What is a systemic inflammatory response (SIRS)?

Systemic Inflammatory Response Syndrome (SIRS)

Systemic Inflammatory Response Syndrome (SIRS) is defined as a dysregulated host response characterized by the presence of at least two of the following criteria: temperature >38°C or <36°C, heart rate >90 beats/minute, respiratory rate >20 breaths/minute or PaCO₂ <32 mmHg, and white blood cell count >12,000/μL or <4,000/μL or >10% immature forms. 1

Definition and Diagnostic Criteria

SIRS represents a clinical manifestation of the body's response to an insult, which can be either infectious or non-infectious in origin. The diagnostic criteria include:

SIRS Criteria	Value
Temperature	>38°C (100.4°F) or <36°C (96.8°F)
Heart rate	>90 beats/minute
Respiratory rate	>20 breaths/minute or PaCO₂ <32 mmHg
White blood cell count	>12,000/μL or <4,000/μL or >10% immature (band) forms

2, 1

Pathophysiology

SIRS involves complex pathophysiological processes:

• Cytokine Release: Proinflammatory cytokines (IL-1, IL-6, TNF-α) are released in response to tissue injury or infection 2
• Immune Activation: Activation of neutrophils, macrophages, and natural killer cells leads to an upregulated innate immune response 2
• Metabolic Changes: Includes:
  • Loss of muscle mass and negative nitrogen balance
  • Increased lipolysis
  • Trace metal sequestration
  • Diuresis 2
• Neuroendocrine Alterations: Increased adrenal secretion of cortisol, adrenaline, and glucagon 2
• Hematologic Changes: Anemia, leukocytosis, and thrombocytosis 2
• Hepatic Changes: Increased blood flow and production of acute phase proteins (e.g., C-reactive protein) 2

Triggers and Causes

SIRS can be precipitated by various conditions:

• Infectious causes: Bacterial, viral, fungal, and parasitic infections (when SIRS is due to infection, it's termed sepsis) 1
• Surgical trauma: The "trauma of surgery" leads to release of stress hormones and inflammatory mediators 2
• Cardiopulmonary bypass: During cardiac surgery, surgical trauma, blood contact with non-physiological surfaces, myocardial ischemia/reperfusion, and hypothermia combine to cause dramatic release of inflammatory mediators 2
• Tissue injury: From trauma, burns, pancreatitis, myocardial or pulmonary infarction 1
• Other causes: Hematoma, venous thrombosis 1

Clinical Significance and Progression

The severity of SIRS correlates with clinical outcomes:

• SIRS can progress to more severe conditions including:

  • Severe sepsis (when infection is the cause)
  • Septic shock
  • Multiple Organ Dysfunction Syndrome (MODS)
  • Acute Respiratory Distress Syndrome (ARDS)
  • Disseminated Intravascular Coagulation (DIC) 1

• The magnitude of the systemic inflammatory response corresponds to surgical outcomes - higher responses correlate with poorer outcomes 2

• C-reactive protein (CRP) is a valuable marker for quantifying the magnitude of the postoperative systemic inflammatory response 2

Management Considerations

Management of SIRS focuses on:

1. Identifying and treating the underlying cause:

   • For infectious causes: Empiric broad-spectrum antibiotics within 1 hour of recognition 1
   • For surgical patients: Source control (drainage of abscesses, debridement of infected/necrotic tissue) 1

2. Supportive care:

   • Fluid resuscitation: Initial bolus of 30 mL/kg crystalloids for hypotension or elevated lactate 1
   • Monitoring: Track vital signs, urine output (target >0.5 mL/kg/hr), WBC count, lactate, and organ function tests 1

3. Prevention strategies:

   • Minimizing surgical trauma (e.g., using laparoscopic approaches when possible) 2
   • Appropriate perioperative nutrition 2, 1
   • Maintaining near-zero fluid balance during major procedures 2, 1
   • Specialized CPB circuits for cardiac surgery patients 1

Clinical Importance

Understanding SIRS is critical because:

• Early recognition allows for prompt intervention before progression to more severe forms 1
• The presence of SIRS criteria helps identify patients at risk for poor outcomes 2, 1
• SIRS represents an important therapeutic target for interventions aimed at modulating the inflammatory response 2

Common Pitfalls

• Overlooking non-infectious causes: While infection is a common trigger, many non-infectious conditions can cause SIRS 2, 1
• Unnecessary antibiotic use: In non-infectious SIRS, sustained systemic antimicrobial prophylaxis should be avoided to prevent antimicrobial resistance 2
• Focusing only on inflammation: Although named "inflammatory response syndrome," SIRS actually has both pro- and anti-inflammatory components that need consideration in management 1
• Delayed recognition: Failure to identify SIRS early may lead to delayed interventions and worse outcomes 1