Gastrointestinal & Hepatic · Topic 13 of 26

The Acute Abdomen & Perforation

Gastrointestinal · surgical abdomen

The Acute Abdomen & Perforation

Peritoneal signs, free air or a postoperative patient who stops progressing mean a surgical problem: resuscitate, give antibiotics, image fast, call surgery.

Key numbers

Upright chest film for free airfree air in 50–75%

Source control plus about 4 days of antibiotics≈ 4 days of antibiotics

01 · Look forGuarding, rigidity and rebound mean peritonitis

Involuntary guarding, rigidity and rebound tenderness signal peritoneal irritation; severe pain in a patient lying still with a silent abdomen suggests peritonitis and needs urgent surgical consultation.

02 · DoUpright chest film for free air

For suspected perforation, order upright chest and abdominal radiographs — free air under the diaphragm appears in 50–75% — then CT if nondiagnostic; never give barium when perforation is possible (water-soluble contrast is safe).

03 · Watch forPostoperative failure to progress: suspect anastomotic leak

Suspect anastomotic leak in any postoperative patient who fails to progress or deteriorates (tachycardia, fever, prolonged ileus, pain, abnormal drain output); get contrast CT, with rectal contrast after colorectal surgery.

Causes & risk factors

Why it happens

Consider extra-abdominal mimics: MI, DKA, PEAbdominal pain can come from outside the abdomen: myocardial infarction, pulmonary embolism, diabetic or alcoholic ketoacidosis, adrenal insufficiency, sickle cell crisis, herpes zoster and testicular torsion.

Presentation

What you'll see

Appendicitis: periumbilical pain migrating to RLQAppendicitis classically starts as epigastric or periumbilical pain with anorexia and nausea, then localizes to McBurney point in the right lower quadrant; Rovsing, psoas and obturator signs support it.

Steroids and old age blunt peritoneal signsOlder adults and patients on corticosteroids or other immunosuppressants may show less pain, tenderness and leukocytosis despite perforation or peritonitis — keep the threshold for imaging low.

Sudden pain: perforation, rupture or torsionAbrupt 'light-switch' onset suggests a perforated ulcer, ruptured ectopic pregnancy or aneurysm, or torsion; pain out of proportion to the examination suggests mesenteric ischemia; back pain with shock suggests a ruptured aortic aneurysm.

Do right now

In this order

First moves

Exclude AAA, perforation, ischemia and ectopic first

→

Give IV analgesia; it does not mask signs

→

Upright chest film for free airfree air in 50–75%

→

Perforation: fluids, antibiotics, emergency surgery

Why each step

Exclude AAA, perforation, ischemia and ectopic firstRule out the immediately lethal causes first — ruptured abdominal aortic aneurysm, perforated viscus, mesenteric ischemia, ruptured ectopic pregnancy — and get a pregnancy test in every patient of childbearing potential.

Give IV analgesia; it does not mask signsDo not withhold analgesia while diagnosing: moderate IV doses (fentanyl 50–100 mcg or morphine 4–6 mg) do not mask peritoneal signs and often make the examination easier.

Upright chest film for free airFor suspected perforation, order upright chest and abdominal radiographs — free air under the diaphragm appears in 50–75% — then CT if nondiagnostic; never give barium when perforation is possible (water-soluble contrast is safe).

Perforation: fluids, antibiotics, emergency surgeryA perforated viscus needs IV fluid and electrolyte resuscitation, broad-spectrum IV antibiotics against intestinal flora, sometimes a nasogastric tube, and immediate surgery — mortality from peritonitis climbs the longer repair is delayed.

Management

Orders

Classic appendicitis: operate; equivocal: contrast CTClassic appendicitis is a clinical diagnosis — imaging only delays surgery and raises perforation risk; with atypical or equivocal findings, obtain contrast CT promptly (graded-compression ultrasound is the radiation-free option, especially in children).

IV antibiotics before every appendectomyGive IV antibiotics (a third-generation cephalosporin is preferred) before appendectomy; none are needed afterward for a nonperforated appendix, while a perforated appendix gets about 4 more days.

Leak with sepsis: resuscitate and reoperateA leak with hemodynamic instability, generalized peritonitis or septic shock needs resuscitation, broad-spectrum antibiotics and prompt operative source control; a stable patient with a contained abscess can be managed with percutaneous drainage and antibiotics.

The rules

Know these

Source control plus about 4 days of antibioticsFor complicated intra-abdominal infection with adequate source control, a fixed course of about 4 days of antibiotics works as well as a longer course continued until fever, leukocytosis and ileus resolve (STOP-IT trial).

Normal labs do not exclude surgical diseaseRoutine blood tests are nonspecific and can be normal in serious disease — a normal WBC does not exclude appendicitis; serum lipase (for pancreatitis) is the useful exception.

Visceral or peritoneal (somatic) pain?

Commonly confused

Trigger	Distention or contraction of a hollow organ
Character	Vague, dull, poorly localized, often midline
Patient behavior	Writhing, trying to get comfortable
Appendicitis example	Early periumbilical pain

More detail

Reference

Show 5 reference facts

Gastric or duodenal perforation presents abruptly with generalized peritonitis and shoulder-tip pain; perforations lower in the bowel are often small and walled off by omentum, so pain builds gradually and stays localized.

A lateral chest film is more sensitive for free air than a PA film; contrast CT diagnoses about 95% of significant abdominal pain but must never delay surgery in a clear-cut case.

C-reactive protein on postoperative days 3–5 helps rule out colorectal anastomotic leak: values below roughly 150 mg/L on day 3 carry a high negative predictive value; trends matter more than one value.

Anastomotic leaks are graded by the treatment they need: grade A no active intervention, grade B treatment without relaparotomy (antibiotics, drainage), grade C reoperation.

An appendiceal abscess is drained percutaneously with interval appendectomy; antibiotics alone often resolve appendicitis but many recur within a year, so appendectomy remains standard, especially with an appendicolith on CT.

Pearl

The patient lying perfectly still has peritonitis; the patient writhing to get comfortable has colic.

Sources · Merck Manual Professional — Acute Abdominal Pain · Merck Manual Professional — Acute Perforation of the Gastrointestinal Tract · Merck Manual Professional — Appendicitis · Sawyer RG et al. Trial of short-course antimicrobial therapy for intraabdominal infection (STOP-IT). N Engl J Med 2015;372:1996–2005 · Prevention, Recognition, and Management of Anastomotic Leakage and Pelvic Sepsis After Rectal Cancer Surgery: A Structured Narrative Review. J Clin Med 2026 · From Recognition to Prevention: Modern Approaches to Complication Reduction in Colorectal Surgery. J Clin Med 2026 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).