A concerned dormitory resident advisor escorts a 20-year-old female undergraduate to the u… — USMLE Step 2 CK

A concerned dormitory resident advisor escorts a 20-year-old female undergraduate to the university health center. The student complains of a constant "buzzing" sound in her ears, dizziness, and a feeling of being "short of breath" since waking up this morning. She admits to consuming an entire bottle of over-the-counter pain relievers last night following a difficult personal event. Her temperature is 37.6°C (99.7°F), blood pressure is 112/68 mmHg, pulse is 118/min, and respiratory rate is 26/min. She is diaphoretic and appears anxious. Laboratory evaluation reveals the following:

Laboratory Parameter	Result
Serum sodium	140 mEq/L
Serum chloride	100 mEq/L
Serum bicarbonate	12 mEq/L
Blood urea nitrogen	16 mg/dL
Serum creatinine	0.8 mg/dL
Arterial pH	7.48
Arterial PaCO2	16 mm Hg
Arterial PaO2	102 mm Hg

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

Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosisAnswer
B
Primary high anion gap metabolic acidosis with appropriate respiratory compensation
C
Primary respiratory alkalosis with appropriate metabolic compensation
D
Mixed primary normal anion gap metabolic acidosis and primary respiratory alkalosis
E
Primary respiratory alkalosis and primary metabolic alkalosis

Answer

Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis

This patient presents with signs of salicylate toxicity (tinnitus, tachypnea, ingestion history). Her arterial blood gas shows an alkalemia (

pH = 7.48

), primarily driven by a decreased PaCO2 (

16\ mm\ Hg

), indicating a primary respiratory alkalosis. However, comprehensive acid-base analysis requires calculating the anion gap:

Na^+ - (Cl^- + HCO_3^-)

, which is

140 - (100 + 12) = 28\ mEq/L

. Because normal is 8-12 mEq/L, a gap of 28 confirms the presence of a primary high anion gap metabolic acidosis (HAGMA). To determine if the low PaCO2 is simply compensation or an independent primary disorder, Winter's formula is applied to the bicarbonate level:

Expected\ PaCO_2 = (1.5 \times 12) + 8 = 26 \pm 2\ mm\ Hg

. Since the patient's measured PaCO2 (

16\ mm\ Hg

) is well below the expected compensatory range (

24-28\ mm\ Hg

), she has an independent, primary respiratory alkalosis occurring simultaneously. This mixed disorder is classic for salicylate toxicity, which directly stimulates the medullary respiratory center and subsequently uncouples oxidative phosphorylation.

Step-by-Step Solution

Evaluate the primary pH disturbance.

Arterial pH is 7.48, indicating an alkalemia. The PaCO2 is low (16 mm Hg), which aligns with the high pH, identifying a primary respiratory alkalosis.

Establishing the initial direction of the pH and the corresponding primary driver is the first step in acid-base analysis.

Calculate the serum anion gap.

Anion\ Gap = Na^+ - (Cl^- + HCO_3^-) = 140 - (100 + 12) = 28\ mEq/L

An anion gap must be calculated in every acid-base problem. An anion gap > 12 mEq/L indicates the presence of a primary high anion gap metabolic acidosis (HAGMA), regardless of the overall pH.

Use Winter's formula to determine the expected PaCO2 for the metabolic acidosis.

Expected\ PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 12) + 8 = 26 \pm 2\ mm\ Hg

This determines if the respiratory response is purely compensatory or if there is a concurrent respiratory disorder.

Compare measured PaCO2 to expected PaCO2.

Measured PaCO2 is 16 mm Hg, which is significantly lower than the expected range of 24-28 mm Hg.

Because the measured PaCO2 is lower than expected, there is an additional process driving down CO2, confirming an independent, primary respiratory alkalosis.

Key Concept

Diagnosis of mixed acid-base disorders using the anion gap and Winter's formula

Estimated Time:1m 30s

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