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 acidosisCevap
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

Cevap

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.

Adım Adım Çözüm

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.

Anahtar Kavram

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.

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