An emergency physician evaluates a 24-year-old man who collapsed during gymnastics practic… — USMLE Step 2 CK

An emergency physician evaluates a 24-year-old man who collapsed during gymnastics practice. He reports profound weakness and muscle cramps over the past three days. His body mass index is $19.5 \text{ kg/m}^2$ . The patient is heavily preoccupied with his body image and fears gaining weight. He admits to consuming massive amounts of food in short periods, followed immediately by self-induced vomiting, a pattern he repeats several times a day. Vital signs are temperature $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure $100/60 \text{ mmHg}$ , pulse $105/\text{min}$ , and respirations $10/\text{min}$ . Physical examination reveals enlarged parotid glands and calluses on the dorsum of his right hand.

Laboratory results are as follows:

Test	Result	Reference Range
Sodium	137 mEq/L	136-145 mEq/L
Potassium	2.6 mEq/L	3.5-5.0 mEq/L
Chloride	88 mEq/L	98-106 mEq/L
Bicarbonate	38 mEq/L	22-28 mEq/L
Blood urea nitrogen	29 mg/dL	8-20 mg/dL
Creatinine	1.2 mg/dL	0.7-1.3 mg/dL

Arterial blood gas (room air):

Parameter	Result	Reference Range
pH	7.53	7.35-7.45
$\text{PaCO}_2$	49 mmHg	35-45 mmHg
$\text{PaO}_2$	95 mmHg	75-105 mmHg

Which of the following best describes this patient's current acid-base status?

A
Mixed metabolic alkalosis and respiratory acidosis
Simple metabolic alkalosis with appropriate respiratory compensationAnswer
C
Uncompensated metabolic alkalosis
D
Simple metabolic acidosis with appropriate respiratory compensation
E
Mixed metabolic alkalosis and respiratory alkalosis

Answer

Simple metabolic alkalosis with appropriate respiratory compensation

This patient's clinical presentation (binge eating, self-induced vomiting, parotid gland enlargement, calluses on the hand known as Russell sign) is classic for bulimia nervosa. Frequent vomiting results in the loss of gastric hydrochloric acid (HCl), leading to a primary hypochloremic, hypokalemic metabolic alkalosis. This is evidenced by the elevated pH (7.53) and elevated bicarbonate (38 mEq/L). The body naturally compensates for metabolic alkalosis via respiratory hypoventilation to retain carbon dioxide, which drops the respiratory rate (10/min in this patient) and raises the PaCO2.

The expected PaCO2 can be calculated using the formula:

\text{Expected PaCO}_2 = 40 + [0.7 \times (\text{HCO}_3^- - 24)]

.
For this patient:

40 + [0.7 \times (38 - 24)] = 40 + [0.7 \times 14] = 40 + 9.8 = 49.8 \text{ mmHg}

.
The patient's actual PaCO2 of 49 mmHg is consistent with this expected value, indicating a simple (fully compensated) disorder without a superimposed secondary respiratory process.

Step-by-Step Solution

Identify the primary acid-base disorder based on the pH and bicarbonate levels.

The pH is 7.53 (alkalemia) and the bicarbonate is elevated at 38 mEq/L, indicating a primary metabolic alkalosis.

Self-induced vomiting in bulimia nervosa causes a direct loss of gastric hydrochloric acid (HCl), leading to a hypochloremic metabolic alkalosis.

Calculate the expected respiratory compensation (expected PaCO2) for the primary metabolic alkalosis.

Using the formula: Expected PaCO2 = 40 + [0.7 * (HCO3 - 24)]. The expected PaCO2 = 40 + [0.7 * (38 - 24)] = 40 + 9.8 = 49.8 mmHg.

To determine whether the patient's respiratory response is appropriate or if there is a superimposed secondary acid-base disorder.

Compare the actual PaCO2 to the expected PaCO2.

The patient's actual PaCO2 is 49 mmHg, which perfectly matches the expected physiological compensation range.

Since the actual PaCO2 matches the calculated expectation, the condition is a simple, compensated metabolic alkalosis without any mixed respiratory disorders.

Key Concept

Patients with bulimia nervosa (purging subtype) frequently develop a hypokalemic, hypochloremic metabolic alkalosis. Accurate diagnosis of their medical complications requires identifying the primary acid-base disorder and calculating the expected respiratory compensation to rule out mixed derangements.

Estimated Time:1m 30s

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