Mechanical Intestinal Obstruction

Definition

Mechanical intestinal obstruction is a physical blockage of the bowel lumen that prevents the normal passage of intestinal contents, leading to:

  • Proximal bowel dilatation
  • Distal bowel collapse
  • Fluid sequestration
  • Electrolyte imbalance
  • Risk of ischemia and perforation

It must be differentiated from functional obstruction (ileus), where there is no physical blockage.

Consequences:

  • ↑ Intraluminal pressure
  • Venous congestion
  • Mucosal edema
  • Bacterial translocation
  • Ischemia (if pressure > capillary perfusion)


2️⃣ Fluid & Electrolyte Shifts 

Up to 6–8 liters/day of GI secretions can be trapped proximally.

Leads to:

  • Hypovolemia
  • Metabolic alkalosis (early vomiting)
  • Hypokalemia
  • Hyponatremia
  • Later metabolic acidosis (ischemia, sepsis)


3️⃣ Strangulation (Surgical Emergency)

If obstruction compromises blood supply:

  • Venous obstruction → edema
  • Arterial obstruction → infarction
  • Bacterial translocation
  • Perforation → peritonitis → septic shock


Classification of Mechanical Obstruction

A. Based on Site

Small Bowel Obstruction (SBO)

Most common (~70–80%)

Large Bowel Obstruction (LBO)

Less common but more dangerous (risk of perforation)


B. Based on Mechanism

Type

Mechanism

Example

Extrinsic

Compression outside bowel

Adhesions

Intrinsic

Wall pathology

Tumor

Intraluminal

Inside lumen

Gallstone ileus

Closed-loop

Two-point obstruction

Volvulus

Strangulated

Vascular compromise

Incarcerated hernia



 Small Bowel Obstruction (SBO)

Common Causes

1️⃣ Postoperative Adhesions (Most common)

  • 60–70% cases
  • History of prior abdominal surgery
  • Recurrent episodes common


2️⃣ Hernias

  • Inguinal (most common)
  • Femoral (high strangulation risk)
  • Umbilical

Always examine hernial orifices in obstruction.


3️⃣ Tumors

4️⃣ Crohn’s Disease–Strictures


5️⃣ Gallstone Ileus

Gallstone ileus

  • Large gallstone enters bowel via cholecystoenteric fistula
  • Typically obstructs ileum

CT: Rigler’s triad

  • Pneumobilia
  • SBO
  • Ectopic gallstone


6️⃣ Intussusception (Adults → Think malignancy)


 Large Bowel Obstruction (LBO)

Common Causes

1️⃣ Colorectal Carcinoma (Most common)

Colorectal cancer

  • Elderly
  • Change in bowel habits
  • Weight loss
  • “Apple-core” lesion on imaging


2️⃣ Volvulus

Sigmoid Volvulus (Most common)

  • Coffee bean sign
  • Seen in elderly, chronic constipation

Cecal Volvulus

  • Younger patients
  • More acute presentation


3️⃣ Diverticular Stricture

Diverticular disease

Chronic inflammation → fibrosis → narrowing


 Closed-Loop Obstruction 

Occurs when a bowel segment is obstructed at two points.

Examples:

  • Volvulus
  • Internal hernia
  • Adhesion band

⚠ High risk of:

  • Rapid distension
  • Ischemia
  • Perforation

CT finding:

  • C-shaped loop
  • Whirl sign
  • Radial mesenteric vessels


🧠 Clinical Presentation

SBO

  • Colicky abdominal pain
  • Vomiting (early)
  • Abdominal distension (mild initially)
  • Obstipation (late)

LBO

  • Distension prominent
  • Constipation early
  • Vomiting late
  • Pain less colicky


🔬 Physical Examination

  • High-pitched tinkling bowel sounds (early)
  • Silent abdomen (late/ischemia)
  • Peritonitis signs → urgent surgery
  • Hernial orifices check


 Laboratory Findings

  • Hemoconcentration
  • Leukocytosis (strangulation)
  • Metabolic alkalosis (early vomiting)
  • Metabolic acidosis (ischemia)
  • Elevated lactate (late sign)


🖥 Imaging

1️⃣ X-Ray Abdomen

SBO:

  • Dilated small bowel (>3 cm)
  • Valvulae conniventes visible
  • Multiple air-fluid levels

LBO:

  • Dilated colon (>6 cm)
  • Cecum >9 cm → perforation risk


2️⃣ CT Abdomen (Gold Standard)

Findings:

  • Transition point
  • Proximal dilatation
  • Distal collapse
  • Pneumatosis (ischemia)
  • Portal venous gas


 Red Flags for Strangulation 

  • Continuous pain (not colicky)
  • Fever
  • Tachycardia
  • Leukocytosis
  • Acidosis
  • Elevated lactate
  • Peritonitis
  • CT: Reduced enhancement


 Management (Guideline-Oriented Approach)

Initial ICU Management

  1. NPO
  2. Large-bore IV access
  3. Aggressive isotonic fluid resuscitation
  4. Electrolyte correction
  5. NG tube decompression
  6. Urinary catheter (monitor urine output)
  7. Broad-spectrum antibiotics (if strangulation suspected)


Conservative Management (SBO Only)

Indicated in:

  • Adhesive SBO
  • No strangulation
  • Hemodynamically stable

Trial: 24–48 hours(~70–80% of adhesive SBO resolves without surgery)


Surgical Indications

Immediate surgery if:

  • Strangulation suspected
  • Peritonitis
  • Closed-loop obstruction
  • Failure of conservative therapy
  • Complete obstruction
  • LBO (usually surgical)


⚠ Cecal Diameter Rule (Exam Favorite)

  • 9 cm → high perforation risk
  • 12 cm → imminent perforation

 SBO vs Ileus 

Feature

Mechanical SBO

Ileus

Pain

Colicky

Dull

Bowel sounds

Hyperactive

Absent

Air-fluid levels

Multiple

Diffuse gas

Transition point

Present

Absent

Management

Often surgical

Supportive


 Complications

  • Strangulation
  • Bowel infarction
  • Perforation
  • Peritonitis
  • Septic shock
  • Short bowel syndrome


📌 Special ICU Considerations

  • Massive third spacing
  • ARDS risk (aspiration)
  • Post-op ileus differentiation
  • Intra-abdominal pressure monitoring
  • Abdominal compartment syndrome


📝 Exam Pearls 

✔ Most common cause of SBO → Adhesions
✔ Most common cause of LBO → Colorectal cancer
✔ Most common volvulus → Sigmoid
✔ Closed-loop obstruction → High mortality
✔ Lactate elevation → Late ischemia
✔ Cecum >12 cm → Perforation risk