We are all familiar with the localized skin infection. The presence of redness, increased warmth, swelling (edema) and pain are signals the immune system is at work.
Local immune response
A localized immune system response results in vasodilation to increase the blood supply to the area affected and causes the warmth we feel and the redness we see. The increased blood supply carries certain immune responders that are attracted to the insult area such as white blood cells that directly kill pathogens such as bacteria, viruses, parasites and fungi, or destroy pathogen infected cells.
Macrocytes (cell eaters) are large white bloods cells that are invited to the party to help clean up the mess.1 Capillaries become leaky to increase the delivery of antibodies and immune cells to the affected tissues thus producing the edema we can see and feel. The pain and tenderness are the result of the swelling and the release of chemicals that stimulate local pain nerves.2
Systemic immune response
The immune system is in the protection business. The complex reaction to invasion or injury depends on a variety of immune defenses that are coordinated by immune system messengers (cytokines). Sometimes this response goes global and becomes a systemic inflammatory response. Think about the last time you had the flu or some other irritating virus. Most likely you had a fever, tachycardia, body aches, nausea and, although you couldn’t feel it, your white blood cell count was likely a little higher than normal. These are all signs that your personal immune system is engaged in a large scale assault on the invaders.
Systemic Inflammatory Response Syndrome
But not all systemic immune responses are due to infection. The Systemic Inflammatory Response Syndrome or SIRS is the body’s system wide response to a non-infectious insult or stressor, for example trauma, pancreatitis, or surgery. Criteria for SIRS are met if two or more of the following four clinical findings are present3:
- Temperature greater than 38°C (100.4°F) or less than 36°C (96.8°F)
- Heart rate (HR) greater than 90 beats per minute (bpm)
- Respiratory rate (RR) greater than 20 breaths per minute or arterial carbon dioxide tension (PaCO2) lower than 32 mm Hg
- White blood cell (WBC) count higher than 12,000/µL or lower than 4000/µL, or 10% immature (band) forms
SIRS gone wild
Regardless of the SIRS trigger, benefit can become detriment if the immune response cascades out of control and damages other organs that are not involved in the original insult or injury. For example, a patient with pancreatitis may experience a systemic immune response (SIRS) to the inflamed pancreas, which hopefully will resolve along with the pancreatitis. However, if this dysfunctional immune system response continues to amplify, it may result in damage to other body systems such as the kidneys, lungs, brain or heart. Damage to more than one organ system is called Mutiple Organ Dysfunction Syndrome or MODS. In such cases, the immune system is a life threatening disease.
Sepsis
By definition, sepsis is an infection that causes a systemic inflammatory response; sepsis = infection + SIRS. Infection is the invasion of normally sterile tissue, fluid or body cavity by a pathogen.3 The patient you are transporting with a productive cough, temperature of 102 degrees, heart rate of 110 and breathing 22 times a minute has sepsis. By the way, be sure to wear your gloves and face mask, you don’t want what she’s got.
Severe sepsis
Sepsis becomes severe if it causes organ dysfunction and inadequate perfusion due to the pathogens themselves, and from the uncontrolled inflammatory response.4 So, if your pneumonia patient from above also has mottled skin, delayed capillary refill and/or an altered mental status, then she has severe sepsis. Other signs of severe sepsis include decreased urinary output, low platelet count or worsening lung function.4 Early antibiotics and aggressive fluid administration is essential to maintaining or restoring adequate perfusion and preventing progression to septic shock.
Septic shock
If your pneumonia patient with severe sepsis receives from 30 to 60 ml/kg of intravenous normal saline4,5 (approximately 2 to 4 liters for the average 70 kg patient) and remains hypotensive and underperfused, then she has graduated to septic shock and will require a vasopressor (dopamine, epinephrine, norepinephrine) to obtain and maintain an adequate perfusion pressure. Her chance of survival decreases as she progresses from sepsis to severe sepsis to septic shock. The risk of death with septic shock can be as high as 40 to 50%.5
Summary
Any illness or injury complicated by SIRS has increased severity and increased opportunity for an adverse outcome. Fortunately, the ability to understand and modify the immune system response continues to evolve in our favor.4 The immune system’s duty is to defend us from outside threats and insure our survival. When the immune system turns traitor, that survival is at risk.
References:
1. Understanding The Immune System, How It Works. http://www.niaid.nih.gov/topics/immunesystem/documents/theimmunesystem.pdf
2. King, D. Inflammation. Available at http://www.siumed.edu/~dking2/intro/inflam.htm#3.2 . Accessed October 10, 2011
3. Levy MM, Fink MP, Marshall JC, Abraham E, Angus D, Cook D, Cohen J. Opal SM, Vincent J, Ramsay G. 2001 SCCM/ESICM/ACCP/ATS/SIS International Sepsis Definitions Conference. Crit Care Med 2003; 31:1250-1256.
4. Annane D, Bellissant E, Cavaillon JM. Septic Shock. Lancet 2005: 364:63-78.
5. Marik PE, Lipman J. The Definition of Septic Shock: Implications for Treatment. Crit Care Resusc 2007: 9(1):101-103
