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Shock Etiology and Classification

Updated: Wednesday, Mar 18,2009, 10:19:17 PM
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Etiology and Classification 
There are several mechanisms of organ hypoperfusion and shock. Shock may be due to low circulating volume (hypovolemic shock), vasodilation (distributive shock), primary decrease in cardiac output (both cardiogenic and obstructive shock), or a combination.
Hypovolemic shock: Hypovolemic shock is caused by a critical decrease in intravascular volume. Diminished venous return (preload) results in decreased ventricular filling and reduced stroke volume. Unless compensated for by increased heart rate, cardiac output decreases.

A common cause is bleeding (hemorrhagic shock), typically from trauma, surgical interventions, peptic ulcer, esophageal varices, or aortic aneurysm. Bleeding may be overt (eg, hematemesis or melena) or concealed (eg, ruptured ectopic pregnancy).
Hypovolemic shock may also follow increased losses of body fluids other than blood (see Table 1: Shock and Fluid Resuscitation: Hypovolemic Shock Caused by Body Fluid Loss).
Hypovolemic shock may be due to inadequate fluid intake (with or without increased fluid loss). Water may be unavailable, neurologic disability may impair the thirst mechanism, or physical disability may impair access.
In hospitalized patients, hypovolemia can be compounded if early signs of circulatory insufficiency are incorrectly ascribed to heart failure and fluids are withheld or diuretics are given.

Distributive shock: Distributive shock results from a relative inadequacy of intravascular volume caused by arterial or venous vasodilation; circulating blood volume is normal. In some cases, cardiac output (and DO2) is high, but increased blood flow through arteriovenous shunts bypasses capillary beds, causing cellular hypoperfusion (demonstrated by decreased O2 consumption). In other situations, blood pools in venous capacitance beds and cardiac output falls.

Distributive shock may be caused by anaphylaxis (anaphylactic shock); bacterial infection with endotoxin release (septic shock); severe injury to the brain or spinal cord (neurogenic shock); and ingestion of certain drugs or poisons, such as nitrates, opioids, and adrenergic blockers. Anaphylactic shock and septic shock often have a component of hypovolemia as well.
Cardiogenic and obstructive shock: Cardiogenic shock is a relative or absolute reduction in cardiac output due to a primary cardiac disorder. Mechanical factors that interfere with filling or emptying of the heart or great vessels explain obstructive shock. Causes are listed in Table 2: Shock and Fluid Resuscitation: Mechanisms of Cardiogenic and Obstructive Shock.

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