Page 439 - Clinical Small Animal Internal Medicine

41 Approach to the Patient with Shock 407

occurring that overlap categories, and it is possible that The classic example of distributive or vasodilatory
VetBooks.ir categorization may change as the underlying pathology shock is anaphylaxis whereby massive vasodilation low­
progresses. Therefore, a singular approach to each spe­
ers blood pressure to such a degree that capillary blood
cific shock category is not possible.
flow decreases. This reduction in capillary blood flow
limits oxygen delivery to the tissues and results in tissue
oxygen debt. In addition to vasodilation, an increase in
Hypovolemic Shock
the permeability of the capillaries results in a higher than
Hypovolemic shock occurs when there is an absolute or normal net movement of water from the intravascular
perceived decrease in the intravascular volume. This is space to the interstitial space, further decreasing intra­
the classic example of shock and is arguably the most vascular volume and worsening hypotension.
common cause of shock in veterinary patients. Neurogenic shock is an uncommon manifestation of
Hypovolemic shock can result from a loss of whole blood, CNS injury specifically in the brain, cervical or thoracic
loss of sodium‐rich fluid (plasma) or in severe cases lack spinal cord and results from a decrease in sympathetic
of adequate water intake. Chronic endocrine diseases can tone leading to significant loss of systemic vascular
occasionally present as a unique subset of hypovolemic resistance. The presumed mechanism behind this rapid
shock. As an example, a patient in a hypoadrenicortical vasodilation is disruption of the autonomic paths in the
(Addisonian) crisis may be hypovolemic but may not spinal cord and is different from “spinal shock,” which
exhibit the typical clinical signs of shock. refers to the temporary loss of function of the spinal cord
Hemorrhagic shock is the classic example of hypov­ after spinal cord trauma.
olemic shock. A distinguishing characteristic that sets
hemorrhagic shock apart from other causes of hypov­
olemic shock is the absolute loss of whole blood, which Obstructive Shock
can occur as overt blood loss from the laceration of a ves­ Obstructive shock results from impairment of blood
sel or covert blood loss resulting from cavitary bleeding. return to the heart. It is typically associated with intraab­
A complicating component of hemorrhagic shock is the dominal disease that sequesters blood within the abdo­
necessary loss of red blood cell mass concurrent with the men and decreases preload (e.g., gastric dilation and
loss of intravascular fluid. Patients suffering from hem­ volvulus [GDV]). Some argue that pericardial effusion
orrhagic shock may only have shock completely amelio­ with cardiac tamponade is a form of obstructive shock.
rated by the administration of both IV fluids and Similar to GDV, during pericardial tamponade the intra­
hemoglobin‐containing products. pericardial pressure is higher than the venous pressure
and results in decreased preload of the right atrium.
Cardiogenic Shock
Cardiogenic shock results from an inability of the heart Septic Shock
to move blood into the pulmonary or systemic circula­
tion. This is classically imagined to be a result of “for­ Septic shock can be considered a form of distributive
ward failure” of the heart due to the heart being unable to shock but has enough unique characteristics that it is
maintain forward blood flow. As in the case of congestive often classified as a separate entity. As a result of infec­
heart failure, this may occur in the face of normal or tion and systemic inflammation, fluid is lost to the
increased preload. Due to the unique etiology and treat­ interstitium, water intake is decreased, systemic vascu­
ment approach to cardiogenic shock, a closer examina­ lar resistance decreases, and myocardial performance is
tion will be made in Chapter 42. impaired. The therapy for septic shock must address
each of these in turn as well as the underlying cause of
sepsis. Septic shock will be investigated in detail in
Distributive Shock Chapter 43.
Distributive shock describes shock that occurs primarily
due to loss of systemic vascular resistance (i.e., blood
pressure). This is most often a result of inappropriate Oxygen Delivery
vasodilation resulting in decreased venous flow and thus
decreased preload. Recall that in the absence of signifi­ At its most basic, there are two primary components of
cant heart disease, preload is positively correlated with oxygen delivery: cardiac output (CO) and arterial oxygen
cardiac output. Besides the decrease in cardiac output content (CaO 2 ). Therefore, a useful way to approach the
that occurs in distributive shock, regional perfusion is treatment of shock is to separate the components of the
disrupted with loss of blood pressure. cardiovascular system into the individual determinants