Defining Systemic Inflammatory Response Syndrome (SIRS) and Sepsis Criteria

The body’s response to inflammation and infection has been characterized by the American College of Chest Physicians and Society of Critical Care Medicine. These organizations defined a four step pathway to determine when a body is responding to inflammation and when that inflammation is caused by an infectious organism. This pathway consists of the Systemic Inflammatory Response System (SIRS), Sepsis, Severe Sepsis, and Septic Shock. This post will explain what these terms mean and how to diagnose them.

  

Systemic inflammatory response syndrome (SIRS) is the term used to describe the physiologic response to inflammation. This inflammation can occur from any source, and does not have to be from an infection. The criteria are listed below and at least two must be present to diagnose SIRS.

Systemic Inflammatory Response Syndrome (SIRS) Criteria:

- Temperature >38⁰C (100.4⁰F) or <36⁰C (96.8⁰F)

- Heart Rate >90

- Respiratory Rate >20 or PaCO₂ <32

- White Blood Cell (WBC) Count >12,000 or <4,000 or >10% bands

*PaCO₂ is measured by an arterial blood gas. Bands refer to immature neutrophils.

Sepsis is diagnosed when a patient meets SIRS criteria, and has a documented or suspected source of infection. Infection means inflammation caused by microorganisms. These microorganisms include bacteria, fungi, viruses, and parasites. Accepted ways to document the presence of an infection are by blood culture, urine culture, sputum culture, imaging showing pneumonia or a perforated viscous, and by the existence of WBCs in normally sterile fluid.

Severe sepsis is diagnosed when a patient meets sepsis criteria, and has signs of end organ damage or hypoperfusion (decreased blood flow). Signs of end organ damage include PaO₂/FiO₂ <300 indicating acute lung injury (<200 indicates ARDS), lack of bowel sounds indicating ileus, decreased urine output (oliguria), increased total bilirubin, and low platelets (thrombocytopenia). Signs of hypoperfusion include an increased lactic acid, low blood pressure, decreased capillary refill, and change in mental status.

Septic shock is diagnosed when a patient meets severe sepsis criteria, and has sepsis-induced hypotension that is refractory to fluid resuscitation. Hypotension is defined as a systolic blood pressure < 90, a mean arterial pressure < 60, or a decrease in systolic blood pressure > 40. A patient is considered to have not responded to fluid resuscitation after 20ml/kg of crystalloid solution has been administered.

The figure below summarizes the sepsis pathway and highlights the fact that it is a progressive diagnosis, with each new step including the criteria of the one behind it. SIRS and sepsis is an important diagnosis to keep in mind with each new patient. If these criteria are applied the diagnosis will be harder to miss.

Sources:

Bone, R., R. Balk, F. Cerra, R. Dellinger, A. Fein, W. Knaus, R. Schein, and W. Sibbald. "Definitions for Sepsis and Organ Failure and Guidelines for the Use of Innovative Therapies in Sepsis. The ACCP/SCCM Consensus Conference Committee. American College of Chest Physicians/Society of Critical Care Medicine." Chest 101.6 (1992): 1644-655. Web. 17 Feb. 2012. <http://chestjournal.chestpubs.org/content/101/6/1644.full.pdf+html>.

Dellinger, R. Phillip, Mitchell M. Levy, Jean M. Carlet, et al. "Surviving Sepsis Campaign: International Guidelines for Management of Severe Sepsis and Septic Shock: 2008." Critical Care Medicine 36.1 (2008): 296-327. Web. 17 Feb. 2012. <http://www.survivingsepsis.org/About_the_Campaign/Documents/Final%2008%20SSC%20Guidelines.pdf>.

Levy, Mitchell M., Mitchell P. Fink, John C. Marshall, Edward Abraham, Derek Angus, Deborah Cook, Jonathan Cohen, Steven M. Opal, Jean-Louis Vincent, and Graham Ramsay. "2001 SCCM/ESICM/ACCP/ATS/SIS International Sepsis Definitions Conference." Critical Care Medicine 31.4 (2003): 1250-256. Web. 17 Feb. 2012. <http://www.springerlink.com/content/7wbtu6v6ly8nvhmk/>.

Loma Linda University. "The STOP Sepsis Bundle Toolkit." CrashingPatient.com. Loma Linda University Medical Center, Sept. 2005. Web. 17 Feb. 2012. <http://crashingpatient.com/wp-content/pdf/Loma%20Linda%20STOP%20Sepsis%20Bundle.pdf>.