What are the common causes of ileus in critically ill adult patients in the ICU?

Ileus Causes in ICU

Primary Etiologic Categories

Ileus in the ICU results from a combination of pharmacologic agents (particularly opioids), metabolic derangements, surgical trauma, and systemic illness that collectively impair intestinal motility. 1, 2, 3, 4

Pharmacologic Causes

• Opioid analgesics are the most significant pharmacologic contributor to ileus in critically ill patients, causing dose-dependent inhibition of intestinal motility through mu-receptor activation in the enteric nervous system 1, 2, 3
• Sedatives (propofol, benzodiazepines) and neuromuscular blocking agents directly impair gastrointestinal transit 1, 5
• Anticholinergic medications, including antihistamines and antipsychotics, reduce bowel motility 3, 5
• Vasopressors contribute to ileus through splanchnic hypoperfusion and reduced intestinal blood flow 1, 5

Metabolic and Electrolyte Disturbances

• Hypokalemia, hypomagnesemia, and hypophosphatemia directly impair smooth muscle contractility 3, 5
• Hyperglycemia delays gastric emptying and reduces intestinal motility 1, 5
• Uremia and metabolic acidosis contribute to gastrointestinal dysmotility 3

Surgical and Traumatic Causes

• Abdominal surgery triggers an exaggerated inflammatory response with local cytokine release that inhibits intestinal neural pathways 3, 6
• Spinal surgery and traumatic spinal cord injury disrupt autonomic innervation of the bowel 6
• Lower extremity joint reconstruction (hip and knee arthroplasty) causes ileus through systemic inflammatory responses and high opioid requirements 6
• Direct bowel manipulation during surgery causes localized inflammation lasting 24-72 hours 3

Critical Illness-Related Factors

• Sepsis and systemic inflammatory response syndrome release cytokines (TNF-α, IL-1, IL-6) that directly inhibit intestinal smooth muscle function 4, 5
• Mechanical ventilation, particularly with positive end-expiratory pressure, increases intra-abdominal pressure and reduces splanchnic perfusion 4, 7
• Prolonged immobilization and bed rest reduce intestinal motility through decreased vagal tone 1, 5
• Gastric hypoperfusion in shock states causes gastroparesis and subsequent ileus 1, 5

Infectious Causes

• Clostridium difficile infection should be suspected in any ICU patient with ileus, leukocytosis, and recent antibiotic exposure, as it can present with ileus or toxic megacolon without diarrhea 1
• Intra-abdominal infections and peritonitis cause localized inflammatory ileus 4
• Pneumonia and other systemic infections contribute through systemic inflammatory mediators 5

Clinical Consequences and Complications

• Intestinal dilatation increases luminal pressure, causing gut wall ischemia and potential bacterial translocation 4
• Intra-abdominal hypertension (IAP >12 mmHg) occurs in up to 20% of critically ill patients with ileus, leading to abdominal compartment syndrome when IAP exceeds 20-25 mmHg 4
• Abdominal fluid sequestration causes severe systemic hypovolemia and hemodynamic instability 4
• Bacterial overgrowth promotes translocation and systemic infections 4
• Feeding intolerance occurs in approximately 50% of mechanically ventilated patients, with 20% developing clinically significant impaired gastrointestinal transit 1, 5, 7

Critical Pitfalls to Avoid

• Do not attribute ileus solely to "postoperative status" without investigating reversible metabolic and pharmacologic causes 3, 5
• Recognize that ileus can occur after minor orthopedic procedures, not just major abdominal surgery 6
• Consider C. difficile even in the absence of diarrhea when patients present with ileus and leukocytosis 1
• Monitor for abdominal compartment syndrome in patients with persistent ileus and abdominal distention by measuring bladder pressures 4
• Avoid routine gastric residual volume monitoring as it does not predict aspiration and may lead to unnecessary feeding interruptions 1