What is Systemic Inflammatory Response Syndrome (SIRS)?

Systemic Inflammatory Response Syndrome (SIRS)

Systemic Inflammatory Response Syndrome (SIRS) is defined clinically by the presence of at least two of the following criteria: temperature >38°C or <36°C, heart rate >90 beats/min, respiratory rate >20 breaths/min or PaCO₂ <32 mmHg, and white blood cell count >12,000/mm³ or <4,000/mm³ or >10% immature (band) forms. 1

Definition and Clinical Criteria

• SIRS represents a clinical expression of the acute phase reaction triggered by various events including infection, trauma, surgery, and other injuries 1, 2
• SIRS is diagnosed solely based on clinical parameters and basic laboratory values that can be obtained at the bedside, without requiring pathological specimens or cultures 1
• The presence of SIRS should prompt a thorough search for the underlying cause rather than being considered a final diagnosis 1

Pathophysiology

• SIRS is mediated by cytokines that induce catabolism of glycogen, fat, and protein stores, leading to release of glucose, free fatty acids, and amino acids into circulation to support tissue healing 3
• The syndrome involves both pro-inflammatory and anti-inflammatory components, not just inflammation as the name suggests 4
• Multiple pathophysiological changes occur during SIRS, including:
  • Neuroendocrine changes: fever, somnolence, fatigue, anorexia, increased adrenal secretion of cortisol, adrenaline, and glucagon 3
  • Hematopoietic changes: anemia, leukocytosis, thrombocytosis 3
  • Metabolic changes: loss of muscle mass, negative nitrogen balance, increased lipolysis, trace metal sequestration, diuresis 3
  • Hepatic changes: increased blood flow, increased acute phase protein production 3

Causes and Triggers

• Infectious causes: sepsis, pneumonia, urinary tract infections 3
• Tissue injury: surgery, trauma, hematoma, venous thrombosis 3
• Cardiovascular events: myocardial infarction, pulmonary infarction 3
• Other medical conditions:
  • Pancreatitis 3, 5
  • Transplant rejection 3
  • Erythroderma 3
  • Subarachnoid hemorrhage 3
  • Chronic salicylate intoxication 6
• Therapy-related causes:
  • Blood products 3
  • Cytokines (especially granulocyte-macrophage colony stimulating factor) 3
  • Anesthetic-related malignant hyperpyrexia (especially halothane) 3
  • Neuroleptic malignant syndrome (e.g., caused by haloperidol) 3
  • Opiates/benzodiazepines 3

Differentiation from Related Conditions

• SIRS differs from sepsis, which requires confirmation of infection plus SIRS criteria 1
• Septic shock is defined as sepsis with circulatory and cellular/metabolic abnormalities requiring vasopressors to maintain mean arterial pressure ≥65 mmHg and lactate >2 mmol/L 1
• Hemorrhagic shock results from inadequate tissue perfusion due to blood loss and requires confirmation of blood loss source 1

Clinical Significance and Complications

• SIRS can lead to multiple organ dysfunction syndrome (MODS) if severe or prolonged 5
• The magnitude of SIRS corresponds to the degree of surgical trauma and poorer surgical outcomes 3
• C-reactive protein (CRP) is the prototypical marker of systemic inflammatory response and can be used to monitor its magnitude 3
• SIRS is strongly associated with infection development, multi-organ failure, and high mortality in conditions like alcoholic hepatitis 3
• Compartmentalization occurs in SIRS, where pathophysiological events differ from organ to organ and from organ to peripheral blood 7

Management Considerations

• Early identification of SIRS is crucial to determine the underlying cause and initiate appropriate treatment before progression to more severe forms 5
• Management should focus on treating the underlying cause while providing supportive care 3, 1
• In surgical patients, maintaining near-zero fluid and electrolyte balance reduces complications by 59% and hospital stay by 3.4 days 3
• Multidisciplinary care involving specialists in hepatology, critical care, infectious disease, and nephrology may be required in complex cases 3

Monitoring and Biomarkers

• Frequent neurological assessment with tools such as the Glasgow Coma Scale or National Institutes of Health Stroke Scale is recommended in conditions like aneurysmal subarachnoid hemorrhage with SIRS 3
• Frequent vital sign monitoring is essential for detection of neurological changes and prevention of secondary complications 3
• Infection-related biomarkers like serum lipopolysaccharide, bacterial DNA, high-sensitivity C-reactive protein, and procalcitonin can help assess risk of infection and mortality in conditions complicated by SIRS 3