arterial blood gas and serum chemistry. The patient is acidemic (pH=7.25pH = 7.25). The primary driver of the acidemia is the low bicarbonate (12 mEq/L12 \text{ mEq/L}), confirming a metabolic acidosis. Next, the anion gap is calculated as Na+−(Cl−+HCO3−)=140−(102+12)=26 mEq/LNa^+ - (Cl^- + HCO_3^-) = 140 - (102 + 12) = 26 \text{ mEq/L}. Because this is significantly higher than the normal range (8−12 mEq/L8-12 \text{ mEq/L}), it is a high anion gap metabolic acidosis. Finally, Winter's formula (1.5×HCO3−+8±21.5 \times HCO_3^- + 8 \pm 2) is used to assess for a mixed disorder. The expected pCO2pCO_2 is (1.5×12)+8=26±2 mm Hg(1.5 \times 12) + 8 = 26 \pm 2 \text{ mm Hg} (range 24−28 mm Hg24-28 \text{ mm Hg}). Since the patient's measured pCO2pCO_2 (26 mm Hg26 \text{ mm Hg}) matches the expected value perfectly, this is a simple, fully compensated high anion gap metabolic acidosis.

Step-by-Step Solution

1
Determine the primary acid-base disturbance by evaluating the pH, pCO2, and bicarbonate.
The pH is 7.257.25 (acidemia), pCO2pCO_2 is 26 mm Hg26 \text{ mm Hg} (low), and HCO3−HCO_3^- is 12 mEq/L12 \text{ mEq/L} (low).
An acidic pH paired with a low bicarbonate identifies the primary process as a metabolic acidosis.
2
Calculate the serum anion gap using the formula: AG=Na+−(Cl−+HCO3−)AG = Na^+ - (Cl^- + HCO_3^-).
AG=140−(102+12)=26 mEq/LAG = 140 - (102 + 12) = 26 \text{ mEq/L}.
Differentiating between a normal anion gap (8−12 mEq/L8-12 \text{ mEq/L}) and a high anion gap guides the differential diagnosis. An AG of 26 mEq/L26 \text{ mEq/L} confirms a high anion gap metabolic acidosis, likely secondary to uremia given the elevated BUN and creatinine.
3
Apply Winter's formula to determine the expected respiratory compensation: Expected pCO2=(1.5×HCO3−)+8±2Expected\ pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2.
Expected pCO2=(1.5×12)+8±2=18+8±2=26±2 mm HgExpected\ pCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2 \text{ mm Hg} (range 24−28 mm Hg24-28 \text{ mm Hg}).
Calculating expected pCO2pCO_2 is necessary to identify if a secondary, superimposed respiratory disorder is present.
4
Compare the measured pCO2pCO_2 with the calculated expected pCO2pCO_2 range.
The measured pCO2pCO_2 is 26 mm Hg26 \text{ mm Hg}, which is exactly equal to the calculated expected value.
Because the measured value falls perfectly within the expected range, there is appropriate respiratory compensation without a superimposed respiratory disorder.

Key Concept

Evaluation of metabolic acidosis requires a step-wise approach: first calculating the anion gap, followed by calculating the expected respiratory compensation using Winter's formula (Expected pCO2=1.5×HCO3−+8±2Expected\ pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2).
Estimated Time:1m 0s
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Nephrology

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