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Difficulty: MediumNephrology

A 35-year-old woman presents to the emergency department with profound weakness in her legs that began earlier today. She reports difficulty standing from a seated position. Her medical history is significant for Sjögren syndrome and recurrent nephrolithiasis. She denies recent diarrhea, vomiting, or changes in her medications. Her temperature is 37.0°C (98.6°F), blood pressure is 110/70 mm Hg, pulse is 88/min, and respiratory rate is 16/min with shallow breaths. Physical examination reveals 2/5 motor strength in the bilateral lower extremities and diminished deep tendon reflexes.

Laboratory results are as follows:

TestValue
Sodium138 mEq/L
Potassium2.1 mEq/L
Chloride114 mEq/L
Bicarbonate14 mEq/L
Blood urea nitrogen14 mg/dL
Creatinine0.9 mg/dL

Arterial blood gas on room air:

TestValue
pH7.23
PCO2PCO_234 mm Hg
PO2PO_285 mm Hg

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

  1. Normal anion gap metabolic acidosis and respiratory acidosisAnswer
  2. B
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Normal anion gap metabolic acidosis and respiratory alkalosis
  4. D
    High anion gap metabolic acidosis and respiratory acidosis
  5. E
    Mixed high anion gap and normal anion gap metabolic acidosis

Answer

Normal anion gap metabolic acidosis and respiratory acidosis
This patient's primary disturbance is a normal anion gap metabolic acidosis (NAGMA), evidenced by a low bicarbonate (14 mEq/L), low pH (7.23), and a normal anion gap (138−[114+14]=10138 - [114 + 14] = 10 mEq/L). In the context of Sjögren syndrome and severe hypokalemia, this is highly characteristic of Type 1 (distal) renal tubular acidosis. To assess respiratory compensation, Winter's formula must be applied: Expected PCO2=(1.5×14)+8±2=29±2Expected\ PCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 mm Hg (range 27-31 mm Hg). Because the patient's measured PCO2PCO_2 is 34 mm Hg, which is higher than the expected range, she has a concomitant respiratory acidosis. This occurs physiologically because her profound hypokalemia (K+=2.1K^+ = 2.1 mEq/L) has caused respiratory muscle weakness (evidenced by shallow breaths and limb weakness), limiting her ability to generate the deep, rapid breathing (Kussmaul respirations) required for adequate respiratory compensation.

Step-by-Step Solution

1
Calculate the serum anion gap.
AG=Na+−(Cl−+HCO3−)=138−(114+14)=10AG = Na^+ - (Cl^- + HCO_3^-) = 138 - (114 + 14) = 10 mEq/L.
To determine whether the primary metabolic acidosis is high anion gap or normal anion gap. An AG of 10 is normal, confirming a normal anion gap metabolic acidosis (NAGMA).
2
Calculate the expected respiratory compensation using Winter's formula.
Expected PCO2=(1.5×14)+8±2=21+8±2=29±2Expected\ PCO_2 = (1.5 \times 14) + 8 \pm 2 = 21 + 8 \pm 2 = 29 \pm 2 mm Hg (Range: 27-31 mm Hg).
To assess if the physiological respiratory response to the metabolic acidosis is appropriate, or if a secondary respiratory disorder exists.
3
Compare the actual PCO2PCO_2 to the expected PCO2PCO_2 range.
Actual PCO2PCO_2 (34 mm Hg) is greater than the maximum expected PCO2PCO_2 (31 mm Hg).
Because the actual PCO2PCO_2 is higher than expected, the patient is relatively hypoventilating, indicating a superimposed respiratory acidosis.

Key Concept

Acid-base interpretation using Winter's formula to identify a mixed normal anion gap metabolic acidosis and respiratory acidosis.
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Topics

Nephrology

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