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Difficulty: MediumNephrology

An 18-year-old college freshman visits the campus health center complaining of a constant "ringing" sound in his ears, generalized nausea, and three episodes of nonbloody emesis since last night. He states he has been feeling extremely overwhelmed by his upcoming final examinations. On physical examination, he appears anxious and is breathing rapidly and deeply.

Vital signs:
Temperature: 37.8∘C37.8^\circ C (100.0∘F100.0^\circ F)
Blood pressure: 112/70112/70 mm Hg
Pulse: 114114/min
Respirations: 2828/min

Laboratory results:
Serum chemistry:
Sodium: 140140 mEq/L
Potassium: 3.83.8 mEq/L
Chloride: 104104 mEq/L
Bicarbonate: 1414 mEq/L
Blood urea nitrogen: 1818 mg/dL
Creatinine: 0.80.8 mg/dL

Arterial blood gas (room air):
pH: 7.477.47
PaCO2PaCO_2: 2020 mm Hg
PaO2PaO_2: 9898 mm Hg

Based on the laboratory results, which of the following best characterizes 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
    Mixed primary high anion gap metabolic acidosis and primary respiratory acidosis

Answer

Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis
This patient's clinical presentation (tinnitus, nausea, tachypnea) is classic for salicylate (aspirin) toxicity. The laboratory values reveal a mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis. First, the anion gap is elevated at 2222 mEq/L (140−[104+14]140 - [104 + 14]), confirming a primary high anion gap metabolic acidosis (HAGMA). Next, Winter's formula is applied to determine if the respiratory response is appropriate: Expected PaCO2=(1.5×14)+8=29±2PaCO_2 = (1.5 \times 14) + 8 = 29 \pm 2 mm Hg. Because the patient's measured PaCO2PaCO_2 (2020 mm Hg) is significantly lower than the expected range (2727-3131 mm Hg), there is an independent, primary respiratory alkalosis driving the PaCO2PaCO_2 down further.

Step-by-Step Solution

1
Calculate the anion gap to check for a primary metabolic acidosis.
AG=Na+−(Cl−+HCO3−)=140−(104+14)=22AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 14) = 22 mEq/L.
An elevated anion gap (>12>12 mEq/L) confirms the presence of a primary high anion gap metabolic acidosis (HAGMA).
2
Apply Winter's formula to determine the expected respiratory compensation for the metabolic acidosis.
Expected PaCO2=(1.5×HCO3−)+8±2=(1.5×14)+8±2=29±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 mm Hg (Range: 2727 to 3131 mm Hg).
This establishes the predicted normal physiologic respiratory response to the patient's low bicarbonate level.
3
Compare the measured PaCO2PaCO_2 to the expected PaCO2PaCO_2 range.
The measured PaCO2PaCO_2 is 2020 mm Hg, which is significantly lower than the expected lower limit of 2727 mm Hg.
A PaCO2PaCO_2 lower than the expected compensatory range indicates that the patient is hyperventilating beyond physiologic compensation, confirming an independent, coexisting primary respiratory alkalosis.

Key Concept

Salicylate toxicity classically presents with a mixed primary 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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Topics

Nephrology

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