Mesenteric ischemia

Pathophysiology - Gut Under Attack

• Primary Insult: Acute reduction in gut blood flow, creating an oxygen supply/demand mismatch.
• Cellular Cascade: Ischemia forces anaerobic metabolism (↑ lactic acidosis). The mucosal barrier, highly sensitive to hypoxia, breaks down within 3 hours, permitting bacterial translocation.

⭐ Pain out of proportion to physical exam is the hallmark sign, stemming from visceral ischemia before transmural infarction causes peritoneal signs.

Etiology & Types - The Four Horsemen

• Acute Arterial Occlusive Disease (~75%): The most common cause.
  • Embolism (~50%): Sudden onset. Typically from atrial fibrillation, lodging in the Superior Mesenteric Artery (SMA).
  • Thrombosis (~25%): Gradual onset. Due to severe atherosclerosis at the SMA origin.
• Non-Occlusive Mesenteric Ischemia (NOMI) (~20%): Splanchnic vasoconstriction from low-flow states (e.g., shock, heart failure, high-dose vasopressors).
• Mesenteric Venous Thrombosis (MVT) (<10%): Clot in mesenteric veins, linked to hypercoagulable states.

⭐ The SMA is the most commonly affected vessel in acute mesenteric ischemia, particularly from an embolic source.

Clinical Features - Pain Out of Proportion

• Hallmark: Acute, severe, diffuse abdominal pain with a strikingly benign physical exam.
• Pain is often sudden, periumbilical, and poorly localized.
• Early stages lack peritoneal signs like guarding or rebound tenderness.
• Associated features: nausea, vomiting, and initial forceful bowel evacuation.
• 📌 Pain out of proportion, AFIB/atherosclerosis, Increased lactate, No peritonitis (early) -> PAIN.

⭐ Late findings (>12 hrs) indicate bowel infarction: peritoneal signs, hematochezia, and shock. Survival rates drop dramatically once peritonitis develops.

Diagnosis - Racing the Clock

• Labs: ↑ Lactate (a late, ominous sign), leukocytosis, metabolic acidosis.
• Imaging Gold Standard: CT Angiography (CTA) of the abdomen and pelvis is the primary diagnostic tool.
  • Identifies vessel occlusion (embolus/thrombus), bowel wall thickening, pneumatosis intestinalis (air in bowel wall), and portal venous gas.
• Conventional Angiography: Invasive, but allows for simultaneous diagnosis and therapeutic intervention (e.g., vasodilator infusion for NOMI).

⭐ The classic presentation is "pain out of proportion to the physical exam." High suspicion is critical in patients with risk factors like atrial fibrillation or peripheral vascular disease.

Management - Code Gut

• Immediate Resuscitation: High-flow O₂, aggressive IV fluids, broad-spectrum antibiotics, and correction of metabolic acidosis.
• Anticoagulation: Start systemic anticoagulation with an IV heparin drip immediately upon suspicion, even before definitive diagnosis.
• Definitive Therapy: Guided by clinical stability and presence of peritonitis.

⭐ A planned "second-look" laparotomy in 24-48 hours is often required to re-evaluate bowel viability after initial resection and revascularization.

High‑Yield Points - ⚡ Biggest Takeaways

• Acute mesenteric ischemia (AMI) presents with severe abdominal pain "out of proportion" to the physical exam, often linked to atrial fibrillation.
• The Superior Mesenteric Artery (SMA) is most commonly affected by an embolic event.
• Chronic mesenteric ischemia causes "intestinal angina": crampy postprandial pain that leads to food fear and weight loss.
• CT angiography (CTA) is the gold standard for diagnosis.
• Metabolic acidosis with an elevated serum lactate is a late and ominous sign.
• Treatment is emergent revascularization and possible bowel resection.