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Difficulty: MediumNephrology

An emergency medical response team transports a young adult male to the hospital after he was discovered lethargic in his dormitory. The patient, a 22-year-old college student with a known psychiatric history, reportedly ingested a large, unknown quantity of over-the-counter pain relievers earlier in the day. Upon arrival at the triage desk, he is tachypneic and diaphoretic. Vital signs include a temperature of 38.1°C (100.6°F), blood pressure of 112/68 mm Hg, heart rate of 115/min, and respiratory rate of 28/min.

Laboratory evaluation is obtained immediately:

Sodium: 140140 mEq/L
Potassium: 4.24.2 mEq/L
Chloride: 103103 mEq/L
Bicarbonate: 1313 mEq/L
Blood urea nitrogen: 1818 mg/dL
Creatinine: 1.01.0 mg/dL

Arterial blood gas parameters on room air:
pH: 7.437.43
PaCO2: 2020 mm Hg
PaO2: 9898 mm Hg

Based on the provided laboratory parameters, which of the following accurately characterizes the primary acid-base disturbance(s) in this patient?

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

Answer

Mixed high anion gap metabolic acidosis and respiratory alkalosis
This patient's clinical presentation (lethargy, tachypnea, ingestion of over-the-counter medication) and laboratory findings are highly characteristic of salicylate (aspirin) toxicity. Salicylates cause a classic mixed acid-base disorder: they directly stimulate the medullary respiratory center to cause a primary respiratory alkalosis, and they disrupt cellular metabolism by uncoupling oxidative phosphorylation, leading to organic acid accumulation and a primary high anion gap metabolic acidosis.

This can be proven mathematically:
1. The pH of 7.437.43 indicates alkalemia, meaning the dominant primary disorder is a respiratory alkalosis (driven by the PaCO2 of 2020 mm Hg).
2. The expected acute metabolic compensation for a PaCO2 drop of 2020 mm Hg (from the baseline of 4040) is a 44 mEq/L drop in HCO3−HCO_3^- (22 mEq/L per 1010 mm Hg drop in PaCO2). The expected HCO3−HCO_3^- is therefore 2020 mEq/L.
3. The patient's actual HCO3−HCO_3^- is 1313 mEq/L, which is far lower than the compensatory 2020 mEq/L. This signifies an additional, independent primary metabolic acidosis.
4. The anion gap is calculated as Na+−(Cl−+HCO3−)=140−(103+13)=24Na^+ - (Cl^- + HCO_3^-) = 140 - (103 + 13) = 24 mEq/L. Since this is greater than the normal maximum of 1212 mEq/L, the metabolic component is specifically a high anion gap metabolic acidosis.

Alternatively, applying Winter's formula to the metabolic acidosis: Expected PaCO2=(1.5×13)+8=27.5±2Expected\ PaCO_2 = (1.5 \times 13) + 8 = 27.5 \pm 2 mm Hg. Because the measured PaCO2 (2020 mm Hg) is well below this range, a concurrent respiratory alkalosis is definitively present.

Step-by-Step Solution

1
Determine the primary acid-base disorder based on the pH.
The pH is 7.437.43, indicating alkalemia. The low PaCO2 (2020 mm Hg) aligns with this, establishing a primary respiratory alkalosis.
Identifying the primary disturbance directs the appropriate compensation formula to use.
2
Calculate the expected metabolic compensation for the primary respiratory alkalosis.
For acute respiratory alkalosis, HCO3−HCO_3^- decreases by 22 mEq/L per 1010 mm Hg drop in PaCO2. A drop from 4040 to 2020 mm Hg (a difference of 2020) should decrease HCO3−HCO_3^- by 44. Expected HCO3−HCO_3^- is 24−4=2024 - 4 = 20 mEq/L.
This determines if the low bicarbonate is solely compensatory or represents a concurrent primary metabolic acidosis.
3
Compare the measured bicarbonate to the expected compensatory value.
The measured HCO3−HCO_3^- (1313 mEq/L) is significantly lower than the expected 2020 mEq/L, confirming an additional primary metabolic acidosis.
A significant deviation from expected compensation indicates a mixed acid-base disorder.
4
Calculate the anion gap to classify the metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)=140−(103+13)=24Na^+ - (Cl^- + HCO_3^-) = 140 - (103 + 13) = 24 mEq/L. Since 24>1224 > 12, it is a high anion gap metabolic acidosis.
Metabolic acidosis must always be classified as normal or high anion gap to narrow the differential diagnosis.

Key Concept

Salicylate toxicity classically causes a mixed acid-base disorder: a primary respiratory alkalosis due to direct medullary respiratory center stimulation, and a primary high anion gap metabolic acidosis due to uncoupling of oxidative phosphorylation.
Estimated Time:1m 30s
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Nephrology

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