PassFinals
General Surgery

Bowel Obstruction

Bowel obstruction stops intestinal contents passing, so gas and fluid pile up above the block. The distended bowel leaks litres into its own lumen and can strangle its own blood supply.

Definition

Impaired passage of intestinal contents along the bowel. Mechanical obstruction has a fixed blockage, seen on CT as a transition point between dilated proximal and collapsed distal bowel. Functional obstruction has no blockage: ileus after surgery, sepsis or opioids, and acute colonic pseudo-obstruction, in which the colon dilates without any lesion.

Epidemiology

Small bowel obstruction accounts for 12 to 16% of emergency surgical admissions and 20% of emergency laparotomies in the UK, about 7000 operations a year. Adhesions after previous surgery are the commonest cause; hernia, tumour, volvulus and gallstone ileus follow. Large bowel obstruction is 60% colorectal cancer, 20% diverticular stricture and 5% volvulus. Up to 30% of colorectal cancers first present as an emergency. Ileus and acute colonic pseudo-obstruction are common mimics in postoperative, frail or severely unwell patients.

Pathophysiology

Above a mechanical block, swallowed air, saliva, gastric juice, bile and pancreatic secretions accumulate. The bowel distends, secretion rises and absorption falls, so litres are sequestered in the lumen while the circulating volume falls. Rising intraluminal pressure then compresses the blood supply to the bowel wall. In a closed loop, obstructed at both ends, pressure climbs fastest: venous and then arterial flow fail, giving ischaemia, bacterial translocation and perforation. Ileus is loss of coordinated peristalsis with no block at all, and acute colonic pseudo-obstruction is the same failure confined to the colon.

First principles

Mechanical block or failed propulsion, and CT tells you which

Mechanical obstruction has a physical blockage: adhesions, a hernia, tumour, volvulus or stricture. On CT it shows a transition point, with dilated bowel above and collapsed bowel below. Functional failure has no such point. Ileus follows surgery, sepsis, opioids or electrolyte disturbance; acute colonic pseudo-obstruction dilates the colon with no lesion at all. The distinction sets the pathway: a closed loop may need theatre within hours, while ileus needs its cause corrected.1,2

The fluid loss arrives before the surgeon does

Obstructed bowel secretes into its own lumen and stops absorbing, while the patient vomits and stops drinking. Litres disappear into the gut without ever leaving the body, so the abdomen distends as the circulation empties. Hypokalaemia, hypochloraemia and acute kidney injury follow. Poor perfusion also makes the bowel wall more likely to become ischaemic, which is why resuscitation is not a holding measure but part of preventing strangulation.1,3

Strangulation is a clock

A closed loop, a twisted segment or a tight hernia neck occludes veins first, then arteries. The wall becomes oedematous and ischaemic, bacteria translocate, and perforation follows. Constant rather than colicky pain, guarding, tachycardia, a rising lactate or a tender irreducible hernia are the signs. RCS England (Royal College of Surgeons of England) asks for theatre as soon as possible and within 6 hours of the suspected onset.4

Level tells you the picture, not the plan

Proximal small bowel obstruction vomits early and distends little. Distal small bowel and colonic obstruction distend markedly and vomit late, sometimes faeculently. Absolute constipation supports a complete distal block, but passing flatus early does not exclude obstruction: bowel below the blockage empties normally. None of this replaces imaging, because the cause and the presence of ischaemia decide the plan.1,4

Resuscitation, imaging and referral run at the same time

NCEPOD (National Confidential Enquiry into Patient Outcome and Death) found that patients whose imaging was delayed were far more likely to have a delayed diagnosis: 61% against 6%. So the fluid, the analgesia, the surgical referral and the scan request all happen in the first hour rather than in sequence. Nothing about renal function is a reason to hold the scan.1,5,6

Presentation

Colicky abdominal pain, distension, vomiting and reduced passage of stool or flatus. The pattern shifts with level: early vomiting proximally, marked distension distally. Ileus and pseudo-obstruction look identical from the end of the bed.1,6,4,7

Cardinal features

  • Colicky central or lower abdominal pain that becomes constant if ischaemia develops
  • Distension, more marked the more distal the obstruction
  • Vomiting, early in proximal small bowel obstruction and late or faeculent in distal obstruction
  • Reduced or absent stool and flatus, although a partial obstruction may still pass both
  • Previous abdominal surgery, a groin or abdominal wall hernia, known malignancy, diverticular disease or recent severe illness
  • High-pitched bowel sounds early in mechanical obstruction; absent sounds in ileus, late obstruction or peritonitis

