Definition of shock: Shock is failure of the circulatory system to adequately perfuse vital organs.

In shock, tissue perfusion and oxygen delivery are insufficient to meet the basal metabolic demands of tissues. Whatever the inciting cause, shock is characterized by low blood flow that is usually accompanied by low blood pressure (hypotension). Situations of inadequate tissue perfusion result in a variety of adjustments to the circulatory system that are directed toward maintaining normal arterial blood pressure and conserving body water.

These compensatory mechanisms may maintain viability of vital organs and sustain the life of the patient, a situation termed "compensated shock". When the adjustments fail to reestablish and maintain perfusion sufficient to sustain vital tissues, a condition of "uncompensated shock" ensues where progressive circulatory collapse leads to increasingly severe disruption of critical cellular metabolic pathways and death.

Clinically, the primary goal of therapy for shock is the rapid restoration of systemic blood flow by replacement of intravascular fluid and the use of drugs that increase vascular tone and support cardiac function.

Causes, Types, and Pathogeneses of Shock

Shock is initiated by anything that severely and usually relatively suddenly decreases cardiac output, blood volume, and/or peripheral vascular resistance. The types of shock can be classified on the basis of the primary general cause:

Cardiogenic shock is caused by insults that negatively affect cardiac output (inhibit the heart’s ability to pump blood). Cardiac output = heart rate x stroke volume, so anything affecting heart rate or contractility can decrease cardiac output.

This is a picture of lymphosarcoma in a cow heart, so heck, this heart isn't working so well anymore. With all these tumor blobs, the heart can't function so good, and there is poor cardiac output.

Myocardial degeneration such as might occur with vitamin E/selenium deficiency in pigs (mulberry heart disease), monensin toxicity in horses, or myocardial infarcts (which occur much more commonly in humans than in domestic animals), can cause heart failure and cardiogenic shock.

In this photograph, a big pale-colored infarct is outlined by arrows. Infarcts are areas where the blood supply has been compromised and we'll go into way more detail very soon in this course about what exactly they are.

Cardiac tamponade, which occurs when fluid (usually blood) accumulates rapidly in the pericardial space and impinges on the ability of the cardiac ventricles to dilate and fill with blood, will cause acute heart failure and resulting cardiogenic shock.

Electrolyte imbalances (such as hyperkalemia in uremic animals) that negatively affect heart rate, can lead to heart failure and cardiogenic shock.

Hypovolemic shock is caused by a sudden severe loss of blood volume. Causes include:

  • acute hemorrhage involving loss of greater than 1/4 to 1/3 of total blood volume. The blood may be lost externally or into internal spaces such as the peritoneal cavity or the alimentary tract.
  • loss of fluid (intravascular and extravascular), which may occur with water deprivation, vomiting, diarrhea, etc.
  • increased vascular permeability leading to loss of intravascular fluid, proteins, and sometimes blood cells. These insults include infections, toxicities, and immune reactions that injure vessels. Specific examples include equine viral arteritis, African swine fever, hog cholera, and the hemorrhagic fevers (e.g., simian hemorrhagic fever, Ebola virus infection)

This horse died of hypovolemic shock because it lost all its blood into the abdominal cavity (hemoperitoneum). Can you see the stream of blood coming out of the small opening in the abdomen as the necropsy is started? The underlying reason was a tumor in the spleen that ruptured.

Distributive (or vasogenic) shock is caused by a sudden severe decrease in peripheral vascular resistance that causes extensive pooling of blood within the venous system and subsequent decreased venous return to the heart.

Severe colonic edema in a horse with endotoxic shock. Much of the fluid formerly within the vascular system is now pooled within the colonic tissues, leading to functional hypovolemia characteristic of distributive shock. So it's a two-fer (two fer one, as in two different reasons for the horse going into shock and dying).

There are some important examples of distributive shock including

Septic shock (endotoxic shock, toxic shock) results from a bacterial infection (localized or systemic) in which large quantities of endotoxin are released into circulation. Endotoxins are complex components of the cell wall of gram-negative bacteria and then can really do a number on the body, and especially on the vasculature.

Endotoxin is bad news for endothelium.

Endotoxin is sometimes referred to as "lipopolysaccharide" because the most toxic part of the complex is this type of molecule. Less commonly, other types of toxins from gram-positive bacteria can cause shock by a similar pathogenesis. We'll be covering sepsis and endotoxin and cytokines and all that other bad stuff in much more detail as we go through this course. May as well learn this diagram now and make your life later this semester easier? Just a suggestion.....

Anaphylactic shock is a systemic manifestation of an acute hypersensitivity (allergic) response. This is an idiosyncratic reaction that occurs in certain predisposed individuals upon exposure to certain antigens (substances, usually proteins, to which the individual is allergic) such as insect stings, foods, medicines, etc.

We will discuss hypersensitivity reactions in more detail in another section of this course, but for the present discussion just remember that upon exposure to these allergens, histamine and other chemicals are released from cells such as mast cells. Histamine and the other substances bind to receptors, causing vasodilation and increased vascular permeability with loss of intravascular fluid.

Neurogenic shock can occur following severe trauma, severe pain, or occasionally following restraint. The pathogenesis involves massive autonomic nervous system discharge which disrupts vasomotor control and there is extensive venocapillary pooling of blood. Which is bad. When all of the blood is in the capillaries, it doesn't get moved around to vital organs.


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