7.2 Intestinal Obstruction, Bowel Ischaemia & Perforated Viscus

Intestinal Obstruction: Etiology & Pathophysiology

Mechanical intestinal obstruction occurs when a physical blockage impedes intraluminal transit; functional pseudo-obstruction causes dilation without a mechanical blockage. It accounts for approximately 15% of all emergency surgical admissions for acute abdominal pain.

Etiological Spectrum: Small vs. Large Bowel

• Small Bowel Obstruction (SBO):
  • Postoperative Adhesions (60% to 70%): Fibrous bands formed between peritoneal surfaces following prior abdominal or pelvic surgery (appendectomy, colorectal resections, gynecological operations).
  • Incarcerated / Strangulated Hernias (15% to 20%): Groin hernias (femoral and indirect inguinal) and ventral/incisional hernias. Hernias remain the leading cause of SBO in patients without prior surgical incisions.
  • Malignancy (5%): Metastatic peritoneal carcinomatosis, gastrointestinal stromal tumors (GIST), adenocarcinoma of the small bowel, or lymphoma.
  • Intraluminal & Congenital Causes: Gallstone ileus (erosion of a large gallstone through a cholecystoenteric fistula into the duodenum, impacting classically at the ileocecal valve; characterized by Rigler's triad: pneumobilia, mechanical SBO, and an ectopic radiopaque gallstone in the right iliac fossa), Crohn's disease strictures, intussusception, and radiation enteritis.
• Large Bowel Obstruction (LBO):
  • Colorectal Malignancy (60% to 70%): Obstructing adenocarcinoma, most commonly situated in the rectosigmoid colon or splenic flexure.
  • Volvulus (10% to 15%): Torsion of a mobile, redundant bowel segment around its mesenteric axis (sigmoid volvulus accounts for 75-80% of volvulus cases; cecal volvulus accounts for 15-20%).
  • Diverticular Stricture (10%): Chronic fibrotic narrowing secondary to recurrent diverticulitis.
  • Other Mechanical Causes: Severe fecal impaction in elderly or bedbound patients and pelvic endometriosis. Ogilvie syndrome is a functional mimic, not a mechanical cause.

The Cardinal Quartet of Bowel Obstruction

1. Colicky Abdominal Pain: Intermittent, cramping abdominal pain synchronized with hyperperistaltic contractions attempting to overcome the mechanical blockage. Paroxysms occur every 3 to 5 minutes in proximal SBO and every 10 to 15 minutes in distal SBO or colonic obstruction. The development of constant, unremitting pain heralds impending bowel strangulation or ischemia.
2. Vomiting: Timing and character depend strictly on the anatomical level of obstruction:
   • Proximal / High SBO: Early, frequent, profuse vomiting containing gastric juices and bile; minimal abdominal distension.
   • Distal / Low SBO: Delayed vomiting; initially bilious, progressing over 24 to 48 hours to orange-brown feculent emesis caused by retrograde bacterial colonization and degradation of stagnant intestinal contents.
   • LBO: Late feature; may be absent if the ileocecal valve is competent.
3. Abdominal Distension: Minimal in high jejunal obstruction; marked and progressive in distal ileal SBO and colonic obstruction.
4. Obstipation (Absolute Constipation): The complete inability to pass either flatus or feces per rectum. Note that early in the course of acute obstruction, the patient may evacuate residual gas and stool stored distal to the transition point.

Physical Examination Findings

• Inspection: Abdominal distension, visible peristaltic waves in thin individuals, and previous laparotomy scars. Careful inspection of the groin and umbilical regions is mandatory to exclude hidden incarcerated hernias.
• Auscultation: Early obstruction features hyperactive, high-pitched, metallic 'tinkling' bowel sounds (borborygmi) echoing within gas-filled dilated loops. In late, neglected, or strangulated obstruction, bowel sounds become hypoactive or entirely silent.
• Digital Rectal Examination (DRE): Essential to evaluate for rectal masses, strictures, fecal impaction, or the presence of gross blood or occult melena.

