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Difficulty: EasyNephrology

A 19-year-old male is brought to the urgent care clinic by his parents due to extreme fatigue, increased thirst, and frequent urination over the past week. Today, he developed nausea and deep, rapid breathing. Laboratory evaluation shows:

Serum sodium: 138 mEq/L138 \text{ mEq/L}
Serum chloride: 104 mEq/L104 \text{ mEq/L}
Serum bicarbonate: 12 mEq/L12 \text{ mEq/L}

Arterial blood gas analysis on room air reveals:
pH: 7.247.24
PaCO2PaCO_2: 26 mm Hg26 \text{ mm Hg}
PaO2PaO_2: 95 mm Hg95 \text{ mm Hg}

Based on these findings, which of the following is the most accurate interpretation of this patient's acid-base status?

  1. Primary metabolic acidosis with adequate respiratory compensationAnswer
  2. B
    Primary metabolic acidosis with concurrent respiratory acidosis
  3. C
    Primary metabolic acidosis with concurrent respiratory alkalosis
  4. D
    Primary respiratory acidosis with adequate metabolic compensation
  5. E
    Primary respiratory alkalosis with adequate metabolic compensation

Answer

Primary metabolic acidosis with adequate respiratory compensation
The correct answer accurately identifies the patient's condition. The pH of 7.24 indicates acidemia, and the low bicarbonate level of 12 mEq/L establishes metabolic acidosis as the primary disorder. To determine if the respiratory compensation is appropriate, Winter's formula is used: Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, the calculation yields (1.5×12)+8=26±2 mm Hg(1.5 \times 12) + 8 = 26 \pm 2 \text{ mm Hg}. Since the patient's actual PaCO2PaCO_2 is exactly 26 mm Hg, the respiratory compensation is adequate, and there is no secondary acid-base disturbance.

Step-by-Step Solution

1
Identify the primary acid-base disturbance by evaluating the pH and serum bicarbonate.
The pH is 7.24 (acidemia) and bicarbonate is 12 mEq/L (low).
A low pH combined with a proportionally low bicarbonate level confirms that the primary process is a metabolic acidosis.
2
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×12)+8±2=26±2 mm HgPaCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2 \text{ mm Hg}.
In primary metabolic acidosis, the lungs rapidly compensate by blowing off CO2CO_2 (hyperventilation) to raise the pH. Winter's formula predicts the expected PaCO2PaCO_2 range for a given bicarbonate level.
3
Compare the actual PaCO2PaCO_2 to the expected PaCO2PaCO_2.
The patient's actual PaCO2PaCO_2 is 26 mm Hg, which falls exactly within the expected range.
Because the actual PaCO2PaCO_2 matches the calculated value, the respiratory compensation is fully adequate, meaning there is no superimposed secondary respiratory disorder.

Key Concept

Applying Winter's formula to evaluate respiratory compensation in primary metabolic acidosis
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Topics

Nephrology

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