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Difficulty: HardNephrology

Emergency medical responders transport a 6262-year-old woman to the emergency department due to progressive lethargy and a four-day history of a productive cough. Her past medical history is significant for severe chronic obstructive pulmonary disease. Upon arrival, her temperature is 38.8∘C38.8^\circ\text{C} (101.8∘F101.8^\circ\text{F}), blood pressure is 88/54 mm Hg88/54\text{ mm Hg}, pulse is 122/min122/\text{min}, and respirations are 26/min26/\text{min}. Oxygen saturation is 85%85\% on room air. Physical examination reveals a somnolent patient with prominent accessory muscle use, diffuse expiratory wheezing, and bilateral rhonchi. Laboratory results are as follows:

ParameterValue
Serum sodium138 mEq/L138\text{ mEq/L}
Serum potassium4.2 mEq/L4.2\text{ mEq/L}
Serum chloride102 mEq/L102\text{ mEq/L}
Serum bicarbonate12 mEq/L12\text{ mEq/L}
Blood urea nitrogen42 mg/dL42\text{ mg/dL}
Serum creatinine1.8 mg/dL1.8\text{ mg/dL}
Serum lactate6.5 mmol/L6.5\text{ mmol/L}

Arterial blood gas analysis (on room air):

ParameterValue
pH7.127.12
PaO2\text{PaO}_255 mm Hg55\text{ mm Hg}
PaCO2\text{PaCO}_238 mm Hg38\text{ mm Hg}

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

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

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
The patient's laboratory results demonstrate a high anion gap metabolic acidosis with a concurrent primary respiratory acidosis. First, the severe acidemia (pH 7.127.12) with a low bicarbonate (12 mEq/L12\text{ mEq/L}) establishes a primary metabolic acidosis. Next, calculating the anion gap (138−[102+12]=24 mEq/L138 - [102 + 12] = 24\text{ mEq/L}) confirms a high anion gap metabolic acidosis (HAGMA), which is driven by lactic acidosis from tissue hypoperfusion. Finally, Winter's formula must be applied to determine if the respiratory compensation is appropriate: Expected PaCO2=(1.5×HCO3−)+8±2\text{Expected PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2. For this patient, the expected PaCO2\text{PaCO}_2 is (1.5×12)+8=26±2 mm Hg(1.5 \times 12) + 8 = 26 \pm 2\text{ mm Hg}. Because the measured PaCO2\text{PaCO}_2 of 38 mm Hg38\text{ mm Hg} is significantly higher than the expected range of 24−28 mm Hg24-28\text{ mm Hg}, there is an inadequate respiratory response. This indicates a superimposed primary respiratory acidosis, a life-threatening complication in COPD patients experiencing acute respiratory failure alongside shock.

Step-by-Step Solution

1
Determine the primary acid-base disorder by evaluating the arterial pH and serum bicarbonate.
The pH is 7.127.12 (acidemia) and the serum bicarbonate is 12 mEq/L12\text{ mEq/L} (markedly low), indicating a primary metabolic acidosis.
Establishing the primary direction of the pH derangement dictates which subsequent diagnostic formulas are required.
2
Calculate the serum anion gap to characterize the metabolic acidosis.
Anion Gap=Na+−(Cl−+HCO3−)=138−(102+12)=24 mEq/L\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 138 - (102 + 12) = 24\text{ mEq/L}. This confirms a high anion gap metabolic acidosis (HAGMA).
Differentiation between high and normal anion gap acidosis is essential for uncovering the underlying etiology (in this case, lactic acidosis from sepsis).
3
Apply Winter's formula to assess the adequacy of the respiratory compensation.
Expected PaCO2=(1.5×HCO3−)+8±2=(1.5×12)+8=26±2 mm Hg\text{Expected PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 12) + 8 = 26 \pm 2\text{ mm Hg}.
Metabolic acidosis stimulates central chemoreceptors to increase alveolar ventilation. Winter's formula predicts the physiologically appropriate PaCO2\text{PaCO}_2 for a given bicarbonate level.
4
Compare the measured PaCO2 to the expected PaCO2 range.
The measured PaCO2\text{PaCO}_2 is 38 mm Hg38\text{ mm Hg}, which is significantly higher than the maximum expected value of 28 mm Hg28\text{ mm Hg}.
Because the patient is retaining more CO2\text{CO}_2 than physiologically expected for her level of metabolic acidosis, a concurrent primary respiratory acidosis is present (consistent with her severe COPD exacerbation).

Key Concept

In patients with a primary metabolic acidosis, Winter's formula (Expected PaCO2=[1.5×HCO3−]+8±2\text{Expected PaCO}_2 = [1.5 \times \text{HCO}_3^-] + 8 \pm 2) must be used to evaluate if the respiratory compensation is adequate. A measured PaCO2\text{PaCO}_2 higher than the expected range indicates a superimposed primary respiratory acidosis.
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Nephrology

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