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Difficulty: MediumNephrology

A 32-year-old woman presents to the ambulatory care clinic reporting four days of frequent, large-volume, watery stools after returning from an international trip. During the interview, she is noticeably distressed and pacing the room. She states she is terrified of acquiring a severe parasitic infection and has been breathing rapidly for the past hour. Physical examination reveals dry mucous membranes and hyperactive bowel sounds. Vital signs include a heart rate of 112/min112/\text{min}, blood pressure of 105/65 mm Hg105/65\ \text{mm Hg}, and respiratory rate of 30/min30/\text{min}. Laboratory results are as follows:

Serum sodium: 138 mEq/L138\ \text{mEq/L}
Serum potassium: 3.4 mEq/L3.4\ \text{mEq/L}
Serum chloride: 116 mEq/L116\ \text{mEq/L}
Serum bicarbonate: 12 mEq/L12\ \text{mEq/L}

Arterial blood gas (room air):
pH: 7.387.38
pCO2pCO_2: 21 mm Hg21\ \text{mm Hg}

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

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

Answer

Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis
The correct answer accurately synthesizes all steps of the acid-base algorithm. The pH is 7.38 (primary acidosis). The low bicarbonate (12 mEq/L) identifies the acidosis as metabolic. The anion gap is 10 mEq/L, defining it as a normal anion gap (hyperchloremic) metabolic acidosis, consistent with bicarbonate loss from diarrhea. Finally, applying Winter's formula yields an expected pCO2 of 24 to 28 mm Hg. The patient's actual pCO2 of 21 mm Hg is significantly lower than this range, indicating she is hyperventilating beyond what is required for compensation. This confirms a superimposed primary respiratory alkalosis, clinically correlating with her acute distress and tachypnea.

Step-by-Step Solution

1
Determine the primary acid-base disorder by evaluating the pH.
The pH is 7.387.38, which is less than 7.407.40 (acidemia).
The direction of the pH deviation from normal (7.40) always indicates the primary disorder. Therefore, the primary process is an acidosis.
2
Examine the serum bicarbonate and pCO2pCO_2 to identify the source of the acidosis.
Serum bicarbonate is low (12 mEq/L12\ \text{mEq/L}) and pCO2pCO_2 is low (21 mm Hg21\ \text{mm Hg}).
A primary metabolic acidosis is characterized by a low bicarbonate level. The decreased pCO2pCO_2 indicates a respiratory response.
3
Calculate the serum anion gap.
Anion Gap=Na−(Cl+HCO3)=138−(116+12)=10 mEq/L\text{Anion Gap} = \text{Na} - (\text{Cl} + \text{HCO}_3) = 138 - (116 + 12) = 10\ \text{mEq/L}.
Determining whether the metabolic acidosis is an anion gap or non-anion gap process narrows the differential. An AG of 1010 is normal (reference range 8−12 mEq/L8-12\ \text{mEq/L}). Severe diarrhea typically causes a normal anion gap metabolic acidosis due to direct bicarbonate loss.
4
Determine if the respiratory compensation is appropriate using Winter's formula.
Expected pCO2=(1.5×12)+8±2=18+8±2=26±2\text{Expected } pCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2 (range 24−28 mm Hg24-28\ \text{mm Hg}).
Winter's formula calculates the expected respiratory compensation for a given degree of metabolic acidosis to identify superimposed respiratory disorders.
5
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2.
The actual pCO2pCO_2 (21 mm Hg21\ \text{mm Hg}) is lower than the expected range (24−28 mm Hg24-28\ \text{mm Hg}).
Because the patient is blowing off more CO2CO_2 than expected for simple compensation, there is a coexisting primary respiratory alkalosis, likely driven by her acute anxiety and hyperventilation.

Key Concept

Diagnosis of mixed acid-base disorders requires sequential analysis: determining the primary disorder via pH, calculating the anion gap, and verifying respiratory compensation using Winter's formula.
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Topics

Nephrology

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