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Difficulty: MediumNephrology

A 54-year-old man is brought to the emergency department by paramedics after being found stuporous in a local park. Beside him, paramedics found several empty bottles of antifreeze. His temperature is 36.5∘C36.5^\circ\text{C} (97.7∘F97.7^\circ\text{F}), blood pressure is 105/65 mmHg105/65\text{ mmHg}, pulse is 98/min98/\text{min}, and respirations are 14/min14/\text{min}. Physical examination reveals a lethargic patient who arouses to painful stimuli but is unable to answer questions. Cardiopulmonary examination is unremarkable.

The following laboratory results are obtained:

Laboratory TestResult
Serum sodium138 mEq/L138\text{ mEq/L}
Serum potassium4.8 mEq/L4.8\text{ mEq/L}
Serum chloride100 mEq/L100\text{ mEq/L}
Serum bicarbonate12 mEq/L12\text{ mEq/L}
Blood urea nitrogen24 mg/dL24\text{ mg/dL}
Serum creatinine1.5 mg/dL1.5\text{ mg/dL}

Arterial blood gas analysis on room air reveals:

ParameterResult
pH7.157.15
PaCO235 mmHg35\text{ mmHg}
PaO288 mmHg88\text{ mmHg}

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

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

Answer

High anion gap metabolic acidosis with a concurrent respiratory acidosis
The correct answer accurately identifies a mixed acid-base disorder. First, the low pH (7.157.15) and low serum HCO3−HCO_3^- (12 mEq/L12\text{ mEq/L}) establish a primary metabolic acidosis. Next, calculating the anion gap (Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-)) yields 138−(100+12)=26 mEq/L138 - (100 + 12) = 26\text{ mEq/L}. An anion gap greater than 14 indicates a high anion gap metabolic acidosis (HAGMA), which is highly consistent with this patient's presentation of likely antifreeze (ethylene glycol) toxicity. To determine if the respiratory response is appropriate, Winter's formula is applied: Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, the expected PaCO2=(1.5×12)+8±2=26±2 mmHgPaCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2\text{ mmHg} (range 24−28 mmHg24-28\text{ mmHg}). The measured PaCO2PaCO_2 is 35 mmHg35\text{ mmHg}, which is significantly higher than the expected range. An inappropriately elevated PaCO2PaCO_2 in the setting of a metabolic acidosis indicates a concurrent primary respiratory acidosis. This mixed disorder is classically seen in severe toxic alcohol ingestions due to CNS depression resulting in profound hypoventilation.

Step-by-Step Solution

1
Determine the primary acid-base disturbance based on pH and HCO3−HCO_3^-.
The pH is 7.157.15 (acidemia) and serum HCO3−HCO_3^- is 12 mEq/L12\text{ mEq/L} (low), indicating a primary metabolic acidosis.
Establishing the initial primary disorder is the first step in analyzing any arterial blood gas.
2
Calculate the anion gap to classify the metabolic acidosis.
Anion gap = Na+−(Cl−+HCO3−)=138−(100+12)=26 mEq/LNa^+ - (Cl^- + HCO_3^-) = 138 - (100 + 12) = 26\text{ mEq/L}. Since this is >14 mEq/L> 14\text{ mEq/L}, it is a high anion gap metabolic acidosis (HAGMA).
HAGMA has a specific differential diagnosis (e.g., MUDPILES) that is distinct from normal anion gap metabolic acidosis.
3
Calculate the expected PaCO2PaCO_2 using Winter's formula to check for respiratory compensation.
Expected PaCO2=(1.5×12)+8±2=26±2 mmHgPaCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2\text{ mmHg} (range: 24 to 28 mmHg24\text{ to }28\text{ mmHg}).
To determine if the body is appropriately compensating via hyperventilation or if a secondary respiratory disorder exists.
4
Compare the calculated expected PaCO2PaCO_2 with the measured PaCO2PaCO_2.
The measured PaCO2PaCO_2 (35 mmHg35\text{ mmHg}) is greater than the expected compensatory range (24−28 mmHg24-28\text{ mmHg}).
An inappropriately high PaCO2PaCO_2 in the setting of severe metabolic acidosis indicates a concurrent primary respiratory acidosis.

Key Concept

In a patient with metabolic acidosis, an inappropriately elevated PaCO2PaCO_2 compared to the calculated expected value using Winter's formula indicates a concurrent primary respiratory acidosis.
Estimated Time:1m 30s
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Nephrology

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