A 28-year-old woman is brought to the emergency department by her roommate due to confusio… — USMLE Step 3

A $28$ -year-old woman is brought to the emergency department by her roommate due to confusion, nausea, and ringing in her ears for the past $6$ hours. She has a history of major depressive disorder and was recently prescribed a new medication for chronic lower back pain. Vital signs show a temperature of $37.8^\circ$ C ( $100.0^\circ$ F), blood pressure of $110/70$ mmHg, pulse of $115$ /min, and respiratory rate of $28$ /min. Physical examination reveals a diaphoretic and anxious woman.

Laboratory studies are obtained:

Complete Blood Count & Serum Chemistry	Result	Reference Range
Hemoglobin	$13.2$ g/dL	$12.0$ – $16.0$ g/dL
Platelets	$210,000/\mu$ L	$150,000$ – $400,000/\mu$ L
Sodium ( $Na^+$ )	$140$ mEq/L	$136$ – $145$ mEq/L
Potassium ( $K^+$ )	$3.8$ mEq/L	$3.5$ – $5.0$ mEq/L
Chloride ( $Cl^-$ )	$104$ mEq/L	$98$ – $106$ mEq/L
Bicarbonate ( $HCO_3^-$ )	$14$ mEq/L	$22$ – $28$ mEq/L
Blood urea nitrogen	$18$ mg/dL	$7$ – $18$ mg/dL
Creatinine	$0.9$ mg/dL	$0.6$ – $1.2$ mg/dL

Arterial Blood Gas (Room Air)	Result	Reference Range
pH	$7.47$	$7.35$ – $7.45$
$PaCO_2$	$20$ mm Hg	$35$ – $45$ mm Hg
$PaO_2$	$98$ mm Hg	$80$ – $100$ 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 respiratory alkalosis with appropriate metabolic compensation
C
Primary high anion gap metabolic acidosis with appropriate respiratory compensation
D
Mixed primary normal anion gap metabolic acidosis and primary respiratory alkalosis
E
Mixed primary high anion gap metabolic acidosis and primary metabolic alkalosis

Answer

The correct interpretation is a mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis.

The patient's presentation of tinnitus, tachypnea, diaphoresis, and confusion is highly characteristic of acute salicylate toxicity. Laboratory interpretation requires a step-wise approach. First, the arterial pH is 7.47 (alkalemia) driven by a low PaCO2 of 20 mm Hg, confirming a primary respiratory alkalosis resulting from direct stimulation of the medullary respiratory center. Second, the serum anion gap must be calculated: 140 - (104 + 14) = 22 mEq/L. This elevated gap indicates an accumulation of unmeasured organic acids, establishing an independent primary high anion gap metabolic acidosis (HAGMA). Finally, checking Winter's formula (1.5 * 14 + 8 = 29 mm Hg) demonstrates that the actual PaCO2 (20 mm Hg) is much lower than the expected compensatory range (27-31 mm Hg), conclusively proving this is a mixed primary disorder.

Step-by-Step Solution

Identify the primary acid-base disturbance by evaluating the pH and PaCO2.

The pH is 7.47 (alkalemia) and the PaCO2 is 20 mm Hg (hypocapnia). Because the low PaCO2 drives the pH in the alkaline direction, a primary respiratory alkalosis is present.

Establishing the primary driver is the essential first step in acid-base interpretation.

Calculate the serum anion gap to check for an underlying metabolic acidosis.

AG = Na - (Cl + HCO3) = 140 - (104 + 14) = 22 mEq/L. Since 22 > 12, a high anion gap metabolic acidosis is also present.

An elevated anion gap reveals an accumulation of unmeasured acids (e.g., salicylic acid, lactic acid), which represents an independent primary metabolic disorder regardless of the pH.

Apply Winter's formula to determine if the respiratory alkalosis is purely compensatory or a separate primary disorder.

Expected PaCO2 = 1.5 * HCO3 + 8 = 1.5 * 14 + 8 = 29 +/- 2 mm Hg. The actual PaCO2 (20 mm Hg) is significantly lower than the expected compensatory range.

A PaCO2 lower than the predicted compensatory level confirms the simultaneous presence of an independent primary respiratory alkalosis, solidifying the diagnosis of a mixed acid-base disorder.

Key Concept

Patients with severe salicylate toxicity typically present with a mixed primary acid-base disorder: a primary respiratory alkalosis (due to direct medullary respiratory center stimulation) and a primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation and accumulation of organic acids).

Alternative Method

Instead of Winter's formula, the Delta Ratio (Change in AG / Change in HCO3) can be evaluated. A ratio around 1.0 (10 / 10 in this case) confirms a pure high anion gap metabolic acidosis component. However, the simultaneous severe alkalemia (pH 7.47) and profound hypocapnia independently mandate the diagnosis of a concurrent primary respiratory alkalosis.

Estimated Time:2m 0s

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