What is Systemic Inflammatory Response Syndrome (SIRS)?

Systemic Inflammatory Response Syndrome (SIRS)

Systemic Inflammatory Response Syndrome (SIRS) is defined as a clinical manifestation of overexuberant acute-phase inflammation 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, 2

Diagnostic Criteria

SIRS is diagnosed when a patient presents with at least two of the following:

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

Pathophysiology

SIRS represents a dysregulated host response characterized by:

• Release of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-8) 2
• Activation of neuroendocrine responses (increased cortisol, adrenaline, glucagon)
• Hematopoietic changes (anemia, leukocytosis, thrombocytosis)
• Metabolic alterations (protein catabolism, negative nitrogen balance, increased lipolysis)
• Hepatic changes (increased blood flow, increased acute phase protein production) 1

Etiology

SIRS can be triggered by:

• Infectious causes: Bacterial, viral, fungal, and parasitic infections
• Non-infectious causes:
  • Tissue injury: Surgery, trauma, burns, hematoma
  • Vascular events: Venous thrombosis, myocardial or pulmonary infarction
  • Therapy-related: Blood products, anesthetic reactions, neuroleptic malignant syndrome
  • Other conditions: Pancreatitis, transplant rejection, adrenal insufficiency, thyroid storm 2

Clinical Significance

SIRS is clinically important because:

1. It represents an early warning sign of potential serious illness
2. The number of SIRS criteria present correlates with mortality risk 2
3. It can progress to more severe conditions:
   • Severe sepsis (when caused by infection with organ dysfunction)
   • Septic shock (sepsis with hypotension despite adequate fluid resuscitation)
   • Multiple Organ Dysfunction Syndrome (MODS)
   • Acute Respiratory Distress Syndrome (ARDS)
   • Disseminated Intravascular Coagulation (DIC) 1, 2

Monitoring and Management

For patients with SIRS, key management steps include:

• Regular monitoring of vital signs and organ function
• Assessment for potential sources of infection
• Early administration of broad-spectrum antibiotics (within 1 hour) if infection is suspected
• Fluid resuscitation with crystalloids (preferably Ringer's lactate) targeting near-zero fluid balance
• Source control if infection is identified (drainage of abscesses, debridement of infected tissue)
• Supportive care for affected organ systems 2

Special Considerations

• Elderly and immunocompromised patients may not exhibit typical SIRS responses despite serious infection 2
• In surgical patients, SIRS scores typically decrease after 24 hours of ICU resuscitation; persistent or worsening SIRS after this period correlates with higher mortality 3
• The magnitude of protein catabolism after surgery corresponds to the degree of systemic inflammatory response and impacts surgical outcomes 1
• C-reactive protein (CRP) is a useful marker to quantify the magnitude of the systemic inflammatory response 1

Pitfalls and Caveats

• SIRS criteria are highly sensitive but lack specificity
• The presence of SIRS does not necessarily indicate infection
• SIRS criteria may be affected by medications (beta-blockers can mask tachycardia)
• In surgical patients, SIRS may be overestimated in the first 24 hours due to effects of surgery, anesthesia, and initial resuscitation 3
• Patients with liver disease may have baseline abnormalities in temperature regulation and white blood cell counts that can confound SIRS criteria 2

SIRS remains a clinically relevant concept for early identification of patients at risk for adverse outcomes, though its definition has evolved over time as our understanding of the inflammatory response has improved.