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Difficulty: EasyNephrology

A 25-year-old woman with a history of type 1 diabetes mellitus presents to the emergency department with generalized weakness, nausea, and abdominal pain for the past 24 hours. She has been unable to eat and has missed her last two doses of insulin. Her blood pressure is 110/70 mm Hg, pulse is 115/min, and respirations are 24/min. Physical examination shows dry mucous membranes and a fruity odor to her breath.

Laboratory studies show:
Serum sodium: 140 mEq/L
Serum potassium: 4.8 mEq/L
Serum chloride: 104 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 28 mg/dL
Serum creatinine: 1.2 mg/dL
Plasma glucose: 450 mg/dL

Arterial blood gas analysis on room air:
pH: 7.25
pCO2: 26 mm Hg
pO2: 95 mm Hg

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

  1. Primary high anion gap metabolic acidosis with appropriate respiratory compensationAnswer
  2. B
    Primary high anion gap metabolic acidosis with concurrent respiratory alkalosis
  3. C
    Primary high anion gap metabolic acidosis with concurrent respiratory acidosis
  4. D
    Primary normal anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Mixed high anion gap metabolic acidosis and primary metabolic alkalosis

Answer

Primary high anion gap metabolic acidosis with appropriate respiratory compensation
This patient presents with classic signs of diabetic ketoacidosis, confirmed by a low pH (7.257.25) and low serum bicarbonate (1212 mEq/L), indicating a primary metabolic acidosis. The anion gap is calculated as Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-), which is 140−(104+12)=24140 - (104 + 12) = 24 mEq/L. Because the normal anion gap is 88 to 1212 mEq/L, this is a high anion gap metabolic acidosis (HAGMA). To determine if respiratory compensation is appropriate, Winter's formula must be applied: Expected pCO2=(1.5×HCO3−)+8±2pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, Expected pCO2=(1.5×12)+8=18+8=26±2pCO_2 = (1.5 \times 12) + 8 = 18 + 8 = 26 \pm 2 mm Hg. The measured pCO2 is 2626 mm Hg, which falls exactly within this range. Therefore, the patient has a primary HAGMA with appropriate respiratory compensation.

Step-by-Step Solution

1
Identify the primary acid-base disturbance using the pH and serum bicarbonate.
pH is 7.25 (acidemia) and bicarbonate is 12 mEq/L (low), indicating a primary metabolic acidosis.
Determining the primary disorder is the first step in basic acid-base analysis.
2
Calculate the anion gap.
Anion Gap = 140−(104+12)=24140 - (104 + 12) = 24 mEq/L.
Metabolic acidosis must be categorized into high anion gap or normal anion gap to narrow down the differential diagnosis.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×12)+8=26±2pCO_2 = (1.5 \times 12) + 8 = 26 \pm 2 mm Hg.
Calculating the expected pCO2 determines if the physiological respiratory response is adequate or if a secondary respiratory disorder exists.
4
Compare the measured pCO2 to the expected pCO2.
The measured pCO2 (26 mm Hg) is exactly within the expected range (24 to 28 mm Hg).
Because the measured pCO2 matches the expected value, there is appropriate respiratory compensation without a superimposed respiratory disorder.

Key Concept

Evaluation of acid-base disorders using the anion gap and Winter's formula to determine primary disturbances and physiological compensation.
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Topics

Nephrology

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