In 1992, the American College of Chest Physicians (ACCP) and the Society of Critical Care Medicine (SCCM) introduced definitions for systemic inflammatory response syndrome (SIRS), sepsis, severe sepsis, septic shock, and multiple organ dysfunction syndrome (MODS). The idea behind defining SIRS was to define a clinical response to a nonspecific insult of either infectious or noninfectious origin. SIRS is defined as 2 or more of the following variables:
- Fever of more than 38°C or less than 36°C
- Heart rate of more than 90 beats per minute
- Respiratory rate of more than 20 breaths per minute or a PaCO2 level of less than 32 mm Hg
- Abnormal white blood cell count (>12,000/µL or < 4,000/µL or >10% bands)
SIRS is nonspecific and can be caused by ischemia, inflammation, trauma, infection, or a combination of several insults. SIRS is not always related to infection. Infection is defined as "a microbial phenomenon characterized by an inflammatory response to the microorganisms or the invasion of normally sterile tissue by those organisms."
Fig: Venn diagram showing overlap of infection, bacteremia, sepsis, systemic inflammatory response syndrome (SIRS), and multiorgan dysfunction.
Bacteremia is the presence of bacteria within the blood stream, but this condition does not always lead to SIRS or sepsis. Sepsis is the systemic response to infection and is defined as the presence of SIRS in addition to a documented or presumed infection. Severe sepsis meets the afore mentioned criteria and is associated with organ dysfunction, hypoperfusion, or hypotension. Sepsis-induced hypotension is defined as "the presence of a systolic blood pressure of less than 90 mm Hg or a reduction of more than 40 mm Hg from baseline in the absence of other causes of hypotension." Patients meet the criteria for septic shock if they have persistent hypotension and perfusion abnormalities despite adequate fluid resuscitation. MODS is a state of physiological derangements in which organ function is not capable of maintaining homeostasis.
Although not universally accepted terminology, severe SIRS and SIRS shock are terms that some authors have proposed. These terms suggest organ dysfunction or refractory hypotension related to an ischemic or inflammatory process rather than to an infectious etiology.