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Radiographic Criteria & The Rule of 3-6-9

Plain Radiography vs. Computed Tomography (CT)

• While supine and erect plain abdominal radiographs are useful initial screening tools, multidetector CT of the abdomen and pelvis with intravenous contrast is the definitive gold standard. CT achieves sensitivity and specificity > 95%, identifying the exact anatomical transition point, determining the underlying etiology, detecting closed-loop mechanisms, and evaluating bowel wall viability.

The Rule of 3-6-9 for Bowel Caliber

On plain films and CT, the upper physiological limit of normal luminal diameter is:

• Small Bowel: Dilated when caliber > 3 cm.
• Colon (Large Bowel): Dilated when caliber > 6 cm.
• Cecum: Dilated when caliber > 9 cm.

Laplace's Law & Cecal Perforation Risk in LBO

In mechanical large bowel obstruction, the integrity of the ileocecal valve is the critical determinant of clinical behavior:

• Competent Ileocecal Valve (Closed-Loop Obstruction, 75% of cases): The valve prevents retrograde decompression of colonic gas and fluid into the compliant small intestine. Intraluminal pressure inside the colon skyrockets.
• Laplace's Law: For a thin-walled cylinder, circumferential wall tension per unit length is T = P x r, and wall stress is P x r / t. For a sphere, wall stress is P x r / (2 x t); P is transmural pressure, r radius and t wall thickness.
• Because the cecum has the largest anatomical radius of any colonic segment and the thinnest muscular wall, it experiences the highest intramural wall tension at any given intraluminal pressure. Progressive dilation, particularly around 12 cm or more, increases ischemia and perforation risk. Diameter alone does not establish absent perfusion: assess pain, tenderness, physiology and CT signs urgently. A distal sigmoid or rectal obstruction can cause diastatic cecal rupture.
• Incompetent Ileocecal Valve (Open-Loop Obstruction, 25%): Colonic contents decompress retrogradely into the small bowel, manifesting as marked small bowel distension resembling distal SBO; cecal tension rises more slowly.

Distinguishing Small vs. Large Bowel on Imaging

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Volvulus: Sigmoid vs. Cecal Volvulus

Volvulus denotes the pathological torsion of a mobile segment of the alimentary tract around its mesenteric axis, rapidly generating a closed-loop obstruction and strangulation.

Sigmoid Volvulus

• Epidemiology: The most common form of colonic volvulus (75-80%). Prevalent in elderly patients, institutionalized or bedbound individuals with chronic constipation, neuropsychiatric disorders (Parkinson's disease, dementia), and users of psychotropic medications that impair colonic motility. In endemic regions (Latin America, Africa), it is linked to high-fiber diets and Chagasic megacolon (Trypanosoma cruzi destroy myenteric plexus ganglia).
• Pathophysiology: A redundant, elongated sigmoid colon possessing a narrow mesenteric base twists around its sigmoid mesentery, causing both venous occlusion and luminal obstruction.
• Radiographic Appearance: Abdominal plain film reveals the pathognomonic 'coffee-bean' sign (or inverted U-tube sign): an enormous, smooth, ahaustral loop of gas-distended sigmoid colon originating in the pelvis with its apex pointing toward the right upper quadrant.
• Management Strategy:
  1. Hemodynamically Stable without Peritonitis: Perform urgent endoscopic detorsion and decompression using a rigid or flexible sigmoidoscope with placement of a soft rectal flatus tube left in situ for 24-48 hours. Decompression is successful in > 75% to 80% of cases, marked by a massive, foul-smelling rush of liquid stool and gas.
  2. Semi-Elective Resection: Because endoscopic detorsion carries a recurrence rate exceeding 50% within the same year, patients should undergo a semi-elective sigmoid colectomy with primary anastomosis during the same hospital admission once the colon is adequately prepped.
  3. Unstable with Peritonitis / Gangrene: Immediate exploratory laparotomy with sigmoid resection and end colostomy (Hartmann's procedure); endoscopic detorsion is strictly contraindicated if mucosal necrosis or perforation is suspected.

