A 27-year-old female is evaluated in the emergency department for severe fatigue, generali… — USMLE Step 2 CK

A 27-year-old female is evaluated in the emergency department for severe fatigue, generalized muscle weakness, and palpitations. She has a history of major depressive disorder and a long-standing intense fear of gaining weight. She exercises vigorously for 2 hours daily and adheres to a strict diet. Her body mass index is $22.0 \text{ kg/m}^2$ . She denies self-induced vomiting but admits to secretly taking "herbal bowel cleansers" containing senna extract multiple times a day for the past year to prevent weight gain. Vital signs are temperature 37.0°C (98.6°F), blood pressure 95/60 mm Hg, pulse 112/min, and respiratory rate 12/min. Physical examination reveals dry oral mucosa and diffusely decreased muscle strength in all extremities. Electrocardiogram shows flattened T waves and prominent U waves.

Laboratory studies:
Sodium: $135 \text{ mEq/L}$
Potassium: $2.4 \text{ mEq/L}$
Chloride: $115 \text{ mEq/L}$
Bicarbonate: $12 \text{ mEq/L}$
Blood urea nitrogen: $28 \text{ mg/dL}$
Creatinine: $1.1 \text{ mg/dL}$

Arterial blood gas analysis on room air:
pH: 7.27
$pCO_2$ : 38 mm Hg
$pO_2$ : 94 mm Hg

Based on the clinical and laboratory findings, which of the following best describes this patient's acid-base status?

Normal anion gap metabolic acidosis with a concurrent respiratory acidosisAnswer
B
Normal anion gap metabolic acidosis with appropriate respiratory compensation
C
High anion gap metabolic acidosis with a concurrent respiratory acidosis
D
High anion gap metabolic acidosis with appropriate respiratory compensation
E
Primary metabolic alkalosis with a concurrent respiratory acidosis

Answer

Normal anion gap metabolic acidosis with a concurrent respiratory acidosis

This patient presents with a purging-type eating disorder utilizing severe laxative abuse. Laxatives cause significant gastrointestinal loss of potassium and bicarbonate, classically leading to a hypokalemic normal anion gap metabolic acidosis (NAGMA). Her anion gap is calculated as

Na^+ - (Cl^- + HCO_3^-) = 135 - (115 + 12) = 8 \text{ mEq/L}

, which is well within normal limits. To assess for mixed acid-base disorders in metabolic acidosis, Winter's formula must be applied: Expected

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

. For this patient, the expected

pCO_2 = (1.5 \times 12) + 8 = 26 \pm 2 \text{ mm Hg}

. Because her measured

pCO_2

(38 mm Hg) is significantly higher than the expected range (24-28 mm Hg), she is hypoventilating relative to her metabolic state, indicating a concurrent primary respiratory acidosis. This hypoventilation (respiratory rate 12/min) is a direct consequence of her profound hypokalemia (

2.4 \text{ mEq/L}

), which causes generalized muscle weakness, including the respiratory muscles.

Step-by-Step Solution

Analyze the pH and bicarbonate levels to determine the primary disorder.

pH is 7.27 (acidemia) and bicarbonate is low (

12 \text{ mEq/L}

), confirming a primary metabolic acidosis.

Establishing the primary acid-base disturbance is the required first step before calculating gaps or compensation.

Calculate the serum anion gap using the formula:

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

135 - (115 + 12) = 135 - 127 = 8 \text{ mEq/L}

. The anion gap is normal (reference range 8-12 mEq/L).

Differentiating between normal and high anion gap metabolic acidosis narrows the differential diagnosis (e.g., GI bicarbonate loss vs. ketoacidosis).

Apply Winter's formula to calculate the expected

pCO_2

: Expected

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

Expected

pCO_2 = (1.5 \times 12) + 8 = 18 + 8 = 26 \pm 2 \text{ mm Hg}

Winter's formula determines if the respiratory system is adequately compensating for the metabolic acidosis.

Compare the measured

pCO_2

to the expected

pCO_2

range.

The measured

pCO_2

is 38 mm Hg, which is significantly higher than the expected range of 24-28 mm Hg. This indicates a concurrent primary respiratory acidosis.

An inappropriately high

pCO_2

in the setting of metabolic acidosis defines a mixed disorder (concurrent respiratory acidosis), which in this case is caused by hypokalemia-induced respiratory muscle weakness.

Key Concept

Patients with eating disorders who abuse laxatives develop a normal anion gap metabolic acidosis (due to GI bicarbonate loss), which can be complicated by respiratory acidosis if severe hypokalemia causes hypoventilation.

Estimated Time:1m 30s

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