Red flags

  • Constant severe pain, guarding or rigidity rather than colic
  • Tachycardia, hypotension, fever, confusion, oliguria or a rising lactate
  • A tender irreducible hernia, or CT showing a closed loop, volvulus or reduced bowel-wall enhancement
  • Free gas, free fluid or pneumatosis on imaging
  • NEWS2 (National Early Warning Score 2) of 7 or more, which NICE NG253 counts as high risk of severe illness or death from sepsis
  • Tenderness over the caecum, or a caecum that is widening on serial films

Investigations

ABCDE, observations and a NEWS2 score

ABCDE is airway, breathing, circulation, disability, exposure. NEWS2 is the National Early Warning Score 2, the UK observation chart score that triggers escalation. NCEPOD requires hydration status to be measured and documented at presentation and again through the admission, because avoidable acute kidney injury was common.

Expected finding: NICE NG253 (suspected sepsis in people aged 16 or over) counts NEWS2 of 7 or more as high risk of severe illness or death, and 5 or 6 as moderate. NICE CG174 flags hypovolaemia at systolic blood pressure below 100 mmHg, heart rate above 90, capillary refill over 2 seconds, respiratory rate above 20, or NEWS of 5 or more.

1,7,3

Bloods and a venous gas with lactate

FBC, U&E, CRP, amylase or lipase, clotting, group and save, and a venous gas with lactate. RCEM (Royal College of Emergency Medicine) asks for lactate and amylase to be prioritised, because they change immediate management and a slow result otherwise delays the CT request.

Expected finding: Hypokalaemia, hypochloraemia, and a raised urea and creatinine from hypovolaemia. A metabolic acidosis with a rising lactate alongside constant pain means ischaemia until proved otherwise. Normal bloods do not exclude early strangulation.

6,1

CT abdomen and pelvis with intravenous contrast

This is the definitive test and NCEPOD's first recommendation. It gives the transition point, the cause, closed loops, ischaemia, perforation and any alternative diagnosis. NCEPOD found it sufficient to diagnose obstruction in 89% of patients, against 32% for a plain abdominal film.

Expected finding: Dilated proximal bowel, collapsed distal bowel, a transition point, and a hernia, tumour, volvulus or closed loop. Reduced bowel-wall enhancement, mesenteric oedema, free fluid or free gas mean ischaemia or perforation. RCS England's key recommendation 1 asks for the radiologist's report within 1 hour of the scan.

1,8

Do not delay the scan for renal function

The Royal College of Radiologists (RCR) and RCEM state jointly that none of these is a prerequisite for an emergency contrast CT. Measuring renal function, pre-existing renal disease, diabetes, metformin, age, and intravenous pre-hydration are all on that list.

Expected finding: The risk of contrast-induced acute kidney injury is outweighed by the risk of missing an ischaemic or perforated bowel. If a senior clinician still judges contrast unsuitable, scan without it rather than not scanning.

5,1

Plain radiographs, and when they still earn their place

An erect chest radiograph answers one question fast: is there free gas under the diaphragm. A plain abdominal film is useful for tracking caecal diameter in pseudo-obstruction, or where CT is not available.

Expected finding: Dilated small bowel sits centrally with valvulae conniventes crossing the full width; dilated colon sits peripherally with haustra that do not. Neither film assesses bowel viability, and neither should delay CT.

4,2

Water-soluble contrast study in adhesional small bowel obstruction

Used after about 48 hours of non-operative management to decide whether to continue or operate. The contrast volume and the timing of the follow-up film are set by your radiology department's protocol; no UK national guideline specifies them.

Expected finding: Contrast reaching the colon predicts resolution without surgery, and the hypertonic contrast is itself therapeutic. Contrast that does not reach the colon points to a complete obstruction that will need an operation.

4

Caecal diameter and the risk of perforation

In large bowel obstruction and in pseudo-obstruction the caecum is the segment that bursts, because it has the largest radius and so the highest wall tension for a given pressure.