Cecal Volvulus

• Epidemiology: Accounts for 15% to 20% of colonic volvulus. Occurs in younger patients (30 to 50 years) possessing an embryologically mobile cecum due to incomplete congenital retroperitoneal fixation of the ascending colon mesentery.
• Radiographic Appearance: An enormous, comma-shaped or kidney-shaped gas-filled loop with its apex pointing toward the left upper quadrant or epigastrium, accompanied by marked small bowel distension and complete collapse of the distal colon.
• Management Strategy: Endoscopic decompression carries a negligible success rate (< 20%) and a high risk of iatrogenic cecal perforation. Therefore, emergency surgical intervention is mandatory: right hemicolectomy with primary ileocolic anastomosis (or temporary stoma in unstable patients). In frail patients with completely viable cecum, cecopexy may be considered.

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Strangulated vs. Simple Obstruction

Distinguishing simple mechanical obstruction from strangulated obstruction is the most critical diagnostic task in acute surgical triage.

• Simple Obstruction: Luminal continuity is blocked, but mesenteric vascular supply to the bowel wall remains intact.
• Strangulated Obstruction: Mechanical obstruction complicated by acute compromise of the mesenteric arterial or venous circulation, progressing from venous thrombosis and submucosal edema to transmural hemorrhagic infarction, gangrene, and bowel perforation.
• Closed-Loop Obstruction: The most dangerous precursor to strangulation, occurring when a segment of bowel is occluded at two points along its axis (e.g., volvulus, an incarcerated hernia where both afferent and efferent limbs enter the neck, or LBO with a competent ileocecal valve). Intraluminal secretions and bacterial gases have no exit, driving intraluminal pressure above capillary perfusion pressure within hours.

Clinical Red Flags of Strangulation

• Transformation of Pain: Transition from intermittent, colicky cramping to continuous, unremitting, severe, agonizing abdominal pain.
• Systemic Signs: Persistent tachycardia (> 100 bpm) out of proportion to fever, high-grade pyrexia (> 38.0°C), hypotension, and diaphoresis.
• Localized Peritonitis: Development of focal involuntary guarding, localized abdominal wall rigidity, or percussion tenderness over the obstructed loops.
• Laboratory Biomarkers: Progressive leukocytosis with a prominent left shift (bandemia > 10%), metabolic acidosis with elevated serum lactate (> 2.0 mmol/L), and elevated serum amylase/LDH.

Management of Adhesive SBO

• Non-Operative Conservative Protocol ('Drip and Suck'): Indicated in patients with simple, partial, or uncomplicated adhesive SBO who display no signs of peritonitis, strangulation, or hemodynamic instability:
  1. Complete bowel rest (NPO).
  2. Nasogastric tube (NGT) decompression on low intermittent suction to evacuate gastric and swallowed air, decompress proximal loops, and prevent pulmonary aspiration.
  3. Aggressive intravenous fluid resuscitation using balanced crystalloids (e.g. Ringer's lactate) titrated to maintain urine output >= 0.5 mL/kg/h, correcting concomitant hypokalemic, hypochloremic metabolic alkalosis.
  4. Indwelling Foley catheter for strict fluid balance monitoring.
  5. Water-Soluble Contrast Challenge (Gastrografin): Administer 50 to 100 mL of water-soluble hyperosmolar contrast via the NGT. Plain abdominal radiographs are obtained at 8, 12, and 24 hours. Gastrografin exerts an osmotic draw, shifting water into the lumen to stimulate peristalsis and reduce mural edema. If contrast reaches the cecum within 24 hours, the obstruction has a > 95% likelihood of resolving non-operatively. If contrast fails to reach the colon after 24 hours, or if clinical deterioration occurs, surgery is indicated.
• Absolute Indications for Emergency Laparotomy:
  • Signs of strangulation or generalized peritonitis.
  • Incarcerated, irreducible hernia.
  • Closed-loop obstruction on CT (whirl sign, two transition points).
  • Bowel ischemia findings on CT (reduced wall enhancement, intramural air [pneumatosis intestinalis], portal venous gas).
  • Failure of conservative management to resolve obstruction within 48 to 72 hours.

