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Zorluk: OrtaNephrology

A 65-year-old woman with a history of severe chronic obstructive pulmonary disease (COPD) is brought to the emergency department due to worsening shortness of breath, productive cough, and confusion over the past 3 days. She has also had decreased oral intake. Her temperature is 38.6°C (101.5°F), blood pressure is 90/55 mm Hg, pulse is 120/min, and respiratory rate is 28/min. Physical examination reveals bilateral diffuse wheezing and coarse crackles at the right lung base.

Laboratory studies show:
Serum sodium: 140 mEq/L
Serum potassium: 4.2 mEq/L
Serum chloride: 102 mEq/L
Serum bicarbonate: 14 mEq/L
Blood urea nitrogen: 35 mg/dL
Serum creatinine: 1.8 mg/dL (baseline 0.9 mg/dL)
Serum lactate: 4.5 mmol/L (normal: 0.5-2.2 mmol/L)

Arterial blood gas analysis on room air reveals:
pH: 7.21
pCO2pCO_2: 36 mm Hg
pO2pO_2: 58 mm Hg

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

  1. A
    High anion gap metabolic acidosis with appropriate respiratory compensation
  2. B
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  3. High anion gap metabolic acidosis with concurrent respiratory acidosisCevap
  4. D
    High anion gap metabolic acidosis with concurrent respiratory alkalosis
  5. E
    Normal anion gap metabolic acidosis with concurrent respiratory acidosis

Cevap

High anion gap metabolic acidosis with concurrent respiratory acidosis
The patient's anion gap is 140−(102+14)=24140 - (102 + 14) = 24 mEq/L, confirming a high anion gap metabolic acidosis. Applying Winter's formula, the expected pCO2pCO_2 is (1.5×14)+8±2=29±2(1.5 \times 14) + 8 \pm 2 = 29 \pm 2 mm Hg (range 27-31 mm Hg). Because the patient's actual pCO2pCO_2 (36 mm Hg) is higher than the expected range, she has a concomitant primary respiratory acidosis, likely secondary to underlying COPD and respiratory fatigue from her severe infection.

Adım Adım Çözüm

1
Calculate the serum anion gap to determine the primary metabolic disorder.
Anion gap = 140−(102+14)=24140 - (102 + 14) = 24 mEq/L.
An elevated anion gap (>12 mEq/L) indicates a primary high anion gap metabolic acidosis, which in this patient is likely due to lactic acidosis secondary to tissue hypoperfusion and sepsis.
2
Use Winter's formula to calculate the expected pCO2pCO_2 for the degree of metabolic acidosis.
Expected pCO2=(1.5×14)+8±2=27pCO_2 = (1.5 \times 14) + 8 \pm 2 = 27 to 3131 mm Hg.
Winter's formula determines if the respiratory compensation (hyperventilation) is appropriate for a simple metabolic acidosis.
3
Compare the expected pCO2pCO_2 to the patient's measured pCO2pCO_2.
Measured pCO2pCO_2 (36 mm Hg) is greater than the expected range (27-31 mm Hg).
An inappropriately elevated pCO2pCO_2 indicates inadequate alveolar ventilation, diagnosing a concurrent primary respiratory acidosis. This is common in patients with severe COPD and respiratory fatigue.

Anahtar Kavram

In a patient with metabolic acidosis, Winter's formula (1.5×HCO3−+8±21.5 \times HCO_3^- + 8 \pm 2) is used to calculate the expected pCO2pCO_2. If the measured pCO2pCO_2 is higher than expected, there is a concurrent primary respiratory acidosis.
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Nephrology

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