Systemic inflammatory response syndrome

SIRS Criteria - The Body's Red Alert

SIRS is diagnosed when ≥2 of the following criteria are met, indicating a widespread inflammatory state.

• Temperature: <36°C (96.8°F) or >38°C (100.4°F)
• Heart Rate: >90 bpm
• Respiratory Rate: >20 breaths/min or PaCO₂ <32 mmHg
• White Blood Cell Count: <4,000/mm³ or >12,000/mm³ or >10% immature bands

📌 Mnemonic: Remember the 4 T's: Temperature, Tachycardia, Tachypnea, Terrific (or Terrible) white count.

⭐ SIRS is a non-specific inflammatory response. It can be triggered by non-infectious conditions like pancreatitis, burns, or trauma. Sepsis is defined as SIRS with a presumed or confirmed source of infection.

Pathophysiology - The Cytokine Storm

• Trigger: An insult (e.g., surgery, trauma, infection) activates the innate immune system, primarily macrophages and endothelial cells.
• Key Mediators: A massive, uncontrolled release of pro-inflammatory cytokines creates the "storm."
  • TNF-α & IL-1: Early, primary mediators causing fever and inflammation.
  • IL-6: Drives the acute-phase response; correlates with morbidity.

⭐ IL-6 is a key driver of the hepatic acute-phase response, stimulating the liver to produce C-reactive protein (CRP), a major inflammatory marker.

Progression - SIRS to Sepsis Spiral

• SIRS → Sepsis: SIRS criteria met due to a suspected or confirmed infection.
• Sepsis → Septic Shock: Sepsis with persistent hypotension requiring vasopressors to maintain MAP ≥ 65 mmHg AND serum lactate > 2 mmol/L despite adequate volume resuscitation.
• Septic Shock → MODS: Development of progressive dysfunction in two or more organ systems.

⭐ The concept of "Severe Sepsis" (sepsis + signs of end-organ damage) is largely replaced by the Sepsis-3 definition, which frames sepsis itself as life-threatening organ dysfunction caused by a dysregulated host response to infection.

Management - Taming the Inferno

• Initial Resuscitation (The "Sepsis Six" within 1 hour):
  • High-flow oxygen to maintain SpO₂ >94%.
  • IV fluid challenge: 30 mL/kg crystalloid for hypotension.
  • IV broad-spectrum antibiotics after blood cultures.
  • Measure serum lactate.
  • Monitor urine output.
• Source Control: The definitive treatment. Drain abscesses, debride tissue, or remove infected hardware.
• Hemodynamic Support:
  • Vasopressors (Norepinephrine first-line) if MAP remains <65 mmHg after fluids.

⭐ Refractory Septic Shock: For patients unresponsive to fluids and vasopressors, consider IV corticosteroids (e.g., hydrocortisone 200 mg/day).

• SIRS is a widespread inflammatory response triggered by infectious (sepsis) and non-infectious causes like trauma, burns, or pancreatitis.
• Diagnosis requires ≥2 criteria: abnormal temperature (<36°C or >38°C), heart rate (>90/min), respiratory rate (>20/min), or WBC count (<4k, >12k, or >10% bands).
• Sepsis is distinguished from SIRS by the presence of a documented or suspected infection.
• Unchecked, it can progress to septic shock and Multiple Organ Dysfunction Syndrome (MODS).