Intra-abdominal Infections

IAI: Overview - Gut Gone Wrong

• Intra-abdominal Infection (IAI): Inflammation within peritoneal cavity due to microorganisms.
• Types:
  • Primary Peritonitis (Spontaneous Bacterial Peritonitis - SBP): No evident intra-abdominal source. Common in liver cirrhosis.
  • Secondary Peritonitis: Due to perforation/inflammation of GI tract (e.g., appendicitis, diverticulitis, PUD). Most common type.
  • Tertiary Peritonitis: Persistent/recurrent infection after initial treatment, often with resistant organisms.
• Classification:
  • Uncomplicated IAI: Infection confined to a single organ.
  • Complicated IAI (cIAI): Infection extends beyond organ into peritoneal space; associated with abscess or peritonitis.
• Common Origins: Appendix, colon, stomach/duodenum, biliary tract.

⭐ Most common cause of secondary peritonitis is perforated appendicitis.

IAI: Pathogens & Process - Belly's Bad Bugs

• Key Pathogens (Often Polymicrobial):
  • Aerobes:
    • Escherichia coli (most frequent Gram-negative)
    • Klebsiella spp.
    • Enterococcus spp. (Gram-positive cocci)
  • Anaerobes:
    • Bacteroides fragilis (most frequent anaerobe)
    • Clostridium spp.
    • Peptostreptococcus
  • Fungi (e.g., Candida): Consider in immunocompromised, prolonged antibiotics, or recurrent IAI.
  • 📌 Mnemonic: "BECK" - Bacteroides, E.coli, Clostridium, Klebsiella.
• Pathogenesis Cascade:
  • Source of microbes:
    • Perforation (e.g., appendicitis, diverticulitis, Peptic Ulcer Disease)
    • Ischemia/Necrosis (e.g., mesenteric ischemia, strangulated bowel)
    • Translocation (e.g., severe pancreatitis, shock states)
  • Peritoneal contamination → Inflammation (Peritonitis).
  • Host defense: Opsonization, phagocytosis, fibrin deposition (walling-off → phlegmon/abscess).

⭐ Escherichia coli is the most common aerobic pathogen, and Bacteroides fragilis is the most common anaerobic pathogen in Intra-abdominal Infections (IAIs).

IAI: Diagnosis - Spotting Sepsis Signs

• Clinical:
  • Local: Abdominal pain, tenderness, guarding, rigidity.
  • Systemic: Fever, tachycardia, hypotension.
• Labs:
  • ↑WBC (left shift), ↑CRP.
  • ↑Serum Lactate (>2 mmol/L = hypoperfusion).
  • ↑Procalcitonin. Blood cultures (pre-antibiotics).
• Imaging:
  • X-Ray (erect): Free air (perforation), air-fluid levels.
  • USG: Fluid, abscesses, organ-specific (e.g., appendicitis, cholecystitis).
  • CT (Contrast): Gold standard for source, extent, complications.
• Sepsis (qSOFA ≥2):
  • Respiratory Rate ≥22/min
  • Altered Mentation (GCS <15)
  • Systolic BP ≤100 mmHg 📌 Mnemonic: HAT (Hypotension, Altered Mental Status, Tachypnea).

⭐ Serum lactate >2 mmol/L is a key indicator of tissue hypoperfusion and adverse outcomes in sepsis. Early qSOFA assessment is vital for timely intervention in IAI cases.

IAI: Treatment - Quelling the Chaos

• Core Principles (Time-Sensitive):

  • Resuscitation: ABCs, IV crystalloids, O2. Target UO > 0.5 ml/kg/hr.
  • Early Empiric Antibiotics: Broad-spectrum IV within 1 hour of sepsis/septic shock diagnosis.
  • Definitive Source Control: Within 6-12 hours. Critical for outcomes.

• Antimicrobial Strategy:

  • CA-IAI (Community-Acquired):
    • Mild-Moderate (e.g., uncomplicated appendicitis, diverticulitis): Ertapenem OR (Ceftriaxone/Cefotaxime + Metronidazole).
    • Severe/High-Risk (e.g., diffuse peritonitis): Piperacillin-Tazobactam OR Carbapenem (Imipenem, Meropenem).
  • HA-IAI (Hospital/Healthcare-associated):
    • Broader coverage for MDROs (Pseudomonas, ESBL): Piperacillin-Tazobactam, Carbapenems. Add Vancomycin/Linezolid if MRSA suspected.
  • Duration: Typically 4-7 days after adequate source control.

• Source Control Modalities:

  • Surgical: Laparotomy/laparoscopy (e.g., appendectomy, resection, repair of perforation).
  • Percutaneous Drainage (PCD): For localized abscesses.

  ⭐ In complicated IAI (e.g., perforated diverticulitis with abscess), achieving source control within 6 hours is associated with significantly improved survival.

• Supportive Measures:

  • Nutritional support (enteral preferred), DVT & stress ulcer prophylaxis.

High‑Yield Points - ⚡ Biggest Takeaways

• Secondary peritonitis: E. coli (aerobic) & B. fragilis (anaerobic) are most common.
• Tertiary peritonitis: Persistent infection, often resistant organisms or fungi.
• Hinchey classification guides diverticulitis management; Stages III & IV often need surgery.
• Appendicitis: Primarily clinical diagnosis; Alvarado score aids, imaging for equivocal cases.
• Source control (drainage/repair/resection) is key, with antibiotics.
• Empirical antibiotics: Must cover gram-negative bacilli & anaerobes.
• Intra-abdominal abscess: Percutaneous drainage (USG/CT guided) is often treatment.