A 72-year-old man with a history of severe chronic obstructive pulmonary disease and a pri… — USMLE Step 2 CK

A $72$ -year-old man with a history of severe chronic obstructive pulmonary disease and a prior appendectomy is evaluated in the hospital for sudden-onset, severe, diffuse abdominal pain and bilious vomiting. He was admitted $4$ days ago for community-acquired pneumonia. A sputum culture collected on admission grew *Streptococcus pneumoniae* that is susceptible to penicillin and ceftriaxone; however, he has been maintained on the empiric intravenous piperacillin-tazobactam started in the emergency department. He has not passed flatus for $24$ hours. Temperature is $38.8^\circ\text{C}$ ( $101.8^\circ\text{F}$ ), blood pressure is $88/54\text{ mm Hg}$ , pulse is $122/\text{min}$ , and respirations are $26/\text{min}$ . Physical examination reveals a markedly distended, tympanitic abdomen with diffuse rebound tenderness and involuntary guarding. Bowel sounds are absent. Lung auscultation reveals decreased crackles at the right base compared to admission.

Laboratory results are as follows:

Test	Value
Leukocyte count	$22,000/\text{mm}^3$ ( $15\%$ bands)
Serum sodium	$140\text{ mEq/L}$
Serum potassium	$4.0\text{ mEq/L}$
Serum chloride	$104\text{ mEq/L}$
Serum bicarbonate	$12\text{ mEq/L}$
Blood urea nitrogen	$30\text{ mg/dL}$
Serum creatinine	$1.5\text{ mg/dL}$
Serum lactate	$6.5\text{ mmol/L}$

Arterial blood gas analysis on room air shows:
pH: $7.27$
pCO $_2$ : $26\text{ mm Hg}$
pO $_2$ : $85\text{ mm Hg}$

An upright abdominal radiograph demonstrates multiple dilated loops of small bowel with prominent air-fluid levels and a paucity of gas in the colon. No free air is visible under the diaphragm.

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

Immediate exploratory laparotomyAnswer
B
Nasogastric tube insertion, bowel rest, and intravenous fluid hydration
C
Endotracheal intubation and mechanical ventilation
D
Broaden antimicrobial therapy to intravenous meropenem and vancomycin
E
Administer intravenous furosemide and nebulized albuterol

Answer

Immediate exploratory laparotomy

The correct intervention is immediate surgical exploration. While conservative management (nasogastric decompression, bowel rest) is appropriate for uncomplicated small bowel obstruction, this patient exhibits definitive signs of strangulation and bowel ischemia. These "red flags" include severe hemodynamic instability, fever, marked peritonitis (rebound and guarding), significant leukocytosis, and an elevated lactate leading to a high-anion-gap metabolic acidosis. In this clinical scenario, any delay in surgical source control drastically increases the risk of bowel perforation and mortality.

Step-by-Step Solution

Interpret the abdominal radiograph and clinical presentation.

Dilated small bowel loops, air-fluid levels, obstipation, and bilious vomiting confirm a diagnosis of small bowel obstruction (SBO).

Establishing the mechanical basis of the patient's gastrointestinal symptoms is the first priority.

Assess for complications of the SBO.

The patient exhibits fever, hypotension, severe rebound tenderness/guarding, and a markedly elevated lactate of 6.5 mmol/L.

These findings are 'red flags' that differentiate a simple SBO from a strangulated (ischemic) SBO.

Evaluate the patient's acid-base status using Winter's formula.

Anion gap is

140 - 104 - 12 = 24

. Expected pCO2 is

(1.5 \times 12) + 8 = 26

. Measured pCO2 is 26 mm Hg, indicating perfect respiratory compensation.

This rules out a concurrent primary respiratory acidosis and confirms that intubation is not acutely indicated for the tachypnea.

Select the definitive management for the diagnosed condition.

Emergency surgical exploration is selected over medical or conservative management.

Strangulated bowel rapidly progresses to perforation and death without mechanical source control (resection of ischemic bowel).

Key Concept

Patients presenting with a small bowel obstruction who develop signs of strangulation or bowel ischemia (e.g., fever, tachycardia, hemodynamic instability, severe localized pain/peritonitis, high-anion-gap metabolic acidosis) require immediate exploratory laparotomy, overriding trials of conservative management.

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