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Difficulty: HardNephrology

A 2020-year-old woman is brought to the emergency department by her roommate due to confusion, nausea, and ringing in her ears that began several hours ago. The roommate found an empty bottle of an over-the-counter analgesic in their shared bathroom. The patient has a medical history of major depressive disorder. Her temperature is 38.0∘38.0^\circ C (100.4∘100.4^\circ F), blood pressure is 112/70112/70 mm Hg, pulse is 114114/min, and respirations are 2828/min. She appears agitated and is breathing deeply and rapidly. Arterial blood gas on room air and serum chemistry results are as follows:

Laboratory ParameterResult
pHpH7.447.44
PaCO2PaCO_21616 mm Hg
PaO2PaO_29898 mm Hg
Sodium (Na+Na^+)140140 mEq/L
Potassium (K+K^+)3.63.6 mEq/L
Chloride (Cl−Cl^-)104104 mEq/L
Bicarbonate (HCO3−HCO_3^-)1010 mEq/L
Blood urea nitrogen1616 mg/dL
Creatinine0.80.8 mg/dL

Which of the following best describes this patient's acid-base status?

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

Answer

Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis
The correct answer accurately identifies two independent primary acid-base disorders using systematic calculation. First, the anion gap is 2626 mEq/L (140−[104+10]140 - [104 + 10]), which defines a high anion gap metabolic acidosis. Second, applying Winter's formula (1.5×10+8=23±21.5 \times 10 + 8 = 23 \pm 2 mm Hg) reveals that adequate respiratory compensation should result in a PaCO2PaCO_2 between 2121 and 2525 mm Hg. Because the patient's actual PaCO2PaCO_2 is 1616 mm Hg, she is 'blowing off' more CO2CO_2 than expected, proving a concurrent primary respiratory alkalosis. This mixed picture, along with tinnitus and hyperthermia, is the hallmark of salicylate toxicity.

Step-by-Step Solution

1
Calculate the Anion Gap to determine the presence and type of metabolic acidosis.
AG=Na+−(Cl−+HCO3−)=140−(104+10)=26AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 10) = 26 mEq/L.
An elevated anion gap (>12>12 mEq/L) establishes the presence of a primary high anion gap metabolic acidosis (HAGMA).
2
Calculate the expected respiratory compensation for the metabolic acidosis using Winter's formula.
Expected PaCO2=1.5×HCO3−+8±2=(1.5×10)+8=23±2PaCO_2 = 1.5 \times HCO_3^- + 8 \pm 2 = (1.5 \times 10) + 8 = 23 \pm 2 mm Hg (Range: 21−2521-25 mm Hg).
This determines if the change in PaCO2PaCO_2 is purely compensatory or if there is a secondary respiratory acid-base disorder.
3
Compare the actual PaCO2PaCO_2 to the expected PaCO2PaCO_2 calculated from Winter's formula.
Actual PaCO2PaCO_2 (1616 mm Hg) is less than the expected range (21−2521-25 mm Hg).
Because the patient is hyperventilating more than expected for her level of metabolic acidosis, there is a concurrent primary respiratory alkalosis.
4
Calculate the Delta Ratio to check for a third hidden disorder.
ΔAG=26−12=14\Delta AG = 26 - 12 = 14. ΔHCO3−=24−10=14\Delta HCO_3^- = 24 - 10 = 14. Ratio = 14/14=1.014/14 = 1.0.
A ratio of 1.01.0 confirms a pure high anion gap metabolic acidosis with no hidden normal anion gap acidosis or metabolic alkalosis.

Key Concept

Salicylate toxicity classically presents with a mixed acid-base disorder: a primary respiratory alkalosis (due to direct stimulation of the medullary respiratory center) and a primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation and accumulation of organic acids).
Estimated Time:2m 0s
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Topics

Nephrology

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