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Septic Shock
James W. Barr, DVM, DACVECC

BluePearl Veterinary Partners, Tampa, FL, USA

Etiology/Pathophysiology with SIRS while SIRS can exist without the presence of
sepsis. As is the case with most aspects of veterinary
The ability of an organism to mount an inflammatory medicine, sepsis/SIRS is not an all‐or‐nothing condi-
response is necessary for the survival of nearly all life tion but rather occurs along a spectrum, the least severe
forms. Through the simple interaction of the body with form of which is a hemodynamically stable patient that
its internal and external environments, a constant bom- fulfills the clinical criteria. Severe sepsis is defined as
bardment with infectious agents and minor injuries sepsis with acute dysfunction of one or more organs.
occurs that is prevented from progressing to systemic Common examples of secondary organ dysfunction
disease by the innate immune system. This process is include the development of a coagulopathy or acute
performed successfully on a continuous basis without kidney injury. The worst permutation of the septic
any conscious effort on the part of the organism or need spectrum is septic shock in which a patient not only ful-
for prior exposure to offending antigens. fills criteria for severe sepsis but also has hypotension
Unfortunately, these same mechanisms, which are and signs of shock that are not reversed with fluid ther-
protective when functioning normally, can become apy and is consequently vasopressor dependent.
unregulated or overactive, resulting in added pathology,
perhaps even worse than the original insult would have Response to Infection or Injury
caused. This idea is reinforced by the fact that patients
often die of sequelae to infections or systemic inflamma- Innate Immunity
tion rather than directly from the primary insult. The immune system has evolved into two complemen-
The systemic inflammatory response syndrome (SIRS) tary systems that work together to fight infection and
is the systemic manifestation of a response to an insult mitigate the damage that occurs in the host: the innate
and may result from anything creating inflammation. immune system and the adaptive immune system. For
Importantly, this can be the result of a systemic condi- convenience, they are often discussed separately, but it
tion (e.g., heat stroke) or a severe local insult (e.g., pan- should be remembered that neither branch works in iso-
creatitis). While systemic inflammation may be necessary lation from the other and that cross‐talk and feedback
to combat the initial insult, when left unregulated it can are constantly occurring between the two branches.
lead to severe consequences. The presence of certain The innate immune system is the portion of the
clinical findings is used to define SIRS and sepsis in both immune system that is nonspecific and responds to any
veterinary and human medicine. Definitions for SIRS insult that may be present without need for preexposure
have been proposed and are currently being used clini- while the adaptive arm of immunity is a more specialized
cally (Table 43.1). system that requires preexposure to an organism or anti-
Since there is little difference between the clinical gen. The adaptive response can have infinite, very spe-
appearance of SIRS and uncomplicated sepsis, the two cific targets but unfortunately, the delay necessary to
terms are often used interchangeably. When SIRS recognize an antigen and synthesize antibodies in order
occurs due to an infectious cause, the term used to to mount a response makes adaptive immunity unsuita-
describe it is sepsis. As a result of these definitions, ble as a primary means of defending the host from an
it means that sepsis always occurs in conjunction acute injury or novel infection.

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical