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Difficulty: MediumNephrology

A 32-year-old woman is brought to the emergency department by her husband due to progressive lethargy and confusion. She has a history of Crohn disease and has been experiencing frequent, voluminous, watery diarrhea for the past four days. Two days ago, she was prescribed oral hydrocodone-acetaminophen for an acute musculoskeletal back injury. Her husband reports she has been taking the pain medication more frequently than prescribed. Temperature is 37.1°C (98.8°F), blood pressure is 92/60 mm Hg, pulse is 112/min, and respirations are 8/min. Physical examination shows dry mucous membranes, decreased skin turgor, and pinpoint pupils. The patient is somnolent but arousable. Laboratory studies are obtained:

TestValue
Sodium136 mEq/L
Potassium2.8 mEq/L
Chloride115 mEq/L
Bicarbonate13 mEq/L
Arterial blood gas on room air:
TestValue
pH7.22
pCO2pCO_234 mm Hg
pO2pO_268 mm Hg

Based on these findings, which of the following best describes this patient's acid-base disorder?

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

Answer

Normal anion gap metabolic acidosis and respiratory acidosis
The correct answer is normal anion gap metabolic acidosis and respiratory acidosis. The patient's severe diarrhea causes a loss of bicarbonate, resulting in a primary metabolic acidosis. The first step in evaluating metabolic acidosis is calculating the anion gap (AG): AG=Na+−(Cl−+HCO3−)AG = Na^+ - (Cl^- + HCO_3^-). In this patient, AG=136−(115+13)=8AG = 136 - (115 + 13) = 8 mEq/L, which is within the normal range (8-12 mEq/L), confirming a normal anion gap metabolic acidosis (NAGMA). The next required step is evaluating for appropriate respiratory compensation using Winter's formula: Expected pCO2=(1.5×HCO3−)+8±2Expected\ pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, the expected pCO2pCO_2 is (1.5×13)+8=27.5±2(1.5 \times 13) + 8 = 27.5 \pm 2 mm Hg (range: 25.5 to 29.5 mm Hg). Because the patient's measured pCO2pCO_2 (34 mm Hg) is higher than the expected range, she is retaining more CO2CO_2 than she should. This indicates a concurrent primary respiratory acidosis. Clinically, this is explained by her recent overuse of hydrocodone-acetaminophen, leading to opiate-induced central respiratory depression (evidenced by lethargy, pinpoint pupils, and a respiratory rate of 8/min).

Step-by-Step Solution

1
Calculate the Anion Gap (AG).
AG=Na+−(Cl−+HCO3−)=136−(115+13)=8AG = Na^+ - (Cl^- + HCO_3^-) = 136 - (115 + 13) = 8 mEq/L.
To determine the mechanism of the primary metabolic acidosis (diarrhea-induced bicarbonate loss vs. unmeasured anions).
2
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×13)+8=19.5+8=27.5±2Expected\ pCO_2 = (1.5 \times 13) + 8 = 19.5 + 8 = 27.5 \pm 2 mm Hg (Range: 25.5 to 29.5 mm Hg).
To determine if the patient has a simple metabolic acidosis or a mixed acid-base disorder.
3
Compare expected pCO2pCO_2 to the measured pCO2pCO_2.
Measured pCO2pCO_2 (34 mm Hg) is greater than the expected range (25.5 - 29.5 mm Hg).
A measured pCO2pCO_2 higher than expected indicates a superimposed primary respiratory acidosis, driven here by opiate toxicity.

Key Concept

Patients with a primary metabolic acidosis must be evaluated for adequate respiratory compensation using Winter's formula. If the measured pCO2pCO_2 is higher than the calculated expected pCO2pCO_2, a concurrent primary respiratory acidosis is present.
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Topics

Nephrology

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