1. Clinical Presentation and Initial Assessment

Complicated intra-abdominal infections (cIAI) result when gastrointestinal flora breach the peritoneum, provoking localized or diffuse peritonitis and systemic inflammation. Early recognition of pain patterns, peritoneal signs, Systemic Inflammatory Response Syndrome (SIRS) criteria, and organ dysfunction is critical for prompting timely imaging and intervention.

A. Signs and Symptoms of cIAI

• Acute Abdominal Pain: Typically constant and severe, worsening with movement. In appendicitis, pain may migrate from the periumbilical region to the right lower quadrant; perforation often leads to diffuse pain.
• Peritoneal Signs: Key indicators include involuntary guarding, rebound tenderness, and abdominal rigidity.
• Systemic Inflammatory Response/Sepsis Indicators:
  • Temperature >38 °C or <36 °C
  • Heart rate >90 beats/minute
  • Respiratory rate >20 breaths/minute or PaCO₂ <32 mm Hg
  • Altered mental status, hypotension (Mean Arterial Pressure <65 mm Hg), or oliguria (low urine output).

B. Laboratory Evaluations

• White Blood Cell Count: Leukocytosis (>12,000/mm³) with a “left shift” (increased immature neutrophils) is common. Conversely, leukopenia (<4,000/mm³) can be a poor prognostic sign indicating severe sepsis.
• C-Reactive Protein (CRP): A non-specific inflammatory marker that begins to rise 6–8 hours after an insult. Its trend over time is more informative than any single absolute value for monitoring response to treatment.
• Procalcitonin (PCT): More specific for bacterial infection than CRP. Serial measurements can be valuable for guiding the de-escalation of antibiotic therapy in conjunction with clinical improvement.

2. Imaging Modalities

Imaging is essential to confirm the diagnosis, define the anatomical extent of the infection, and guide source control procedures. Contrast-enhanced CT is the gold standard, while ultrasound and MRI serve important adjunctive roles in select patient populations.

A. Computed Tomography (CT)

• Protocol: The standard is an abdominopelvic CT with intravenous contrast administered in the portal venous phase (60-70 second delay), with 3–5 mm slice thickness. Oral or rectal contrast may be added if bowel perforation is suspected.
• Key Findings:
  • Abscess: A rim-enhancing fluid collection, typically ≥3 cm.
  • Perforation: Extraluminal (free) air or leakage of oral/rectal contrast.
  • Inflammation: Bowel wall thickening (>3 mm), mesenteric fat stranding, and free peritoneal fluid.

B. Ultrasound

• Uses: Excellent for rapid, bedside assessment of free fluid in hemodynamically unstable patients (e.g., FAST exam) and for real-time guidance of percutaneous drainage procedures.
• Limitations: Highly operator-dependent. Its utility is often limited by overlying bowel gas and patient obesity. Sensitivity is approximately 70% for fluid collections >50 mL.

C. Magnetic Resonance Imaging (MRI)

• Indications: A valuable alternative for patients with a severe IV contrast allergy, in pregnancy to avoid radiation, or in cases of complex postoperative anatomy where its superior soft-tissue contrast is beneficial.
• Limitations: Longer acquisition time makes it less suitable for unstable patients. It also has limited availability in many emergency departments.

3. Classification and Severity Scoring

Stratifying cIAI by acquisition setting and physiologic derangement is crucial to inform empiric antibiotic therapy, determine the urgency of source control, and estimate prognosis.

A. Community- vs. Healthcare-Associated cIAI

• Community-Acquired (CA-cIAI): Infection develops in a patient without recent healthcare exposure (symptom onset <48 hours from admission). These infections have a lower risk of multidrug-resistant (MDR) organisms.
  • Empiric Therapy Example: Piperacillin-tazobactam or ertapenem.
• Healthcare-Associated (HA-cIAI): Infection develops ≥48 hours after hospital admission or following recent healthcare instrumentation. These carry a higher risk of MDR pathogens like ESBL-producing organisms and Pseudomonas aeruginosa.
  • Empiric Therapy Example: Meropenem, potentially with additional agents for Pseudomonas or Enterococcus coverage based on local data.

B. Physiologic Severity Scores

• APACHE II Score: A complex score (0–71) based on 12 physiologic variables, age, and chronic health status. A score ≥10 predicts higher mortality and treatment failure, identifying patients who may require more aggressive care.
• SOFA (Sequential Organ Failure Assessment) Score: Scores six organ systems (respiratory, coagulation, liver, cardiovascular, CNS, renal) on a scale of 0–4. An acute increase of ≥2 points from baseline is a key component of the Sepsis-3 definition.

C. Risk Stratification and Management Algorithm

A systematic approach integrates clinical assessment, imaging, and scoring to guide timely interventions.

Key Principles of Management

• Early and effective source control (e.g., percutaneous drainage or surgery) combined with prompt antibiotic initiation (ideally within 1 hour of sepsis recognition) are the cornerstones of successful cIAI management and are proven to improve outcomes.
• Empiric antibiotic regimens must be customized based on whether the infection is community- or healthcare-associated, patient-specific risk factors for resistance, and local institutional antibiograms.