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Difficulty: MediumNephrology

A 32-year-old woman is evaluated in the urgent care clinic due to severe nausea, diffuse abdominal discomfort, and a persistent "ringing" sensation in her ears for the past day. She reports a history of chronic lower back pain and admits to taking a large handful of over-the-counter analgesic tablets recently to manage a severe flare-up.

Vital signs show a temperature of 38.1°C (100.6°F), blood pressure of 114/72 mm Hg, pulse of 116/min, and respiratory rate of 28/min. She appears flushed and diaphoretic. Lungs are clear to auscultation.

Laboratory results are as follows:

TestResult
Arterial Blood Gas (room air)
pH7.44
PaCO₂19 mm Hg
PaO₂96 mm Hg
Serum Chemistry
Sodium141 mEq/L
Potassium3.6 mEq/L
Chloride104 mEq/L
Bicarbonate12 mEq/L
Creatinine0.8 mg/dL

Which of the following best characterizes this patient's current acid-base disturbance?

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

Answer

Mixed primary metabolic acidosis and primary respiratory alkalosis
The patient's presentation (tinnitus, tachypnea, fever, nausea) following an overdose of an over-the-counter analgesic is highly suspicious for salicylate (aspirin) toxicity. Salicylate toxicity characteristically causes a mixed acid-base disorder. First, calculate the anion gap: 141−(104+12)=25141 - (104 + 12) = 25 mEq/L. This is elevated, confirming a primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation and accumulation of organic acids). Next, determine if the respiratory response is purely compensatory by applying Winter's formula: Expected PaCO2=1.5×12+8=26±2PaCO_2 = 1.5 \times 12 + 8 = 26 \pm 2 mm Hg (range 24-28 mm Hg). The patient's actual PaCO2PaCO_2 is 19 mm Hg. Because the actual PaCO2PaCO_2 is lower than the expected compensatory range, there is a concurrent primary respiratory alkalosis (due to direct stimulation of the medullary respiratory center by salicylates). These two opposing primary disorders explain why the pH (7.44) is remarkably close to normal.

Step-by-Step Solution

1
Calculate the anion gap to assess the metabolic component.
AG = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 141−(104+12)141 - (104 + 12) = 2525 mEq/L.
An elevated anion gap (>12 mEq/L) definitively identifies the presence of a primary high anion gap metabolic acidosis.
2
Apply Winter's formula to determine the expected respiratory compensation for the metabolic acidosis.
Expected PaCO2=1.5×HCO3−+8±2PaCO_2 = 1.5 \times HCO_3^- + 8 \pm 2 = 1.5×12+8±21.5 \times 12 + 8 \pm 2 = 26±226 \pm 2 (range 24-28 mm Hg).
Winter's formula determines if the respiratory response is purely compensatory or if an independent respiratory disorder exists.
3
Compare the actual PaCO2 to the expected PaCO2 range.
Actual PaCO2PaCO_2 is 19 mm Hg, which is significantly lower than the expected 24-28 mm Hg.
A PaCO2PaCO_2 lower than expected indicates an additional primary respiratory alkalosis.
4
Synthesize the laboratory findings with the clinical presentation.
The patient has both a high anion gap metabolic acidosis and a primary respiratory alkalosis, classic for salicylate toxicity.
Salicylates directly stimulate the medullary respiratory center (respiratory alkalosis) and uncouple oxidative phosphorylation (metabolic acidosis), often resulting in a near-normal pH due to opposing forces.

Key Concept

Evaluation of mixed acid-base disorders using the anion gap and Winter's formula.
Estimated Time:1m 30s
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Topics

Nephrology

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