Bowel Obstruction & Ileus

The organizing distinction on this whole page is mechanical versus functional. A mechanical obstruction is a physical blockage — a mass, an adhesion band, a hernia trapping a loop — that intestinal contents cannot pass. Ileus is the functional counterpart: the bowel is open, but propulsion has been inhibited — "intolerance of oral intake due to inhibition of the gastrointestinal propulsion without signs of mechanical obstruction."

Mechanical obstructions are further split by what is happening to the blood supply. A simple obstruction is a blockage without ischemia. A strangulated obstruction has compromised blood flow, bowel ischemia, and potential necrosis — a surgical emergency. Strangulation particularly occurs in closed-loop obstructions, where a segment is obstructed at two points (a twisted volvulus is the classic example) and in hernias. And a partial obstruction may still allow some stool or flatus through early on — which is exactly why continued output does not rule an obstruction out.

One more cousin to know: acute colonic pseudo-obstruction (Ogilvie syndrome) — massive colonic dilation with no transition point and no anatomical blockage, from autonomic dysregulation with decreased parasympathetic activity, typically involving the cecum to the splenic flexure. It matters because its treatment (neostigmine) is the opposite of useless in that setting but does not work for ordinary postoperative ileus.

Above a blockage, the bowel distends with swallowed air, secretions, and gas made by bacteria in the stagnant segment. Rising intraluminal pressure first impairs venous outflow, producing bowel-wall edema. The increasingly permeable wall then lets fluid and electrolytes sequester into the bowel lumen and third-space into the peritoneal cavity — liters of fluid the circulation can no longer use. Add vomiting on top, and the result is hypovolemia with hyponatremia, hypokalemia, and metabolic alkalosis. The patient is drying out from the inside.

If the obstruction persists, pressure eventually exceeds arterial inflow — note the ordering: veins first, arteries later — and the wall becomes ischemic, then necrotic, then perforates, spilling contents into the peritoneum: peritonitis and sepsis. Even before perforation, the thickened, inflamed, ischemic wall lets gut bacteria (predominantly E. coli) translocate across it into the circulation. Meanwhile gas-forming bacteria in the obstructed segment add to the dilation, feeding the pressure loop.

This cascade is why bowel obstruction is a fluid-and-perfusion emergency, not just a plumbing complaint — and why the first treatments are an NG tube to decompress from above and isotonic IV fluid to refill the circulation, before anyone talks about an operating room.

In SBO, the classic triad is abdominal pain, vomiting, and distension — colicky pain that surges with each peristaltic wave fighting the blockage. Where along the small bowel matters: a proximal obstruction produces significant vomiting, minimal distension, and rapid dehydration and electrolyte imbalance; a distal obstruction produces more pronounced distension, delayed vomiting, and slower systemic decline. The vomitus itself evolves — it may start as gastric contents and become bilious as the obstruction sits more distally.

In LBO — most often obstructing colorectal cancer — everything hinges on the ileocecal valve. If the valve is competent, the obstructed colon becomes a closed loop with nowhere to decompress: the cecum, which has the largest diameter and thinnest wall, dilates progressively and is at greatest perforation risk (Laplace’s law — a larger diameter needs less pressure to keep stretching). Progressive right iliac fossa pain in an LBO patient signifies impending cecal perforation. If the valve is incompetent, colon contents decompress backward into the small bowel — buying time, delaying the acute picture, and eventually producing vomiting.

The subtypes with their own mechanics: volvulus — a segment twists around its mesentery, creating a closed loop with a strangled blood supply, presenting abruptly with distension and tympany (sigmoid volvulus in older men, cecal in younger women); intussusception — one segment telescopes into the next, dragging its mesentery and compressing the blood supply. In adults intussusception is rare (1–5% of obstructions) but serious: over 90% have a pathological lead point and more than half of the associated neoplasms are malignant, which is why adult management is surgical.

Postoperative ileus is a neuroimmune and pharmacologic event, in phases. First the neurogenic phase: surgical handling of the bowel fires the sympathetic nervous system, which suppresses neuromuscular function (with nitric oxide, calcitonin-gene-related peptide, and corticotropin-releasing factor among the messengers). Then the inflammatory phase: resident macrophages in the bowel wall release IL-6, TNF-alpha, neuropeptides, and nitric oxide, sustaining the inhibitory sympathetic tone. Layered over both is the pharmacologic phase: postoperative opioids bind mu and delta receptors — which in the gut predominate in the myenteric and submucosal plexuses — inhibiting gastric emptying and peristalsis, increasing fluid absorption, and tightening the anal sphincter against the defecation reflex.

The timeline is testable: ileus typically declares itself the third to fifth day after surgery and usually lasts 2 to 3 days. Bowel segments recover in order — small bowel first (0–24 h), stomach (24–48 h), colon last (48–72 h) — so ileus persisting beyond about 72 hours is considered pathologic ("prolonged ileus"). Roughly 10–25% of patients develop it after abdominal surgery, depending on the procedure — lower with laparoscopy, higher with open lower-abdominal operations.

And here is the loop nurses can actually close: hypokalemia causes ileus, and ileus treatment causes hypokalemia. Vomiting and NG suction waste hydrogen, chloride, and potassium (a contraction alkalosis with secondary hyperaldosteronism driving renal K⁺ wasting) — and low potassium is itself one of the named causes of ileus. Replacing potassium is therefore a treatment, not just a correction.