Expected finding: Serial films track the trend. RCS England states that tenderness over the caecum implies impending perforation, and that sign should trigger surgical review rather than another film. The diameter thresholds come from the ACPGBI (Association of Coloproctology of Great Britain and Ireland) consensus guidelines, which are paywalled. Confirm the figure there or in local policy before acting on it.

2,4

Management

StepDetailSource
Resuscitate the circulationGive 0.9% sodium chloride or Hartmann's 500 mL intravenously over less than 15 minutes if the patient is hypovolaemic. NICE CG174 indicators: systolic blood pressure below 100 mmHg, heart rate above 90, capillary refill over 2 seconds, or NEWS2 of 5 or more. Reassess after every bolus, catheterise and chart hourly urine output.3,1NICE CG174, Intravenous fluid therapy in adults in hospital, recommendations 1.2.1 and 1.3.1; NCEPOD Acute Bowel Obstruction
Replace potassium and ongoing lossesVomiting strips potassium and chloride. Prescribe a ready-mixed bag such as potassium chloride 0.3% in sodium chloride 0.9%, which contains 40 mmol potassium per litre; never add potassium to a bag yourself. Usual replacement is 20 mmol over 2 to 3 hours with cardiac monitoring, into a large vein, never faster than 20 mmol/hour. Replace nasogastric output volume for volume.9,3Electronic Medicines Compendium SmPC, Potassium Chloride 0.3% and Sodium Chloride 0.9% Solution for Infusion; NICE CG174 recommendations 1.4.1 and 1.5.1
Nil by mouth and nasogastric decompressionKeep the patient nil by mouth. Pass a nasogastric tube on free drainage if they are vomiting or the stomach is distended. NCEPOD gives relief of vomiting as the indication; RCS England says aspirate it regularly to reduce aspiration risk. Measure and chart the output, because it sets the replacement fluid.1,4NCEPOD Acute Bowel Obstruction, chapter 4; RCS England and ASGBI Emergency General Surgery commissioning guide, small bowel obstruction pathway
Analgesia, and do not withhold it to preserve signsPain relief is a priority for NCEPOD and RCEM, not something to defer until the surgeon has seen the abdomen. Morphine 5 mg by slow intravenous injection every 4 hours, titrated to response, or 10 mg by mouth or intramuscularly; 5 mg if frail or elderly. The BNF lists acute abdomen among morphine's contraindications: re-examine afterwards.1,6,10NCEPOD recommendation 4; RCEM advisory statement on emergency laparotomy; BNF morphine monograph, acute pain in adults
Antiemetic: cyclizine, never metoclopramideCyclizine 50 mg up to three times a day, by mouth, intramuscularly or intravenously. Metoclopramide is contraindicated: it is prokinetic and the BNF contraindication list names gastro-intestinal obstruction and perforation. Systemic antimuscarinics such as hyoscine butylbromide carry the same contraindication. Ondansetron 4 mg intramuscularly or by slow intravenous injection is the alternative, with caution in subacute obstruction.11,12,13,14BNF monographs for cyclizine, metoclopramide hydrochloride, hyoscine butylbromide and ondansetron
Get the scan and the senior in parallelRequest CT of the abdomen and pelvis with intravenous contrast at once; do not wait for creatinine, metformin review or pre-hydration. RCEM table 1 sets first surgical review by ST3 or above at 30 minutes from referral for suspected peritonitis, perforation, infarction or strangulated hernia. Otherwise 60 minutes, or 30 if septic.5,6,1RCR and RCEM joint advisory statement on emergency CT and iodinated contrast, 2023; RCEM advisory statement on emergency laparotomy, October 2024, table 1, adapted there from RCS England, The High-Risk General Surgical Patient, 2018
