Anaphylactic Shock: Pathophysiology and Manifestations

After going through what hypovolemic shock is, we’ll go into anaphylactic shock – definition, pathophysiology, pharmacology, and manifestations.

So, let’s do it.

Volume and Resistance

Before going through what anaphylactic shock is, let’s get into the essential factors that influence the different types of shock. With shock, there are only two elements that are affected – volume and resistance.


With a hypovolemic shock, the volume is affected due to rapid blood and fluid loss from the body, which then leads to a decrease in blood pressure, resulting in low oxygen. Pressure is equivalent to blood volume which is why, when a person is bleeding out due to trauma, the body will eventually go into hypovolemic shock if no intervention took place.

If there is decreased blood volume, the body will compensate by increasing resistance through vasoconstriction; shunting blood to the heart, brain, and lungs.


Anaphylactic shock, on the other hand, causes hyperdilation of the blood vessels, resulting in decreased resistance. When this happens, the allergic reaction has become severe and requires immediate medical attention.


What happens during an anaphylactic reaction? For example, a client comes to the ER complaining of an allergic reaction due to a bee sting or shellfish ingestion. The blood vessels will instantly precipitate hyperdilation, causing a massive drop in blood pressure. Blood volume is not impacted; however, since resistance is distressed, perfusion is greatly affected, leading to low arterial pressure.

Compensatory Mechanism

How does the body compensate with decreased perfusion, resulting in decreased oxygenation? There is not a lot of blood circulating through the body due to the huge dilated blood vessels that limit the distribution of blood to different parts of the body. Therefore, the body will compensate through the following:

  • Breathing faster to get more oxygen inside the body, and in the blood.
  • Increased heart rate to pump blood around faster.
  • Cyanotic, pale skin due to decreased capillary refill, especially to the distal portions of the body.
  • Decreased urinary output is also expected because the kidneys are not filtering enough blood and are not allowing fluid to leave the body.
  • Hypoperfusion of the brain, leading to confusion, dizziness, and even loss of consciousness.

Fixing Anaphylactic Shock

Epinephrine is the best medication used reverse anaphylaxis. Clients who are known to have allergic outbursts usually carry around epinephrine. The body naturally manufactures adrenaline, and epinephrine is the bottled version of it. Epinephrine is a sympathomimetic drug that vasoconstricts the blood vessels, shunting blood to the primary organs like the lungs, heart, and brain to appropriately deliver oxygen.

Shock in a Nutshell

Whether it’s due to volume loss or decreased resistance, shock, no matter the type, eventually leads to low pressure. If there’s low pressure, there’s decreased perfusion which then results in low oxygen distribution to different parts of the body. With this understanding of the pathophysiology, you can now pinpoint the various manifestations exhibiting how the body compensates with the allergic reaction.

Regarding fixing the type of shock, if it’s caused by volume depletion, the fix is increasing volume. On the other hand, if shock is caused by decreased resistance like anaphylactic shock, constricting the blood vessels through epinephrine is critical.