Special Surgery 26. Acute and Chronic Intestinal Ischemia

I. Vascular Anatomy and Basic Concepts

Definition

• Intestinal ischemia = insufficient blood supply to bowel wall
• Can affect small intestine, colon or both
• Acute intestinal ischemia = sudden perfusion failure → rapid bowel infarction risk
• Chronic intestinal ischemia = recurrent reversible postprandial ischemia due to fixed arterial stenosis
• Most dangerous form: acute mesenteric ischemia (AMI), usually involving SMA territory

Blood Supply

• Celiac trunk:

• Foregut: stomach, duodenum proximal to papilla, liver, spleen, pancreas

• Superior mesenteric artery (SMA):

• Midgut: distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, proximal 2/3 transverse colon
• Main vessel in acute mesenteric ischemia

• Inferior mesenteric artery (IMA):

• Hindgut: distal 1/3 transverse colon, descending colon, sigmoid colon, upper rectum

• Collateral circulation:

• Pancreaticoduodenal arcades: celiac-SMA collateral
• Marginal artery of Drummond: SMA-IMA collateral along colon
• Arc of Riolan / meandering mesenteric artery: central SMA-IMA collateral
• Internal iliac-rectal collaterals: distal colonic/rectal protection

Watershed Areas

• Splenic flexure / Griffith point: SMA-IMA border zone
• Rectosigmoid junction / Sudeck point: IMA-superior rectal collateral zone
• Watershed areas are vulnerable in low-flow states
• Important for ischemic colitis

Classification

1. Acute mesenteric ischemia

• Arterial embolism
• Arterial thrombosis
• Non-occlusive mesenteric ischemia (NOMI)
• Mesenteric venous thrombosis

2. Ischemic colitis

• Usually transient hypoperfusion of colon
• Often left-sided and watershed-related

3. Chronic mesenteric ischemia / intestinal angina

• Atherosclerotic stenosis of mesenteric arteries
• Symptoms usually require severe SMA stenosis and often multivessel disease

II. Acute Intestinal Ischemia

Definition

• Sudden interruption or severe reduction of intestinal blood flow
• Leads to mucosal injury → transmural infarction → perforation if untreated
• Time-critical surgical and vascular emergency
• Mortality is high, especially after bowel necrosis and sepsis develop

Causes and Risk Factors

Arterial Embolism

• Embolus usually lodges in SMA distal to middle colic artery origin
• Sudden severe pain
• Risk factors:

• Atrial fibrillation
• Recent myocardial infarction
• Valvular heart disease
• Endocarditis
• Cardiac thrombus

• Often little collateral adaptation → rapid ischemia

Arterial Thrombosis

• Thrombosis on chronic atherosclerotic stenosis, usually proximal SMA origin
• Often history of chronic mesenteric ischemia symptoms
• Risk factors:

• Atherosclerosis
• Smoking
• Diabetes, hypertension, dyslipidemia
• Peripheral/coronary/cerebrovascular disease

• Can involve large bowel length because proximal inflow is lost

Non-Occlusive Mesenteric Ischemia (NOMI)

• No fixed arterial or venous occlusion
• Severe splanchnic vasoconstriction due to low-flow state
• Typical setting:

• Shock
• Severe heart failure
• Sepsis
• Major surgery or ICU patient
• Hemodialysis hypotension
• High-dose vasopressors
• Digoxin/cocaine/vasoconstrictive drugs

Mesenteric Venous Thrombosis

• Venous outflow obstruction → bowel wall edema → impaired arterial inflow
• Often more subacute onset than arterial occlusion
• Risk factors:

• Inherited thrombophilia
• Malignancy
• Myeloproliferative disease
• Oral contraceptives, pregnancy, postpartum state
• Portal hypertension/cirrhosis
• Pancreatitis, intra-abdominal infection
• Recent surgery or trauma

Clinical Features

• Classical early sign:

• Severe abdominal pain out of proportion to physical findings
• Soft abdomen early despite severe pain

• Nausea, vomiting
• Diarrhea, urge to defecate
• Bloody stool / hematochezia if mucosal sloughing occurs
• Abdominal distension and ileus later
• Peritonitis = bowel infarction/perforation until proven otherwise
• Late signs: fever, tachycardia, hypotension, shock, sepsis, multiorgan failure