Consultant review, documented risk and escalation planningNCEPOD requires consultant review within 14 hours of admission, and within 1 hour if predicted mortality exceeds 10%. Calculate the risk before theatre with the NELA (National Emergency Laparotomy Audit) calculator. A Rockwood Clinical Frailty Scale score of 5 or more triggers a treatment escalation plan and a documented resuscitation decision.1,6NCEPOD recommendations 2, 7 and 10; RCEM advisory statement on emergency laparotomy
Treat sepsis by the NG253 numbersA NEWS2 of 7 or more is high risk of severe illness or death. NICE NG253 then requires broad-spectrum intravenous antibiotics within 1 hour of that score. The sepsis fluid bolus is 250 mL of a balanced crystalloid over 10 to 15 minutes, repeated to a 1000 mL maximum with reassessment after each.7NICE NG253, Managing suspected sepsis, recommendations 1.8.3 to 1.8.9
Antibiotic choiceNG253 gives no national regimen: use your trust's antimicrobial policy. A typical intra-abdominal choice is co-amoxiclav 1.2 g intravenously every 8 hours. If penicillin-allergic, metronidazole 500 mg intravenously every 8 hours with gentamicin 5 to 7 mg/kg once daily, adjusted to levels. Take blood cultures before the first dose.15,16,17,18NICE NG253, Antibiotic therapy, intravenous fluid and oxygen, recommendations 1.9.2 and 1.9.7; BNF monographs for co-amoxiclav, metronidazole and gentamicin
Source control: theatre for strangulation, perforation or a failed trialSuspected ischaemia or strangulation needs theatre as soon as possible and within 6 hours of the suspected onset; peritonitis in large bowel obstruction the same. RCS England sets source control underway within 3 hours in septic shock and 6 hours otherwise. The operation may be adhesiolysis, hernia repair, detorsion, resection or a stoma.4,8RCS England and ASGBI Emergency General Surgery commissioning guide; RCS England, The High-Risk General Surgical Patient, 2018, key recommendation 8
Time-limited conservative trial for adhesional obstructionOnly if the patient is stable, has no peritonism, and CT shows no closed loop, obstructed hernia or ischaemia. RCS England allows up to 72 hours of nil by mouth, fluids, decompression and serial examination. A water-soluble contrast study at about 48 hours helps: contrast reaching the colon predicts resolution. Do not push past 120 hours.4,1RCS England and ASGBI Emergency General Surgery commissioning guide, adhesional obstruction section
Malignant large bowel obstructionCancer causes about 60% of large bowel obstruction. NICE NG151 says consider stenting when treatment is palliative, and offer either stenting or emergency surgery when potentially curative treatment is suitable. Stenting is not an option in free perforation or generalised peritonitis. The patient should be under a colorectal surgeon within 24 hours.19,4,2NICE NG151 recommendations 1.3.1 and 1.3.2; RCS England and ASGBI Emergency General Surgery commissioning guide, large bowel obstruction pathway
Ileus and acute colonic pseudo-obstructionDiagnoses of exclusion, made only after CT. Correct sodium, potassium, magnesium and calcium, stop opioids and antimuscarinics, mobilise. Continue supportive treatment while the patient is stable, the caecum is not enlarging and there is no peritonism. If that fails, the specialist gives neostigmine under cardiac monitoring or decompresses colonoscopically. The paywalled ACPGBI guideline holds the timings and neostigmine dose.2,1ACPGBI consensus guidelines in emergency colorectal surgery, acute colonic pseudo-obstruction section
Nutrition and recoveryNCEPOD requires a MUST (Malnutrition Universal Screening Tool) score on admission and weekly, with dietitian review once the diagnosis is made. Avoid prolonged starvation. After surgery watch for ileus, anastomotic leak, abscess and wound or chest infection, and arrange colorectal follow-up and cancer staging if malignancy caused it.1,19NCEPOD recommendation 6; NICE NG151, Colorectal cancer

