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Difficulty: HardNephrology

A 22-year-old woman is brought to the emergency department 4 hours after intentionally ingesting an unknown quantity of pills. She complains of ringing in her ears, nausea, and diffuse abdominal pain. Her temperature is 37.8°C (100.0°F), blood pressure is 110/70 mm Hg, pulse is 115/min, and respirations are 26/min.

Laboratory studies show:
TestValue
Serum sodium140 mEq/L
Serum potassium3.5 mEq/L
Serum chloride104 mEq/L
Serum bicarbonate14 mEq/L
Blood urea nitrogen18 mg/dL
Serum creatinine0.9 mg/dL
Arterial blood gas analysis on room air:
TestValue
pH7.42
PaCO222 mm Hg
PaO298 mm Hg

Which of the following best characterizes this patient's primary acid-base disturbance(s)?

  1. A
    High anion gap metabolic acidosis with appropriate respiratory compensation
  2. Mixed high anion gap metabolic acidosis and primary respiratory alkalosisAnswer
  3. C
    Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis
  4. D
    Primary respiratory alkalosis with appropriate metabolic compensation
  5. E
    Mixed high anion gap metabolic acidosis and primary metabolic alkalosis

Answer

Mixed high anion gap metabolic acidosis and primary respiratory alkalosis
This patient has a mixed acid-base disorder characteristic of acute salicylate toxicity (indicated by tinnitus, nausea, and hyperventilation). Step 1: The anion gap is 140−(104+14)=22140 - (104 + 14) = 22, confirming a high anion gap metabolic acidosis. Step 2: To check for appropriate respiratory compensation, Winter's formula is applied: ExpectedPaCO2=(1.5×14)+8±2=29±2Expected PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2. Step 3: The patient's measured PaCO2 is 22 mm Hg, which is lower than the expected 27-31 mm Hg range. This indicates an excessive respiratory drive beyond what is needed for compensation, signifying an independent primary respiratory alkalosis. Step 4: The delta-delta ratio is (22−12)/(24−14)=1.0(22 - 12) / (24 - 14) = 1.0, which rules out any additional primary metabolic alkalosis or non-anion gap metabolic acidosis.

Step-by-Step Solution

1
Calculate the anion gap to determine if a primary metabolic acidosis is present.
AnionGap=Na+−(Cl−+HCO3−)=140−(104+14)=22 mEq/LAnion Gap = Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 14) = 22 \text{ mEq/L}.
An elevated anion gap (>12 mEq/L) establishes the presence of a primary high anion gap metabolic acidosis (AGMA).
2
Apply Winter's formula to determine the expected respiratory compensation for the metabolic acidosis.
ExpectedPaCO2=(1.5×HCO3−)+8±2=(1.5×14)+8±2=29±2 mm HgExpected PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 \text{ mm Hg}. The expected range is 27 to 31 mm Hg.
Winter's formula determines if the patient's respiratory drive is appropriately compensating for the metabolic acidosis or if there is a secondary respiratory disorder.
3
Compare the measured PaCO2 to the expected PaCO2.
The measured PaCO2PaCO_2 is 22 mm Hg, which is lower than the expected range of 27-31 mm Hg.
Because the measured PaCO2 is lower than the expected appropriate compensation, an independent primary respiratory alkalosis is present.
4
Calculate the delta-delta ratio to evaluate for an additional covert metabolic disorder.
ΔAG/ΔHCO3−=(22−12)/(24−14)=10/10=1.0\Delta AG / \Delta HCO_3^- = (22 - 12) / (24 - 14) = 10 / 10 = 1.0.
A ratio between 1 and 2 indicates a pure high anion gap metabolic acidosis without a concurrent primary metabolic alkalosis or non-anion gap metabolic acidosis.

Key Concept

Patients with salicylate toxicity typically present with a mixed high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation) and a primary respiratory alkalosis (due to direct stimulation of the medullary respiratory center).
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Nephrology

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