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Acute Mesenteric Ischaemia: Hyperacute Recognition & Interventions

Acute mesenteric ischemia (AMI) is a life-threatening vascular catastrophe characterized by abrupt interruption of blood supply to the small intestine and colon, carrying an overall mortality rate of 50% to 70%.

Etiological Classifications

1. Superior Mesenteric Artery (SMA) Embolism (40% to 50%): Sudden lodgment of a cardiogenic embolus (atrial fibrillation, left ventricular mural thrombus post-myocardial infarction, infective endocarditis) into the SMA. Because the SMA arises at an acute angle from the aorta, emboli typically lodge 3 to 10 cm distal to the SMA origin, often sparing the proximal jejunum and proximal colon because early SMA branches remain patent.
2. SMA In Situ Thrombosis (25% to 30%): Superimposed acute thrombosis over a pre-existing, severe atherosclerotic plaque, usually at the ostium or origin of the SMA. Patients often provide an antecedent history of chronic mesenteric ischemia ('intestinal angina': severe postprandial cramping pain, food fear, and progressive weight loss).
3. Non-Occlusive Mesenteric Ischemia (NOMI, 20%): Severe splanchnic arterial vasoconstriction triggered by systemic low-flow states (cardiogenic shock, septic shock, cardiac bypass surgery, or high-dose vasopressor infusions such as norepinephrine or epinephrine) in patients with diffuse atherosclerotic disease.
4. Mesenteric Venous Thrombosis (MVT, 5% to 10%): Thrombosis of the superior mesenteric vein secondary to inherited thrombophilias (Factor V Leiden, prothrombin gene mutation, antithrombin III deficiency), malignancy, cirrhosis with portal hypertension, or intra-abdominal sepsis.

The Hallmark Clinical Triad

• Agonizing Pain Out of Proportion to Physical Findings: The classic pathognomonic presentation is severe, catastrophic, generalized abdominal pain in a patient whose abdominal examination reveals a completely soft, non-tender abdomen without guarding or rigidity. This occurs because the visceral pain receptors are stimulated by acute ischemia, but the overlying somatic parietal peritoneum has not yet become inflamed.
• As transmural infarction develops over the ensuing 6 to 12 hours, the ischemic mucosal barrier breaks down, resulting in bowel evacuation, bloody diarrhea ('currant-jelly' stools), generalized peritonitis, board-like abdominal rigidity, septic shock, and profound metabolic acidosis with skyrocketing serum lactate levels.

Diagnostic Gold Standard: CT Mesenteric Angiography

• Any clinical suspicion of AMI mandates immediate multiphase CT Mesenteric Angiography (CTA) with non-contrast, arterial, and portal venous phases. Standard non-contrast CT or plain films are utterly inadequate.
• CTA Stigmata: Filling defect or abrupt cutoff in the SMA trunk; lack of bowel wall enhancement; bowel wall thickening or 'paper-thin' dilation; mesenteric stranding; mesenteric vascular engorgement; and late signs including pneumatosis intestinalis (gas inside bowel wall) and portal venous gas.