Illustrations

Mechanism of proximal distensionDiagram showing a mechanical transition point or non-peristalsing bowel with gas and fluid accumulating proximally, while highlighting that strangulation threatens the blood supply.PassFinals · original
Abdominal X-ray in bowel obstructionErect abdominal radiograph showing dilated bowel loops in intestinal obstruction; use it for pattern recognition but emphasise that prompt contrast CT is the preferred definitive test when appropriate.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0
CT showing a closed-loop obstructionAxial CT image demonstrating a closed loop of strangulated bowel with reduced wall enhancement and mesenteric oedema; image recognition must be linked to urgent senior surgical review.Mikael Häggström, Wikimedia Commons · CC0

Differentials

Postoperative or metabolic ileus

Diffuse small and large bowel dilatation with no transition point, after surgery, sepsis, opioids or electrolyte disturbance.

Acute colonic pseudo-obstruction

Marked colonic dilatation with no mechanical lesion, typically in a frail or severely unwell inpatient. CT must exclude obstruction first.

Sigmoid or caecal volvulus

Abrupt onset, a coffee-bean loop on plain film and a whirl sign on CT. Sigmoid volvulus is often decompressed endoscopically.

Toxic megacolon

Colonic dilatation with systemic toxicity in severe colitis, usually with bloody diarrhoea rather than absolute constipation.

Faecal impaction or severe constipation

Faecal loading throughout the colon with no transition point. Rectal examination finds hard stool; exclude obstruction before giving laxatives.

Acute mesenteric ischaemia

Pain out of proportion to a soft abdomen, atrial fibrillation or vascular disease, and a lactic acidosis that outruns the clinical signs.

Complications

  • Hypovolaemia, acute kidney injury, hypokalaemia and hypochloraemia
  • Aspiration of vomit
  • Strangulation, bowel ischaemia and necrosis
  • Perforation, faecal peritonitis, sepsis and septic shock
  • Malnutrition from prolonged starvation
  • Postoperative ileus, anastomotic leak, abscess and recurrent adhesional obstruction

Prognosis

Simple adhesional obstruction usually settles without an operation. Outcome worsens sharply with delay, frailty, malignancy, strangulation and perforation. Thirty-day mortality after surgery for small bowel obstruction is 10.6% in UK practice. Emergency large bowel surgery kills 10 to 20%, rising to 40% once the colon has perforated. Thirty-eight per cent of NCEPOD's frail patients died during the admission. The modifiable factors are prompt contrast CT, resuscitation, senior review and timely source control.

Guidelines

  • Acute Bowel Obstruction: Delay in Transit (NCEPOD, 2020)
  • Consensus guidelines in emergency colorectal surgery (ACPGBI, 2021)
  • The High-Risk General Surgical Patient: Raising the Standard (Royal College of Surgeons of England, 2018)
  • Suspected sepsis in people aged 16 or over (NG253) (NICE, 2025)
  • Colorectal cancer: diagnosis and management (NG151) (NICE, 2020)
  • Intravenous fluid therapy in adults in hospital (CG174) (NICE, 2013)

References

  1. NCEPOD (National Confidential Enquiry into Patient Outcome and Death), Acute Bowel Obstruction: Delay in TransitPublished 9 Jan 2020
  2. Association of Coloproctology of Great Britain and Ireland (ACPGBI), consensus guidelines in emergency colorectal surgery, Colorectal Disease, free full textPublished 20 Jan 2021
  3. NICE, Intravenous fluid therapy in adults in hospital: recommendations (CG174)Published 10 Dec 2013 | Updated 5 May 2017
  4. Royal College of Surgeons of England (RCS England) and Association of Surgeons of Great Britain and Ireland (ASGBI), Emergency General Surgery commissioning guide, small and large bowel obstruction pathwaysPublished 1 Jan 2014
  5. Royal College of Radiologists (RCR) and Royal College of Emergency Medicine (RCEM), joint advisory statement on emergency CT scans and intravenous iodinated contrast agentsPublished 31 May 2023
  6. Royal College of Emergency Medicine, Advisory Statement regarding the management of adults presenting to the Emergency Department who may require an emergency laparotomyPublished 1 Oct 2024
  7. NICE, Suspected sepsis in people aged 16 or over: managing suspected sepsis (NG253)Published 19 Nov 2025
  8. Royal College of Surgeons of England, The High-Risk General Surgical Patient: Raising the StandardPublished 1 Dec 2018
  9. Electronic Medicines Compendium, Summary of Product Characteristics: Potassium Chloride 0.3% w/v and Sodium Chloride 0.9% w/v Solution for Infusion BPUpdated 1 Jun 2023
  10. BNF, Morphine
  11. BNF, Cyclizine
  12. BNF, Metoclopramide hydrochloride
  13. BNF, Hyoscine butylbromide
  14. BNF, Ondansetron
  15. NICE, Suspected sepsis in people aged 16 or over: antibiotic therapy, intravenous fluid and oxygen (NG253)Published 19 Nov 2025
  16. BNF, Co-amoxiclav
  17. BNF, Metronidazole
  18. BNF, Gentamicin
  19. NICE, Colorectal cancer: recommendations, acute left-sided large bowel obstruction (NG151)Published 29 Jan 2020 | Updated 15 Dec 2021

Evidence checked: 2026-08-07

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.