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Difficulty: MediumNephrology

Paramedics transport a 68-year-old male resident of a skilled nursing facility to the acute care clinic due to profound lethargy and weakness over the past 24 hours. According to the accompanying transfer paperwork, the patient has experienced large-volume, watery stools for five consecutive days following a treatment course of clindamycin for cellulitis. His daily medications include a transdermal fentanyl patch for severe osteoarthritis and oral acetaminophen. Initial triage vital signs show a temperature of 37.1 C (98.8 F), blood pressure of 88/58 mm Hg, heart rate of 114/min, and a respiratory rate of 9/min. Physical assessment is notable for somnolence, a delayed capillary refill of 4 seconds, and dry oral mucosa. Blood is drawn for an arterial blood gas and serum chemistry panels with the following results:

Lab TestValue
Serum sodium135 mEq/L135 \text{ mEq/L}
Serum potassium2.9 mEq/L2.9 \text{ mEq/L}
Serum chloride112 mEq/L112 \text{ mEq/L}
Serum bicarbonate14 mEq/L14 \text{ mEq/L}
Blood urea nitrogen42 mg/dL42 \text{ mg/dL}
Serum creatinine1.9 mg/dL1.9 \text{ mg/dL}
Arterial blood gas (room air):
ParameterValue
pHpH7.257.25
PaCO2PaCO_235 mm Hg35 \text{ mm Hg}
PaO2PaO_278 mm Hg78 \text{ mm Hg}

Which of the following is the most accurate interpretation of this patient's current acid-base status?

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

Answer

The most accurate interpretation is a normal anion gap metabolic acidosis with a concurrent respiratory acidosis.
The patient's lab results show a primary metabolic acidosis (pH<7.35pH < 7.35 and low [HCO3−][HCO_3^-]). The anion gap is calculated as 135−(112+14)=9 mEq/L135 - (112 + 14) = 9 \text{ mEq/L}, which is normal. To determine if respiratory compensation is appropriate, Winter's formula must be applied: Expected PaCO2=1.5×14+8=29±2 mm HgExpected\ PaCO_2 = 1.5 \times 14 + 8 = 29 \pm 2 \text{ mm Hg}. The patient's measured PaCO2PaCO_2 is 35 mm Hg35 \text{ mm Hg}. Since the measured PaCO2PaCO_2 is higher than the expected range of 27−31 mm Hg27-31 \text{ mm Hg}, the patient has a concurrent respiratory acidosis. This clinical picture is consistent with severe diarrhea causing bicarbonate loss (normal anion gap metabolic acidosis) alongside fentanyl-induced hypoventilation (respiratory acidosis).

Step-by-Step Solution

1
Analyze the pH and serum bicarbonate to identify the primary disorder.
The pHpH is 7.257.25 (acidemia) and [HCO3−][HCO_3^-] is 14 mEq/L14 \text{ mEq/L} (low). This indicates a primary metabolic acidosis.
Establishing the primary acid-base disturbance is the necessary first step before evaluating compensation.
2
Calculate the serum anion gap using the formula: AG=[Na+]−([Cl−]+[HCO3−])AG = [Na^+] - ([Cl^-] + [HCO_3^-]).
AG=135−(112+14)=135−126=9 mEq/LAG = 135 - (112 + 14) = 135 - 126 = 9 \text{ mEq/L}. This is a normal anion gap (normal range is typically 8−12 mEq/L8-12 \text{ mEq/L}).
Differentiating between normal and high anion gap metabolic acidosis helps narrow down the etiology. Here, bicarbonate loss from severe diarrhea causes a normal anion gap metabolic acidosis (NAGMA).
3
Calculate the expected respiratory compensation using Winter's formula: Expected PaCO2=(1.5×[HCO3−])+8±2Expected\ PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2.
Expected PaCO2=(1.5×14)+8=21+8=29±2Expected\ PaCO_2 = (1.5 \times 14) + 8 = 21 + 8 = 29 \pm 2. The expected range is 2727 to 31 mm Hg31 \text{ mm Hg}.
Winter's formula determines if the respiratory response (hyperventilation blowing off CO2) is adequate for the degree of metabolic acidosis.
4
Compare the expected PaCO2PaCO_2 to the patient's measured PaCO2PaCO_2.
The measured PaCO2PaCO_2 (35 mm Hg35 \text{ mm Hg}) is greater than the expected maximum of 31 mm Hg31 \text{ mm Hg}.
Because the actual PaCO2PaCO_2 is higher than the expected compensatory range, the patient is retaining CO2. This indicates an independent, superimposed primary respiratory acidosis, likely secondary to fentanyl-induced respiratory depression (evidenced by a respiratory rate of 9/min).

Key Concept

In patients with metabolic acidosis, Winter's formula must be used to calculate expected PaCO2 to rule out a mixed acid-base disorder; an actual PaCO2 higher than expected signifies a concurrent respiratory acidosis.
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Nephrology

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