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Difficulty: MediumNephrology

An alert 42-year-old female is evaluated in the urgent care clinic for persistent weakness, severe muscle cramps, and three days of voluminous, watery stools after attending a community barbecue. Her past medical history is significant for severe obesity and obstructive sleep apnea, for which she uses continuous positive airway pressure (CPAP) inconsistently at night. Vital signs show a temperature of 37.4°C (99.3°F), pulse of 108/min, respirations of 22/min, and blood pressure of 102/64 mmHg. Physical examination reveals dry mucous membranes and decreased skin turgor. The following laboratory values are reported:

TestResult
Sodium140 mEq/L140 \text{ mEq/L}
Potassium3.1 mEq/L3.1 \text{ mEq/L}
Chloride114 mEq/L114 \text{ mEq/L}
Bicarbonate14 mEq/L14 \text{ mEq/L}
Blood urea nitrogen32 mg/dL32 \text{ mg/dL}
Serum creatinine1.2 mg/dL1.2 \text{ mg/dL}
Arterial pH7.247.24
Arterial pCO2pCO_252 mmHg52 \text{ mmHg}
Arterial pO2pO_278 mmHg78 \text{ mmHg}

Based on the provided laboratory values and clinical presentation, which of the following best characterizes this patient's acid-base status?

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

Answer

The patient has a mixed acid-base disorder consisting of a normal anion gap metabolic acidosis and a respiratory acidosis.
The correct answer characterizes the presence of two distinct acid-base disorders. First, the patient's arterial pH is 7.247.24 with a low bicarbonate (14 mEq/L14 \text{ mEq/L}), confirming a metabolic acidosis. The anion gap is 140−(114+14)=12140 - (114 + 14) = 12, which is a normal anion gap metabolic acidosis, classically caused by severe gastrointestinal bicarbonate losses (diarrhea). Second, to verify if the respiratory compensation is adequate, Winter's formula must be applied: Expected pCO2=1.5×[HCO3−]+8±2\text{Expected } pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2. For a bicarbonate of 14 mEq/L14 \text{ mEq/L}, the expected pCO2pCO_2 is 27−31 mmHg27-31 \text{ mmHg}. The patient's measured pCO2pCO_2 is 52 mmHg52 \text{ mmHg}. Because the actual pCO2pCO_2 is much higher than the expected pCO2pCO_2, the patient is hypoventilating relative to her metabolic state, indicating a concurrent respiratory acidosis. Her history of severe obesity and obstructive sleep apnea strongly suggests baseline CO2CO_2 retention (obesity hypoventilation syndrome), explaining the respiratory component.

Step-by-Step Solution

1
Analyze the pH and primary derangement.
The pH is 7.247.24 (acidemia). Both the HCO3−HCO_3^- (14 mEq/L14 \text{ mEq/L}) and the pCO2pCO_2 (52 mmHg52 \text{ mmHg}) are contributing to acidemia, indicating a mixed disorder or at least a primary metabolic acidosis with abnormal pCO2pCO_2.
Establishing the direction of pH change and corresponding variables dictates the primary disorder.
2
Calculate the anion gap (AG).
AG=Na+−(Cl−+HCO3−)=140−(114+14)=12 mEq/L\text{AG} = Na^+ - (Cl^- + HCO_3^-) = 140 - (114 + 14) = 12 \text{ mEq/L}.
Differentiates the cause of the metabolic acidosis. An AG of 1212 is normal, consistent with bicarbonate loss from severe diarrhea.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=1.5×[HCO3−]+8±2=(1.5×14)+8=29±2 mmHg\text{Expected } pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = (1.5 \times 14) + 8 = 29 \pm 2 \text{ mmHg}.
Determines if the patient's ventilation is appropriately compensating for the metabolic acidosis.
4
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2.
The actual pCO2pCO_2 is 52 mmHg52 \text{ mmHg}, which is significantly higher than the expected range of 27−31 mmHg27-31 \text{ mmHg}.
An actual pCO2pCO_2 greater than the expected pCO2pCO_2 indicates a superimposed, independent respiratory acidosis (likely due to the patient's obesity hypoventilation and sleep apnea).

Key Concept

Identifying mixed acid-base disorders using the anion gap and Winter's formula for respiratory compensation.
Estimated Time:1m 30s
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Topics

Nephrology

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