Examkin

Question

Difficulty: HardInterpreting Laboratory Results

A 28-year-old woman is brought to the emergency department by her roommate due to confusion, ringing in her ears, and rapid breathing for the past 66 hours. She has also had multiple episodes of non-bilious vomiting. The roommate reports finding an empty bottle of over-the-counter pain medication in their shared bathroom. On physical examination, the patient appears agitated and diaphoretic. Vital signs are temperature 37.8∘37.8^\circ C (100.0∘100.0^\circ F), blood pressure 110/70110/70 mm Hg, heart rate 112112/min, and respiratory rate 2828/min.

Laboratory studies show:

Serum chemistry
Sodium: 140140 mEq/L
Potassium: 3.43.4 mEq/L
Chloride: 9696 mEq/L
Bicarbonate: 1414 mEq/L
Blood urea nitrogen: 2020 mg/dL
Creatinine: 1.21.2 mg/dL

Arterial blood gas (room air)
pH: 7.457.45
PaCO 2_2: 2020 mm Hg
PaO 2_2: 9696 mm Hg

Which of the following is the most accurate interpretation of this patient's acid-base status?

  1. Mixed primary elevated anion gap metabolic acidosis, primary respiratory alkalosis, and primary metabolic alkalosisAnswer
  2. B
    Mixed primary elevated anion gap metabolic acidosis and primary respiratory alkalosis
  3. C
    Primary elevated anion gap metabolic acidosis with appropriate respiratory compensation
  4. D
    Primary respiratory alkalosis with appropriate metabolic compensation
  5. E
    Mixed primary non-anion gap metabolic acidosis, primary respiratory alkalosis, and primary metabolic alkalosis

Answer

Mixed primary elevated anion gap metabolic acidosis, primary respiratory alkalosis, and primary metabolic alkalosis
The correct answer identifies a triple acid-base disorder based on systematic interpretation of the lab values. First, the anion gap is 3030 mEq/L (140−[96+14]140 - [96 + 14]), confirming a primary elevated anion gap metabolic acidosis. Second, Winter's formula demonstrates an expected PaCO 2_2 of 2929 mm Hg; the actual PaCO 2_2 of 2020 mm Hg falls below this expected range, confirming a concurrent primary respiratory alkalosis. Finally, the delta-delta gap calculation gives a corrected bicarbonate of 3232 mEq/L (14+[30−12]14 + [30 - 12]). Because this corrected value exceeds normal levels (2424-2626 mEq/L), an independent primary metabolic alkalosis is also confirmed.

Step-by-Step Solution

1
Calculate the serum anion gap to evaluate for metabolic acidosis.
Anion Gap (AG) = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 140−(96+14)=30140 - (96 + 14) = 30 mEq/L.
An elevated AG (normal 88-1212 mEq/L) confirms the presence of a primary high anion gap metabolic acidosis, which in this case is caused by salicylate toxicity and resulting lactic acidosis.
2
Apply Winter's formula to determine the expected respiratory compensation for the metabolic acidosis.
Expected PaCO 2_2 = (1.5×HCO3−)+8=(1.5×14)+8=29±2(1.5 \times HCO_3^-) + 8 = (1.5 \times 14) + 8 = 29 \pm 2 mm Hg.
Because the measured PaCO 2_2 (2020 mm Hg) is significantly lower than the expected range (2727-3131 mm Hg), an independent primary respiratory alkalosis is also present (due to direct salicylate stimulation of the medullary respiratory center).
3
Perform a delta-delta (Δ/Δ\Delta/\Delta) gap calculation to reveal any hidden metabolic disorders.
Δ\Delta AG = 30−12=1830 - 12 = 18 mEq/L. Corrected HCO3−HCO_3^- = Measured HCO3−HCO_3^- + Δ\Delta AG = 14+18=3214 + 18 = 32 mEq/L.
Since the corrected HCO3−HCO_3^- (3232 mEq/L) is greater than the normal baseline of 2424-2626 mEq/L, a concurrent primary metabolic alkalosis must be present, which is clinically consistent with her history of severe vomiting.

Key Concept

Patients with severe salicylate toxicity frequently present with a mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis; concurrent vomiting can add a third primary disorder (metabolic alkalosis), which is unmasked by calculating the delta-delta gap.
Rate this question

Topics

Interpreting Laboratory Results

More questions from this topic