What is the initial assessment and management of an adult presenting with acute peritonitis characterized by abdominal pain, guarding, fever, tachycardia, and possible hypotension?

Initial Assessment and Management of Acute Peritonitis in Adults

For an adult presenting with acute peritonitis characterized by abdominal pain, guarding, fever, tachycardia, and possible hypotension, immediately initiate aggressive fluid resuscitation, administer broad-spectrum intravenous antibiotics within the first hour, and proceed to emergency surgical source control without delay—delays beyond 24 hours significantly increase mortality. 1, 2

Immediate Resuscitation (Within First Hour)

Begin rapid intravenous fluid resuscitation immediately upon recognizing peritonitis or septic shock, targeting a mean arterial pressure of 65-70 mmHg. 1 Use crystalloid solutions as first-line agents, infusing rapidly to restore intravascular volume while monitoring for pulmonary edema. 1 In patients with septic shock (hypotension requiring vasopressors plus lactate >2 mmol/L), norepinephrine is the first-line vasopressor. 1

• Critical caveat: Avoid fluid overload, as aggressive crystalloid resuscitation in peritonitis predisposes to bowel edema, increased intra-abdominal pressure, and abdominal compartment syndrome—a potentially lethal complication. 1
• Target hemodynamic stability but balance against the risk of intra-abdominal hypertension (IAP >20 mmHg with organ dysfunction). 1

Diagnostic Workup

Obtain blood cultures and initiate diagnostic imaging only if it will not delay surgical intervention. 1, 2

• In patients with diffuse peritonitis and obvious signs of perforation (rigid abdomen, rebound tenderness, guarding), proceed directly to the operating room without imaging—further diagnostic tests are unnecessary and delay definitive treatment. 1
• For hemodynamically stable patients without clear diffuse peritonitis, obtain a CT scan with IV contrast to identify the source and extent of infection. 1 CT has the highest sensitivity and specificity for detecting peritonitis compared to ultrasound or plain radiography. 2
• Plain abdominal radiography showing free air confirms perforation but has lower sensitivity than CT. 2

Key clinical findings to document:

• Abdominal rigidity strongly suggests peritonitis (present in 74-95% of cases). 2
• Tachycardia occurs in 62.5% of patients. 2
• Fever >38.5°C is present in only 38% of cases—absence does not exclude peritonitis. 2
• Hypotension, lactic acidosis, oliguria, or altered mental status indicate septic shock requiring immediate intervention. 2

Antibiotic Therapy (Initiate Within First Hour)

Administer broad-spectrum intravenous antibiotics immediately once peritonitis is suspected or diagnosed—in septic shock, every hour of delay increases mortality. 1, 2, 3

Community-Acquired Peritonitis (No Recent Healthcare Exposure)

• First-line: Piperacillin-tazobactam 4.5 g IV every 6 hours for critically ill or septic patients. 3
• Alternative for non-critically ill patients: Piperacillin-tazobactam 3.375 g IV every 6 hours. 3
• This regimen covers common pathogens including E. coli, Klebsiella, Streptococcus species, and anaerobes (Bacteroides fragilis). 2, 3

Hospital-Acquired or Healthcare-Associated Peritonitis

• First-line for critically ill/septic shock: Meropenem 1-2 g IV every 8 hours, imipenem-cilastatin 500 mg-1 g IV every 6-8 hours, or doripenem 500 mg IV every 8 hours. 2, 3
• Carbapenems are mandatory for patients with prior antibiotic exposure (≥5 days), hospitalization >1 week, or risk factors for extended-spectrum beta-lactamase (ESBL)-producing organisms. 2, 3
• Add empiric antifungal coverage (echinocandins) for hospital-acquired peritonitis due to high risk of Candida species. 2, 3

Pharmacokinetic optimization: Critically ill patients require higher loading doses of hydrophilic beta-lactam antibiotics due to sepsis-induced plasma dilution, independent of renal function. 3

Surgical Source Control (Emergency Intervention)

Proceed to emergency laparotomy immediately for patients with diffuse peritonitis—delays beyond 24 hours independently predict mortality and need for relaparotomy. 1, 2, 3 Operating room latency ≥60 hours is an independent predictor of death. 2

Surgical Objectives

• Drain infected foci and fluid collections. 1, 2
• Control ongoing peritoneal contamination by resection or repair of perforated viscus. 1, 2
• Remove infected organs and debride necrotic tissue. 1, 2
• Restore anatomic continuity when physiologically feasible. 1

Damage Control Surgery Considerations

In patients with severe physiological derangement (hypotension requiring high-dose vasopressors, coagulopathy, hypothermia), perform abbreviated laparotomy and leave the abdomen open. 1, 2 Defer intestinal anastomosis in unstable patients. 1 The open abdomen prevents abdominal compartment syndrome in the setting of visceral edema and incomplete source control. 1

Non-Operative Management (Highly Selected Cases Only)

Non-operative management may be considered ONLY in hemodynamically stable patients responding to antibiotics with:

• Perforated diverticulitis with abscess <4 cm diameter. 2
• Peri-appendiceal phlegmon or small abscess. 2
• Small perforated peptic ulcer with minimal contamination. 2
• CT findings of pericolic air only without diffuse peritonitis or distant free air. 2

Absolute contraindications to non-operative management: Distant free air, hemodynamic instability, or diffuse peritonitis mandate immediate surgical intervention. 2

Duration of Antibiotic Therapy

Limit antibiotics to 3-5 days post-operatively in patients with adequate source control—a fixed 4-day regimen is optimal based on highest-quality evidence. 2, 3 Longer courses do not improve outcomes and increase antimicrobial resistance and Clostridioides difficile infection risk. 2, 3

• If fever, leukocytosis, or peritonitis signs persist beyond 5-7 days, obtain abdominal CT to evaluate for residual infection, abscess, or inadequate source control. 2, 3
• De-escalate to narrow-spectrum agents within 24-48 hours once culture results and susceptibilities are available. 2, 3

Microbiological Sampling

Collect peritoneal fluid (minimum 1-2 mL) intraoperatively and inoculate directly into aerobic and anaerobic blood culture bottles before starting antibiotics. 2, 3 This allows targeted therapy and de-escalation based on culture results. 2

Common Pitfalls to Avoid

• Do not delay surgery for imaging in patients with obvious diffuse peritonitis—mortality increases with each hour of delay. 1, 2
• Do not continue antibiotics beyond 5 days without investigating for inadequate source control. 2
• Do not use narrow-spectrum antibiotics in hospital-acquired peritonitis—resistance patterns demand broader coverage. 2
• Do not attempt primary fascial closure in patients with visceral edema, incomplete source control, or hemodynamic instability requiring high-dose vasopressors—this leads to abdominal compartment syndrome. 1
• Do not over-resuscitate with fluids—balance hemodynamic goals against the risk of bowel edema and intra-abdominal hypertension. 1