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Difficulty: EasyNephrology

A 32-year-old woman presents to the urgent care clinic with profound weakness and lightheadedness. She reports having large-volume, watery stools up to 8 times a day for the past 4 days after returning from a camping trip. She has not been able to tolerate oral intake due to nausea.

Vital signs show a temperature of 37.1°C (98.8°F), blood pressure of 98/60 mm Hg, pulse of 115/min, and respiratory rate of 22/min. Physical examination reveals dry mucous membranes, flat neck veins, and delayed capillary refill.

Laboratory testing reveals the following:

TestResult
Sodium136 mEq/L
Potassium3.1 mEq/L
Chloride111 mEq/L
Bicarbonate15 mEq/L
Blood urea nitrogen (BUN)30 mg/dL
Creatinine1.2 mg/dL
Arterial blood gas analysis on room air:
TestResult
pH7.26
pCO2pCO_230 mm Hg
pO2pO_295 mm Hg

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

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

Answer

Normal anion gap metabolic acidosis with appropriate respiratory compensation
This patient has a history of severe diarrhea, which causes profound gastrointestinal loss of bicarbonate, leading to a metabolic acidosis. The step-wise evaluation of her acid-base status confirms this diagnosis:

1. Primary Disorder: The arterial pH is 7.26, indicating acidemia. The serum bicarbonate (HCO3−HCO_3^-) is appropriately decreased at 15 mEq/L, confirming a primary metabolic acidosis.

2. Anion Gap: The serum anion gap is calculated as Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-). For this patient, the gap is 136−(111+15)=10136 - (111 + 15) = 10 mEq/L. Since normal is typically 8-12 mEq/L, she has a normal anion gap metabolic acidosis (NAGMA).

3. Compensation: To determine if her respiratory compensation is adequate, Winter's formula is used: Expected pCO2=(1.5×[HCO3−])+8±2pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2.
Expected pCO2=(1.5×15)+8±2=30.5±2pCO_2 = (1.5 \times 15) + 8 \pm 2 = 30.5 \pm 2 mm Hg.
Because her measured pCO2pCO_2 is exactly 30 mm Hg, her respiratory response falls perfectly within the expected range (28.5 to 32.5 mm Hg). Thus, she has appropriate respiratory compensation with no superimposed mixed acid-base disorders.

Step-by-Step Solution

1
Identify the primary acid-base disorder using the pH and standard laboratory values.
The pH is 7.26 (acidemia). The HCO3−HCO_3^- is low (15 mEq/L).
Matching the low pH with a low bicarbonate level confirms that the primary disturbance is a metabolic acidosis.
2
Calculate the serum anion gap to determine the etiology of the metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 136−(111+15)=10136 - (111 + 15) = 10 mEq/L.
An anion gap of 10 mEq/L is within the normal range (8-12 mEq/L), indicating a normal anion gap metabolic acidosis (NAGMA), which is consistent with this patient's history of severe diarrhea.
3
Apply Winter's formula to determine if the respiratory compensation is appropriate.
Expected pCO2=(1.5×15)+8±2=22.5+8±2=30.5±2pCO_2 = (1.5 \times 15) + 8 \pm 2 = 22.5 + 8 \pm 2 = 30.5 \pm 2 mm Hg (range: 28.5 to 32.5 mm Hg).
Because the patient's measured pCO2pCO_2 (30 mm Hg) falls within this expected range, there is purely appropriate respiratory compensation without a superimposed respiratory disorder.

Key Concept

The step-wise approach to acid-base disorders requires identifying the primary disturbance via pH, calculating the anion gap for metabolic acidosis, and utilizing Winter's formula (Expected pCO2=1.5×[HCO3−]+8±2pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2) to check for appropriate respiratory compensation.
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Topics

Nephrology

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