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Zorluk: OrtaNephrology

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 acidosisCevap
  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

Cevap

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.

Adım Adım Çözüm

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.

Anahtar Kavram

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

Nephrology

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