Emergency medical personnel transport a 21-year-old female university student to the emerg… — USMLE Step 2 CK

Emergency medical personnel transport a 21-year-old female university student to the emergency department after her roommates found her profoundly confused and breathing rapidly. The roommates report that she has been studying intensely for final exams over the past week and had complained of a persistent "ringing in her ears" earlier in the day. She has no known past medical history and takes no prescribed medications.

Vital signs reveal:
* Temperature: 38.2°C (100.8°F)
* Blood pressure: 112/68 mm Hg
* Heart rate: 115/min
* Respiratory rate: 28/min

Physical examination shows an agitated, diaphoretic patient with prominent hyperreflexia.

Initial laboratory test results are shown below:

Test	Result
Sodium ( $Na^+$ )	141 mEq/L
Potassium ( $K^+$ )	3.9 mEq/L
Chloride ( $Cl^-$ )	103 mEq/L
Bicarbonate ( $HCO_3^-$ )	14 mEq/L
Blood urea nitrogen	18 mg/dL
Creatinine	1.0 mg/dL

An arterial blood gas sample obtained on room air reveals:

Test	Result
pH	7.44
$pCO_2$	21 mm Hg
$pO_2$	98 mm Hg

Based on the provided laboratory data, which of the following best describes the acid-base disturbance present in this patient?

Mixed 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 normal anion gap metabolic acidosis and primary respiratory alkalosis
E
Triple acid-base disorder: high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis

Answer

The patient has a mixed high anion gap metabolic acidosis and primary respiratory alkalosis.

The correct answer identifies both the primary high anion gap metabolic acidosis and the primary respiratory alkalosis. The patient's clinical presentation—tinnitus, hyperthermia, tachypnea, and agitation—is classic for acute salicylate (aspirin) toxicity. Salicylates cause two distinct primary acid-base disturbances: they directly stimulate the medullary respiratory center causing primary hyperventilation (respiratory alkalosis), and they uncouple oxidative phosphorylation, leading to the accumulation of organic acids like lactic acid and ketoacids (high anion gap metabolic acidosis).

Mathematically, the diagnosis is confirmed in three steps:
1. Anion Gap:

141 - (103 + 14) = 24

. The elevated gap (>12) confirms a primary High Anion Gap Metabolic Acidosis.
2. Winter's Formula: Expected

pCO_2

for the metabolic acidosis is

1.5 \times 14 + 8 = 29 \pm 2

mm Hg. Because the actual

pCO_2

(21 mm Hg) is lower than 27 mm Hg, an independent, concurrent primary respiratory alkalosis exists.
3. Delta Gap: The gap difference is

24 - 12 = 12

. Adding this to the measured bicarbonate yields a corrected

HCO_3^-

14 + 12 = 26

mEq/L. Since this falls into the normal range (22-26 mEq/L), no third disorder is present.

Step-by-Step Solution

Calculate the Anion Gap (AG).

AG = Na^+ - (Cl^- + HCO_3^-) = 141 - (103 + 14) = 24

mEq/L.

An elevated anion gap (>12) confirms the presence of a primary High Anion Gap Metabolic Acidosis (HAGMA), regardless of the net pH.

Use Winter's formula to determine if respiratory compensation is appropriate for the metabolic acidosis.

Expected

pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2

mm Hg.

This establishes the baseline

pCO_2

expected strictly from metabolic compensation. Since the patient's actual

pCO_2

(21 mm Hg) is significantly lower than the expected range (27-31 mm Hg), there is a concurrent primary respiratory alkalosis.

Calculate the Delta Gap to evaluate for a third occult disorder (metabolic alkalosis or NAGMA).

Delta Gap = Actual AG (24) - Normal AG (12) = 12. Corrected

HCO_3^-

= Measured

HCO_3^-

(14) + Delta Gap (12) = 26 mEq/L.

A corrected bicarbonate between 22 and 26 mEq/L indicates that there is no concurrent non-anion gap metabolic acidosis or metabolic alkalosis present.

Key Concept

Salicylate toxicity classically presents with a mixed acid-base disorder: a primary respiratory alkalosis (from direct medullary stimulation) and a primary high anion gap metabolic acidosis (from uncoupled oxidative phosphorylation). Diagnosing this requires calculating the anion gap and applying Winter's formula, even when the net pH is alkalemic.

Estimated Time:1m 30s

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