Emergency Management Protocol

1. Resuscitation: Aggressive crystalloid hydration, correction of acidosis, and avoidance of splanchnic vasoconstrictors (vasopressin and alpha-agonists should be minimized; dobutamine or milrinone preferred for inotropic support).
2. Systemic Anticoagulation: In arterial thromboembolism or mesenteric venous thrombosis, start therapeutic unfractionated heparin unless contraindicated (a usual regimen is 80 units/kg bolus, then 18 units/kg/h, adjusted by aPTT/anti-Xa). Anticoagulation in NOMI is individualized.
3. Empiric Broad-Spectrum IV Antibiotics: Cover translocating enteric organisms.
4. Revascularization & Bowel Resection:
   • SMA Embolus: Emergency surgical open embolectomy via a transverse arteriotomy using a Fogarty balloon catheter, or percutaneous mechanical catheter thrombectomy/aspiration.
   • SMA Thrombosis: Surgical revascularization (aortomesenteric or iliac-mesenteric bypass graft) or endovascular stenting.
   • Mesenteric Venous Thrombosis: Anticoagulation is first-line without peritonitis; deterioration or infarcted bowel requires urgent reassessment and possible intervention.
   • NOMI: Correct the low-flow cause and optimize perfusion; specialists may use catheter-directed vasodilators. Resect infarcted bowel.
   • Bowel Viability Assessment: Following arterial revascularization when indicated, non-viable, frankly necrotic bowel is resected. Questionable bowel is left in situ, and a mandatory planned 'second-look' laparotomy is scheduled within 24 to 48 hours to reassess mucosal viability before permanent reconstruction.

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Perforated Peptic Ulcer & Perforated Viscus

Visceral perforation represents full-thickness disruption of a hollow intra-abdominal organ, producing rapid contamination of the peritoneal cavity with chemical and bacterial contents.

Clinical Presentation of Perforated Peptic Ulcer

• Typically involves an anterior duodenal bulb ulcer (duodenal ulcers on the posterior wall erode into the gastroduodenal artery, causing massive gastrointestinal hemorrhage rather than free perforation) or a prepyloric/gastric antral ulcer.
• The Catastrophic Event: Sudden onset of explosive, excruciating, generalized abdominal pain ('like a knife stabbing into the belly'). Patients lie completely motionless with shallow respirations because any abdominal movement, coughing, or inspiration shifts the inflamed parietal peritoneum.
• Physical Examination:
  • Board-Like Abdominal Rigidity: Reflex tonic muscular contraction of the entire rectus abdominis and abdominal wall.
  • Loss of Hepatic Dullness: On percussion over the right mid-axillary line, normal liver dullness is replaced by tympany due to free subdiaphragmatic gas.
  • Tachycardia, tachypnea, cool clammy extremities, and fever.

Diagnostic Imaging

• Erect Chest Radiograph (CXR): Demonstrates pneumoperitoneum (free subdiaphragmatic air) in > 75% to 80% of patients, visualized as a thin, radiolucent sickle-shaped crescent beneath the right hemidiaphragm (or bilaterally).
• Left Lateral Decubitus Abdominal Radiograph: For critically ill patients unable to stand erect; free gas rises and outlines the lateral liver border against the abdominal wall.
• Contrast-Enhanced CT Abdomen/Pelvis: The most sensitive modality (> 98%), detecting subtle extraluminal air bubbles, focal gastroduodenal wall thickening, and free intra-abdominal fluid.

Emergency Surgical Protocol

1. Resuscitation with two large-bore IVs, balanced crystalloids, broad-spectrum IV antibiotics, continuous nasogastric suction, and high-dose intravenous proton pump inhibitors (omeprazole 80 mg bolus).
2. Operative Repair (Laparoscopic or Open Laparotomy):
   • Graham Patch Omentopexy (Cellan-Jones Repair): The gold standard surgical repair. A viable, pedicled flap of vascularized greater omentum is placed over the duodenal perforation and secured with three or four full-thickness, absorbable sutures tied loosely over the omental plug without strangulating its vascular supply.
   • Copious peritoneal lavage with 6 to 10 liters of warm sterile saline to eliminate chemical and bacterial contamination.
   • Postoperative testing for Helicobacter pylori (and eradication therapy) alongside cessation of ulcerogenic medications (NSAIDs, aspirin, steroids).