A 62-year-old man presents to the emergency department with a 2-week history of worsening … — USMLE Step 2 CK

A $62$ -year-old man presents to the emergency department with a $2$ -week history of worsening nausea, early satiety, and frequent vomiting. He notes that the vomitus often contains undigested food eaten several hours earlier. He reports a $5$ -kg ( $11$ -lb) weight loss over the past month. His past medical history is notable for severe osteoarthritis of the knees, for which he takes high-dose ibuprofen daily. Temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $105/65\text{ mmHg}$ , pulse is $108/\text{min}$ , and respiratory rate is $14/\text{min}$ . Physical examination reveals dry mucous membranes and decreased skin turgor. The abdomen is distended in the epigastric region, and a splashing sound is heard over the upper abdomen when the patient is gently rocked back and forth. There is no rebound tenderness or guarding. Laboratory studies are as follows:

Laboratory Parameter	Result
Sodium	$132\text{ mEq/L}$
Potassium	$2.8\text{ mEq/L}$
Chloride	$88\text{ mEq/L}$
Bicarbonate	$34\text{ mEq/L}$
Blood urea nitrogen	$42\text{ mg/dL}$
Creatinine	$1.5\text{ mg/dL}$

Which of the following is the most appropriate initial step in the management of this patient?

Intravenous administration of $0.9\%$ sodium chloride and potassium chlorideAnswer
B
Immediate exploratory laparotomy
C
Administration of broad-spectrum intravenous antibiotics
D
Application of Winter's formula to determine expected arterial carbon dioxide
E
Measurement of serum thyroid-stimulating hormone and free thyroxine levels

Answer

Intravenous administration of 0.9% sodium chloride and potassium chloride

The correct answer accurately prioritizes fluid and electrolyte resuscitation. In gastric outlet obstruction, recurrent vomiting of gastric contents leads to a profound loss of fluid, hydrogen, and chloride. The resulting volume contraction stimulates the renin-angiotensin-aldosterone axis, which promotes sodium reabsorption in exchange for potassium and hydrogen excretion, worsening both hypokalemia and metabolic alkalosis. Administration of intravenous

0.9\%

sodium chloride restores intravascular volume, stopping aldosterone-driven losses, while providing the necessary chloride to allow renal excretion of excess bicarbonate. Potassium chloride directly repletes the profound cellular potassium deficit.

Step-by-Step Solution

Analyze the clinical presentation and physical examination findings.

The patient has a history of chronic NSAID use, vomiting of undigested food, weight loss, and a positive succussion splash. This constellation is diagnostic of gastric outlet obstruction (GOO), likely secondary to a peptic ulcer stricture.

Identifying the mechanical nature of the upper gastrointestinal obstruction is necessary to guide therapy.

Interpret the laboratory values in the context of the clinical picture.

The labs show a hypokalemic (

2.8\text{ mEq/L}

), hypochloremic (

88\text{ mEq/L}

) metabolic alkalosis (bicarbonate

34\text{ mEq/L}

) with prerenal acute kidney injury (BUN:Cr ratio

> 20:1

Prolonged vomiting of gastric hydrochloric acid leads to direct loss of hydrogen and chloride. Subsequent volume depletion triggers aldosterone release, which attempts to retain sodium at the expense of wasting more potassium and hydrogen into the urine (contraction alkalosis).

Determine the most appropriate initial management step.

Aggressive intravenous fluid resuscitation with normal saline (

0.9\%\text{ NaCl}

) and potassium chloride supplementation must be initiated immediately.

Volume expansion with chloride-rich fluid turns off the renin-angiotensin-aldosterone system, allowing the kidneys to excrete excess bicarbonate. Potassium must be replenished to correct total body deficits and stop paradoxical aciduria. This medical resuscitation is required before proceeding to upper endoscopy or surgery.

Key Concept

Gastric outlet obstruction classically presents with nonbilious vomiting, early satiety, a succussion splash, and a hypokalemic, hypochloremic metabolic alkalosis. Initial management mandates aggressive fluid resuscitation with intravenous normal saline and potassium chloride prior to diagnostic evaluation or definitive surgical intervention.

Estimated Time:1m 30s

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