Page 570 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

558 FLUID THERAPY

TABLE 23-1 Classification of Shock Syndromes

Classification Subclassification Description Causes
Circulatory Hypovolemic Decreased blood volume Trauma, hemorrhage, dehydration
Cardiogenic Cardiac origin, decreased forward Congestive heart failure, cardiac tamponade,
flow arrhythmias
Distributive Vascular origin, dysfunction that alters Vasodilatation, caval syndrome
delivery
Metabolic Intracellular energy production Hypoglycemia, cyanide toxicity
malfunction
Hypoxic Decreased oxygen content of arterial Anemia, methemoglobinemia, carbon
blood monoxide toxicity

Hypovolemic shock can occur by loss of intravascular common pathophysiologic event and does not necessarily
volume of any etiology, including dehydration, blood end with survival after a simple universal initial treatment.
loss, and third-space loss of fluids. Cardiogenic shock Each cause of shock sets in motion a complex series of
can occur as a result of any cardiac abnormality that events that include neural and hormonal responses, as well
causes pump failure such as heart disease, myocardial as numerous inflammatory cascades.
injury, cardiac tamponade, or arrhythmias. Distributive To confuse matters further, there are several named
shock occurs when cardiac function and blood volume shock syndromes that are commonly described in the vet­
are not affected, but there is failure of the vascular erinary literature that include an etiologic descriptor. One
tree to allow appropriate delivery (either globally or example is anaphylactic shock, which is a form of shock
locally) and can occur through loss of neurohormonal that is triggered by exposure to an offending allergen that
input (e.g., sympathetic trunk transection, relative adre­ triggers significant mast cell degranulation. Release of
nal insufficiency, catecholamine imbalance), inflamma­ numerous chemokines, especially histamine, causes wide­
tory mediator release (e.g., sepsis, endotoxemia, and spread vasodilatation that could be classified within the
anaphylaxis), or interference with blood delivery (e.g., distributive framework discussed above. In addition,
thromboembolism, caval syndrome, gastric dilatation- histamine causes leak of protein and fluid from within
volvulus (GDV) syndrome, acute portal hypertension). the intravascular space, which, if of sufficient magnitude
Hypoxic causes of shock all have a decrease in the oxy­ would be considered hypovolemic shock. Numerous
gen content of the blood in common. Anemia, as well as other clinical designations or descriptors of shock exist
alterations in hemoglobin form or function (e.g., carbon and each have multiple potential pathophysiologic
monoxide toxicity, methemoglobinemia), can all cause classifications that might contribute to inadequate
hypoxic shock. Failure of gas exchange in the lungs can cellular energy production. Additional examples include:
also be a significant cause of hypoxemic shock. septic shock (can include hypoxic, hypovolemic,
Metabolic causes of shock are linked together through cardiogenic, distributive, and metabolic components)
some failure of intracellular energy production despite or neurogenic shock (can include distributive, cardio­
normal oxygen delivery to the cell. Hypoglycemia is genic, or metabolic components).
perhaps the most common cause of this form of shock, Perhaps the most useful feature of these schemes is to
but toxins such as cyanide could also be a cause. In addi­ force the clinician to consider potential dysfunction of
tion, cytokines may play a role in mitochondrial dysfunc­ multiple components of the cardiovascular and intracellu­
tion in patients with sepsis and contribute some part in lar energy production systems and thus consider therapies
failure of adequate intracellular energy production. that support each component of dysfunction.
Although such categorical organization of shock Additional disagreement also centers on the use of the
syndromes is helpful to understand the complex nature term distributive shock. The term distributive has been
of shock, it is crucial to recognize that most clinical forms used to describe various types of high-flow shock under
of shock may encompass several classifications. The utility the assumption that blood flow is not normally
of anatomic or functional categories of shock is question­ distributed to tissue beds. Used in this context, “distrib­
able when viewed from a clinical perspective, and utive shock” is a theoretical designation of a type of shock
some have argued that such a classification is misleading syndrome that is not defined by criteria that can be easily
because clinicians may approach a one-dimensional, measured in a clinical setting. Maldistribution of or
easy-to-understand representation of shock with a simplis­ heterogeneous blood flow has been documented in
tic one-dimensional approach to therapy. 130 Unfortu­ people in clinically accessible microscopic vascular beds
nately, shock is complex; it does not begin with a (e.g., mucous membranes, sclera, liver, nail bed). Direct