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Difficulty: MediumInterpreting Laboratory Results

Emergency medical services bring a 24-year-old female to the emergency department because of altered mental status and reported ringing in her ears. According to her roommate, the patient was found in her bedroom surrounded by several empty bottles of over-the-counter analgesics. On physical examination, she is diaphoretic and confused. Her temperature is 37.8°C (100.0°F), pulse is 118/min, respirations are 30/min, and blood pressure is 112/68 mm Hg. The lungs are clear to auscultation. A radial artery blood sample and a peripheral venous blood sample are drawn. Laboratory results are as follows:

Arterial blood gas (room air)
pH\text{pH}: 7.42
PaO2\text{PaO}_2: 98 mm Hg
PaCO2\text{PaCO}_2: 20 mm Hg
HCO3−\text{HCO}_3^-: 13 mEq/L

Serum chemistries
Sodium: 140 mEq/L
Potassium: 3.8 mEq/L
Chloride: 104 mEq/L
Bicarbonate: 13 mEq/L
Blood urea nitrogen: 16 mg/dL
Creatinine: 0.9 mg/dL

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

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

Answer

Primary metabolic acidosis and primary respiratory alkalosis
The correct answer accurately synthesizes the multi-step interpretation of the provided laboratory data. Step 1: Calculate the anion gap. AG=Na+−(Cl−+HCO3−)=140−(104+13)=23\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (104 + 13) = 23. A normal anion gap is approximately 8 to 12. Since 23 > 12, a primary high anion gap metabolic acidosis is present. Step 2: Determine appropriate respiratory compensation using Winter's formula. Expected PaCO2=(1.5×HCO3−)+8±2=(1.5×13)+8=27.5±2\text{PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 13) + 8 = 27.5 \pm 2. The expected PaCO2\text{PaCO}_2 range is 25.5 to 29.5 mm Hg. Step 3: Compare expected to measured PaCO2\text{PaCO}_2. The patient's actual PaCO2\text{PaCO}_2 is 20 mm Hg, which is lower than expected. This indicates an independent process driving down carbon dioxide, confirming a superimposed primary respiratory alkalosis. Finally, calculating the delta gap (ΔAG=23−12=11\Delta\text{AG} = 23 - 12 = 11) and adding it to the measured bicarbonate (11+13=2411 + 13 = 24) yields a normal baseline bicarbonate, ruling out a third metabolic disorder. This mixed acid-base profile is classic for salicylate toxicity, which directly stimulates the medullary respiratory center (causing respiratory alkalosis) and uncouples oxidative phosphorylation (causing high anion gap metabolic acidosis).

Step-by-Step Solution

1
Calculate the serum anion gap to screen for a primary metabolic acidosis.
Anion Gap = 140−(104+13)=23140 - (104 + 13) = 23.
An elevated anion gap (greater than 12) definitively establishes the presence of a primary high anion gap metabolic acidosis (HAGMA), regardless of the overall pH.
2
Use Winter's formula to determine the expected respiratory compensation for the identified metabolic acidosis.
Expected PaCO2=(1.5×13)+8±2=19.5+8±2=27.5±2\text{PaCO}_2 = (1.5 \times 13) + 8 \pm 2 = 19.5 + 8 \pm 2 = 27.5 \pm 2 (Range: 25.5 to 29.5 mm Hg).
Winter's formula determines what the carbon dioxide level should be if the hyperventilation is purely compensatory for the metabolic acidosis.
3
Compare the actual measured PaCO2\text{PaCO}_2 to the expected PaCO2\text{PaCO}_2.
The measured PaCO2\text{PaCO}_2 (20 mm Hg) is lower than the expected range (25.5 to 29.5 mm Hg).
Because the measured carbon dioxide is driven lower than the expected compensatory range, there is an independent, concomitant primary respiratory alkalosis.

Key Concept

Interpretation of mixed acid-base disorders requires calculating the anion gap and using Winter's formula to uncover underlying primary processes that may mask each other in the overall pH.
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Interpreting Laboratory Results

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