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.

In a nutshell

Bowel obstruction is a mechanical blockage or failed propulsion; the two emergencies are hypovolaemia and strangulation. Resuscitate with 500 mL crystalloid over less than 15 minutes, decompress with a nasogastric tube, and get prompt CT with intravenous contrast.

Classic presentation

A 68-year-old with a midline laparotomy scar, two days of colicky central pain, vomiting, distension and no flatus, with a tympanitic abdomen and tinkling bowel sounds.

Key points

  • Small bowel obstruction is 12 to 16% of emergency surgical admissions and 20% of emergency laparotomies in the UK, about 7000 operations a year.
  • Large bowel obstruction is 60% cancer, 20% diverticular stricture and 5% volvulus. As many as 30% of colorectal cancers initially present as emergencies.
  • Emergency surgery for large bowel obstruction kills 10 to 20%, against under 5% electively, and 40% once the colon has perforated.
  • Frailty is the strongest prognostic sign here. NCEPOD found 38% of patients scoring 5 to 9 on the Rockwood frailty scale died in hospital, against 3% scoring 1 to 4.
  • Contrast CT diagnosed obstruction in 89% of NCEPOD's patients and a plain abdominal film in only 32%. Neither film assesses bowel viability.
  • The caecum bursts first because its radius is largest, so tenderness there or a widening caecum on serial films means stop managing conservatively.
  • The 30-day mortality after emergency laparotomy in England and Wales is just over 9%, and half of these patients are judged high risk.

First-line investigation

CT abdomen and pelvis with intravenous contrast, with FBC, U&E, CRP and a venous gas with lactate. Score NEWS2 (National Early Warning Score 2) on arrival and repeat it.

Management

Resuscitate, decompress, relieve symptoms

  • 0.9% sodium chloride or Hartmann's 500 mL intravenously over less than 15 minutes if hypovolaemic. That means systolic below 100 mmHg, pulse above 90, or NEWS2 of 5 or more.3
  • Nil by mouth. Nasogastric tube on free drainage if vomiting or distended; aspirate regularly, chart the output, then replace it volume for volume.1,4
  • Morphine 5 mg by slow intravenous injection every 4 hours, titrated to response, or 10 mg by mouth or intramuscularly. Use 5 mg if frail or elderly.10,1
  • Cyclizine 50 mg up to three times a day by mouth, intramuscularly or intravenously. Never metoclopramide: it is prokinetic and contraindicated in obstruction.11,12
  • Ready-mixed potassium chloride 0.3% in sodium chloride 0.9% is 40 mmol per litre. Give 20 mmol over 2 to 3 hours; never exceed 20 mmol/hour.9,3

Image and stratify

  • CT abdomen and pelvis with intravenous contrast now. The Royal Colleges of Radiologists and of Emergency Medicine say creatinine, metformin, age and pre-hydration are not prerequisites.5,1
  • FBC, U&E, CRP, amylase and a venous gas with lactate. A rising lactate with constant rather than colicky pain means ischaemia until proved otherwise.6
  • Examine every hernial orifice and every scar. A tender irreducible hernia is a strangulated hernia and belongs in the immediate-surgery group.6
  • First surgical review by ST3 or above within 30 minutes of referral if peritonitis, perforation, infarction or strangulated hernia is suspected; 60 minutes otherwise, 30 if septic.6

Treat sepsis, then choose the branch

  • NEWS2 of 7 or more is high risk; under NICE NG253, broad-spectrum intravenous antibiotics are due within 1 hour of calculating that score.7
  • The sepsis fluid bolus is 250 mL of balanced crystalloid over 10 to 15 minutes, repeated to a 1000 mL maximum, reassessing after each one.7
  • Local antimicrobial policy decides the drug. Typically co-amoxiclav 1.2 g intravenously every 8 hours; if penicillin-allergic, metronidazole 500 mg 8-hourly with gentamicin 5 to 7 mg/kg daily.16,17,18,15
  • Stable adhesional obstruction with no closed loop, hernia or ischaemia on CT: non-operative for up to 72 hours, water-soluble contrast study at about 48 hours.4

Source control

  • Suspected strangulation or ischaemia: theatre as soon as possible and within 6 hours of the suspected onset. Peritonitis in large bowel obstruction, the same.4
  • In septic shock, source control must be underway within 3 hours; within 6 hours for sepsis without shock.8
  • Left-sided malignant large bowel obstruction: NICE NG151 says consider stenting if treatment is palliative, and offer stenting or emergency surgery if potentially curative.19
  • Pseudo-obstruction only after CT excludes a lesion. Stable, no peritonism and a caecum that is not enlarging: supportive treatment, then neostigmine or colonoscopic decompression.2

Recovery, nutrition and the risk conversation

  • Consultant review within 14 hours of admission; within 1 hour if predicted mortality exceeds 10%. Calculate it with the NELA (National Emergency Laparotomy Audit) risk calculator.1,6
  • Rockwood Clinical Frailty Scale 5 or more, on a 1 to 9 scale where 5 upwards counts as frail: escalation plan, resuscitation decision, multidisciplinary discussion.1
  • MUST (Malnutrition Universal Screening Tool) score on admission and weekly, dietitian review once the diagnosis is made, and no prolonged starvation.1
  • After surgery: ileus, anastomotic leak, abscess, wound and chest infection. Arrange colorectal follow-up and cancer staging if malignancy caused the obstruction.19,1

Exam traps

  • Metoclopramide is contraindicated in mechanical obstruction: it is prokinetic, and the BNF names gastro-intestinal obstruction and perforation in its contraindications. Prescribe cyclizine instead.
  • Hyoscine butylbromide is contraindicated too. The BNF bars all systemic antimuscarinics in gastro-intestinal obstruction, intestinal atony and paralytic ileus.
  • The sepsis bolus is 250 mL, not 500 mL. NICE NG253 changed it in 2025; the 500 mL bolus in CG174 is for hypovolaemia.
  • Passing flatus or stool does not exclude obstruction, because bowel below the blockage empties normally before absolute constipation appears.
  • Do not delay contrast CT for creatinine, metformin, age or pre-hydration. The Royal Colleges of Radiologists and Emergency Medicine say none is a prerequisite.
  • Neostigmine treats pseudo-obstruction, never mechanical obstruction, and needs cardiac monitoring because it causes bradycardia.
  • A tender irreducible hernia in an obstructed patient is strangulation. It sits in the immediate-surgery group: surgical review by ST3 or above within 30 minutes.
  • Ondansetron carries a BNF caution in subacute intestinal obstruction, so cyclizine stays first choice.

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

Key sources

  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

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.