Diagnostics

Laboratory Tests

• No laboratory test excludes acute mesenteric ischemia
• Leukocytosis
• High CRP
• Metabolic acidosis
• Elevated lactate:

• Important severity marker
• Often late; normal lactate does not exclude early ischemia

• Hemoconcentration/dehydration
• Renal dysfunction
• D-dimer may be elevated but is nonspecific
• Blood gas, electrolytes, coagulation profile, type and screen

Imaging

• CT angiography abdomen/pelvis = diagnostic test of choice
• Do not delay CTA for mild renal dysfunction if AMI is suspected
• CTA evaluates:

• Arterial embolus or thrombosis
• Venous thrombosis
• Bowel wall enhancement
• Bowel dilatation and ileus
• Mesenteric edema or ascites
• Pneumatosis intestinalis
• Portal venous gas
• Free air/perforation
• Alternative diagnosis

• Plain abdominal X-ray:

• Low sensitivity
• May show ileus, air-fluid levels, pneumatosis or free air late

• Angiography:

• Mainly therapeutic now: endovascular thrombolysis, aspiration, stent, vasodilator infusion

Treatment

Immediate General Treatment

• High suspicion → urgent surgical + vascular/interventional radiology consultation
• NPO
• Large-bore IV access
• Fluid resuscitation and correction of shock
• Oxygen, ICU-level monitoring if severe
• NG tube if ileus/vomiting
• Broad-spectrum IV antibiotics covering Gram-negative and anaerobic bacteria
• Systemic anticoagulation with unfractionated heparin unless contraindicated
• Correct acidosis, electrolytes and organ hypoperfusion
• Avoid excessive vasopressors if possible; restore mesenteric perfusion

Revascularization / Source Control

• Peritonitis, perforation or obvious necrosis → emergency laparotomy
• Resect clearly necrotic bowel
• Preserve borderline bowel if possible and plan second-look operation
• Second-look laparotomy/laparoscopy often after 24-48 h if viability uncertain
• Restore blood flow when possible:

• Arterial embolism: surgical embolectomy or endovascular therapy depending on stability/local expertise
• Arterial thrombosis: endovascular stent/angioplasty or open bypass/endarterectomy
• Hybrid approach: laparotomy for bowel assessment + endovascular revascularization
• Mesenteric venous thrombosis: anticoagulation first if no peritonitis; surgery if infarction/peritonitis
• NOMI: treat shock/low-flow state, reduce vasoconstrictors, intra-arterial vasodilator in selected cases, resect necrotic bowel if present

Postoperative / Follow-Up Principles

• Continue anticoagulation according to cause
• Investigate embolic source: ECG, echocardiography, atrial fibrillation
• Investigate thrombophilia in venous thrombosis when appropriate
• Monitor for short bowel syndrome after extensive resection
• Nutritional support: enteral if possible, parenteral if needed
• Risk factor control: smoking cessation, statin, antiplatelet therapy for atherosclerosis

III. Ischemic Colitis

Definition

• Ischemic colitis = hypoperfusion injury of colon
• Most common form of intestinal ischemia overall
• Often transient and non-gangrenous
• Can progress to gangrene, perforation or stricture

Causes and Risk Factors

• Low-flow states: dehydration, shock, heart failure, dialysis hypotension
• Atherosclerosis
• Major vascular surgery, especially aortic surgery
• Embolic or thrombotic occlusion of colonic vessels
• Drugs: vasoconstrictors, cocaine, some constipation-inducing drugs
• Hypercoagulable states
• Most common sites: left colon, splenic flexure, sigmoid colon

Clinical Features

• Crampy abdominal pain, often left-sided
• Urgent defecation
• Bloody diarrhea or hematochezia
• Mild-moderate tenderness
• Fever, peritonitis or shock suggests severe/gangrenous disease

Diagnosis

• CT abdomen/pelvis with IV contrast:

• Segmental colonic wall thickening
• Submucosal edema/thumbprinting
• Pericolic fluid/stranding
• Pneumatosis or free air in severe disease

• Colonoscopy:

