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Difficulty: MediumNephrology

A 45-year-old woman is brought to the emergency department by her husband due to progressive lethargy over the past 12 hours. The patient has a history of Crohn disease and has been experiencing severe, watery diarrhea for the past five days. Yesterday, she took multiple doses of a friend's prescription oxycodone to help manage her severe abdominal cramping. On physical examination, her temperature is 37.1°C (98.8°F), blood pressure is 105/65 mmHg, heart rate is 110/min, and respiratory rate is 8/min. She is somnolent but arouses to verbal stimuli. Mucous membranes are dry.

Laboratory studies show the following:

ParameterValue
Sodium138 mEq/L
Potassium3.2 mEq/L
Chloride112 mEq/L
Bicarbonate14 mEq/L
Arterial pH7.20
Arterial pCO240 mmHg
Arterial pO268 mmHg

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

  1. Concurrent normal anion gap metabolic acidosis and respiratory acidosisAnswer
  2. B
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Concurrent elevated anion gap metabolic acidosis and respiratory acidosis
  4. D
    Elevated anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Primary respiratory acidosis with compensatory metabolic alkalosis

Answer

Concurrent normal anion gap metabolic acidosis and respiratory acidosis
The patient's laboratory values reveal an acidemia (pH 7.20) with a low bicarbonate (14 mEq/L), indicating a primary metabolic acidosis. The anion gap is calculated as Na+−(Cl−+HCO3−)=138−(112+14)=12 mEq/LNa^+ - (Cl^- + HCO_3^-) = 138 - (112 + 14) = 12 \text{ mEq/L}. Because a normal anion gap is typically 8-12 mEq/L, she has a normal anion gap metabolic acidosis, primarily driven by her severe diarrhea (gastrointestinal bicarbonate loss). Next, Winter's formula must be applied to determine if the respiratory compensation is adequate: ExpectedpCO2=(1.5×HCO3−)+8±2Expected pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, ExpectedpCO2=(1.5×14)+8±2=29±2 mmHgExpected pCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 \text{ mmHg}. Her measured pCO2 is 40 mmHg, which is significantly higher than the expected range of 27-31 mmHg. This indicates that she is retaining carbon dioxide, representing a concurrent respiratory acidosis. This is clinically supported by her depressed respiratory rate (8/min) following oxycodone ingestion.

Step-by-Step Solution

1
Calculate the anion gap.
138−(112+14)=12 mEq/L138 - (112 + 14) = 12 \text{ mEq/L}
To determine the etiology of the metabolic acidosis. A result of 12 mEq/L is a normal gap, which is consistent with severe bicarbonate loss from diarrhea.
2
Calculate the expected pCO2 using Winter's formula.
(1.5×14)+8=29±2 mmHg(1.5 \times 14) + 8 = 29 \pm 2 \text{ mmHg}
To assess if the respiratory system is appropriately compensating for the metabolic acidosis.
3
Compare measured pCO2 to the expected pCO2.
The measured pCO2 (40 mmHg) is significantly higher than the expected range (27-31 mmHg).
A higher than expected pCO2 indicates inadequate respiratory compensation, defining a concurrent primary respiratory acidosis (due to opiate-induced hypoventilation).

Key Concept

Patients with a primary metabolic acidosis require calculation of expected pCO2 using Winter's formula to identify whether a secondary respiratory acid-base disorder is present.
Estimated Time:1m 30s
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Topics

Nephrology

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