Necrotising Soft Tissue Infection

Pneumoperitoneum is free gas within the peritoneal cavity. In a patient with acute abdominal pain and peritonitis, assume gastrointestinal perforation until proven otherwise.

Common surgical causes include perforated gastroduodenal ulcer, perforated diverticulitis, perforated malignancy, bowel ischaemia, traumatic perforation and anastomotic leakage.

Not all free intraperitoneal gas requires surgery. Expected postoperative gas, recent laparoscopy, peritoneal dialysis and selected thoracic or gynaecological causes can produce pneumoperitoneum without visceral perforation. Clinical context determines significance.

Perforated viscus typically produces sudden severe abdominal pain followed by localised or generalised peritoneal irritation.

Look for:

• Involuntary guarding or rigidity
• Increasing abdominal distension
• Fever and persistent tachycardia
• Reduced bowel sounds from ileus
• Hypotension, oliguria or altered mentation in advanced sepsis

A patient with free air but no pain, no peritonism, normal physiology and a recent recognised benign cause should be assessed differently from a patient with rigid abdomen and shock.

• CT abdomen and pelvis with IV contrast: preferred in a stable patient; confirms free intraperitoneal gas and, more importantly, attempts to identify the perforation site and associated contamination.
• Plain chest or abdominal radiography may demonstrate subdiaphragmatic free gas but has substantially lower sensitivity. A negative radiograph does not exclude perforation.
• Laboratory studies assess physiological consequences rather than diagnose pneumoperitoneum. Obtain full blood count, renal function, electrolytes and lactate where significant infection or hypoperfusion is suspected.

Management is determined by whether the pneumoperitoneum represents an uncontrolled perforation.

A patient with peritonitis, sepsis or haemodynamic deterioration requires immediate surgical review, resuscitation, broad spectrum antibiotics and source control.

For community acquired gastrointestinal perforation:

Ceftriaxone 2 g IV every 24 hours plus metronidazole 500 mg IV every 12 hours is reasonable in a stable lower risk patient.

For severe sepsis, major contamination or healthcare associated perforation:

Piperacillin tazobactam 4.5 g IV loading dose followed by 4.5 g IV every 8 hours by extended infusion, with renal adjustment.

Provide balanced crystalloid, correct electrolytes, insert a nasogastric tube when there is vomiting or major distension and crossmatch blood when major surgery is anticipated.

Definitive treatment depends on the source and may involve primary repair, resection with anastomosis, diversion, drainage or damage control surgery.

Do not delay theatre for additional imaging when an unstable patient has convincing generalised peritonitis and an operative source is strongly suspected.

Nonoperative observation is appropriate only when the patient is physiologically stable, has no peritonitis and there is a convincing nonsurgical explanation for the free gas. Serial examination is mandatory because deterioration changes the strategy immediately.

After adequate source control for complicated intra abdominal infection, antimicrobial therapy should generally be limited to 4 days, provided clinical recovery is appropriate.