• Confirms mucosal ischemia if no peritonitis/perforation
• Findings: pale mucosa, petechiae, cyanosis, ulceration, segmental injury
• Use minimal insufflation; avoid if peritonitis or gangrene suspected

• Labs: leukocytosis, CRP, lactate in severe disease
• Stool studies if infectious colitis is a differential

Treatment

• Most mild/moderate cases: conservative treatment

• Bowel rest
• IV fluids and correction of hypotension
• Stop vasoconstrictive/offending drugs
• Broad-spectrum antibiotics if moderate/severe disease or systemic signs
• Serial abdominal exams and labs

• Surgery indicated for:

• Peritonitis
• Perforation
• Gangrenous colon
• Ongoing sepsis
• Massive bleeding
• Clinical deterioration despite conservative treatment
• Later symptomatic ischemic stricture

• Operation: resection of nonviable colon, often with stoma in unstable/contaminated cases

IV. Chronic Intestinal Ischemia

Definition

• Chronic mesenteric ischemia / intestinal angina = recurrent postprandial bowel ischemia
• Usually due to progressive atherosclerotic stenosis of mesenteric arteries
• Symptoms develop when collateral reserve is insufficient
• Classically requires severe SMA disease and often at least two major mesenteric vessels affected

Etiology and Risk Factors

• Atherosclerosis: most common
• Risk factors:

• Smoking
• Diabetes mellitus
• Hypertension
• Dyslipidemia
• Coronary/peripheral/cerebrovascular disease
• Older age

• Less common causes:

• Vasculitis
• Fibromuscular dysplasia
• Median arcuate ligament syndrome: celiac artery compression
• Radiation-induced stenosis

Clinical Features

• Postprandial abdominal pain:

• Starts 15-60 min after eating
• Lasts 1-3 h
• Often epigastric or periumbilical

• Food fear / sitophobia
• Weight loss due to reduced intake
• Nausea, bloating, diarrhea or constipation
• Abdominal bruit may be heard but is not reliable
• Acute-on-chronic thrombosis can present as acute mesenteric ischemia

Diagnosis

• Diagnosis = compatible symptoms + significant mesenteric arterial stenosis + exclusion of other causes
• CT angiography:

• First-line anatomic test in many centers
• Shows celiac/SMA/IMA stenosis or occlusion
• Assesses calcification, collaterals and alternative abdominal pathology

• MR angiography:

• Alternative if CT contrast/radiation is problematic

• Duplex ultrasound:

• Screening/follow-up tool
• Increased flow velocity suggests stenosis
• Operator-dependent and limited by bowel gas/body habitus

• Endoscopy/colonoscopy often used to exclude ulcer, malignancy, IBD or other causes of weight loss/pain
• Catheter angiography mainly if endovascular treatment is planned

Treatment

Medical / Supportive Treatment

• Smoking cessation
• Statin therapy
• Antiplatelet therapy for atherosclerosis
• Control hypertension, diabetes and dyslipidemia
• Nutritional support if significant weight loss
• Avoid prolonged delay to revascularization in symptomatic patients because acute infarction risk exists

Revascularization

• Indicated for symptomatic chronic mesenteric ischemia
• Goal: restore postprandial intestinal perfusion and prevent acute thrombosis
• Target vessel: SMA is most important; celiac may also be treated depending on anatomy
• Endovascular treatment:

• Angioplasty + stenting
• First-line in most older/high-risk patients
• Lower early morbidity than open surgery
• Higher restenosis/reintervention risk than open bypass

• Open surgical revascularization:

• Bypass, endarterectomy or transaortic endarterectomy in selected cases
• Consider in young/low-risk patients, long occlusions, failed endovascular treatment or unsuitable anatomy
• More durable but higher initial operative risk

Exam-Focused Summary

1. Acute mesenteric ischemia: severe pain out of proportion → CTA immediately
2. Normal early abdomen or normal lactate does not exclude AMI
3. Peritonitis = bowel infarction → emergency laparotomy
4. Treatment = resuscitation + antibiotics + heparin + revascularization +/- bowel resection
5. Venous thrombosis without peritonitis → anticoagulation first
6. NOMI → correct shock/low-flow state and reduce vasoconstriction
7. Chronic intestinal ischemia: postprandial pain + food fear + weight loss → CTA/MRA and revascularization