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Difficulty: EasyNephrology

A triage nurse urgently calls a physician to evaluate an 8080-year-old man who was just brought to the hospital by his son. The son reports that the patient suddenly developed excruciating abdominal pain two hours ago but now appears extremely lethargic and confused. The patient has a history of medication nonadherence for his paroxysmal atrial fibrillation. On physical examination, the abdomen is soft and non-tender to deep palpation. He is tachypneic with deep, rapid respirations.

Initial laboratory studies show:
Serum Na+\text{Na}^+: 140 mEq/L140 \text{ mEq/L}
Serum Cl−\text{Cl}^-: 100 mEq/L100 \text{ mEq/L}
Serum HCO3−\text{HCO}_3^-: 10 mEq/L10 \text{ mEq/L}

Arterial blood gas analysis on room air reveals a pH of 7.227.22 and a pCO2\text{pCO}_2 of 23 mm Hg23 \text{ mm Hg}.

Based on these findings, which of the following is the most accurate description of this patient's acid-base status?

  1. High anion gap metabolic acidosis with appropriate respiratory compensationAnswer
  2. B
    High anion gap metabolic acidosis with a concurrent primary respiratory alkalosis
  3. C
    High anion gap metabolic acidosis with a concurrent primary respiratory acidosis
  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 presents with a primary metabolic acidosis, evidenced by a pH of 7.22 and a low serum bicarbonate of 10 mEq/L. The next step in any metabolic acidosis is to calculate the anion gap: 140−(100+10)=30 mEq/L140 - (100 + 10) = 30 \text{ mEq/L}. An anion gap greater than 12 indicates a high anion gap metabolic acidosis (HAGMA), which in this patient's clinical context (pain out of proportion to exam, atrial fibrillation off anticoagulation) is highly suspicious for lactic acidosis secondary to acute mesenteric ischemia. Finally, Winter's formula (Expected pCO2=1.5×[HCO3−]+8±2\text{Expected pCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2) is applied to check for a mixed disorder. Calculating (1.5×10)+8(1.5 \times 10) + 8 yields an expected pCO2\text{pCO}_2 of 23 mm Hg23 \text{ mm Hg}. Because the patient's measured pCO2\text{pCO}_2 is exactly 23 mm Hg23 \text{ mm Hg}, the respiratory compensation is appropriate, meaning there is no superimposed secondary respiratory disorder.

Step-by-Step Solution

1
Determine the primary acid-base disorder by looking at the pH and bicarbonate.
The pH is 7.22 (acidemia) and the serum bicarbonate is low at 10 mEq/L, confirming a primary metabolic acidosis.
Acidemia (pH < 7.35) combined with a matched drop in bicarbonate defines a primary metabolic acidosis.
2
Calculate the serum anion gap.
Anion Gap=Na+−(Cl−+HCO3−)=140−(100+10)=30 mEq/L\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (100 + 10) = 30 \text{ mEq/L}.
Determining if the anion gap is normal (8-12 mEq/L) or elevated (>12 mEq/L) restricts the differential diagnosis. Here, the gap is highly elevated, indicating the presence of unmeasured anions (likely lactate due to acute mesenteric ischemia).
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×HCO3−)+8±2=(1.5×10)+8=23±2 mm Hg\text{Expected pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 10) + 8 = 23 \pm 2 \text{ mm Hg}.
Winter's formula evaluates whether the patient's respiratory response (hyperventilation) is adequate for the degree of metabolic acidosis.
4
Compare the measured pCO2 to the expected pCO2.
The measured pCO2\text{pCO}_2 is 23 mm Hg23 \text{ mm Hg}, which falls perfectly within the expected range (21−25 mm Hg21-25 \text{ mm Hg}).
Because the measured value matches the expected value, there is no superimposed secondary respiratory disorder (neither concurrent respiratory acidosis nor alkalosis).

Key Concept

Evaluation of acid-base disorders using the anion gap calculation and Winter's formula.
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Topics

Nephrology

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