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.

What You Need to Know about Sympathomimetic Drugs

Here, we’ll be going right into the anaphylactic drugs.

While eating at a restaurant, your client has suddenly developed an anaphylactic reaction after devouring a plateful of mixed seafood. After a couple of minutes, the person goes into a severe allergic reaction. The moment a person experiences an anaphylaxis due to something he or she ate, inhaled, or came in contact with, the result would be decreased oxygen perfusion to the body.

A Background on Anaphylaxis

It doesn’t matter if it’s a heart attack, a stroke, a deep vein thrombosis, or a pulmonary embolism; for as long as the pathways for blood-rich oxygen are blocked, the body becomes compromised.

In the case of anaphylaxis, the blood vessels become wider, causing low pressure which is equated to low perfusion and essentially, low oxygen supply. Therefore, the goal of anaphylactic drugs is to reverse this consequence.

Sympathomimetic Drugs

The first type of anaphylactic drug is the sympathomimetic drug. Sympathomimetic drugs are meant to mimic the role of the sympathetic nervous system, acting on the fight and flight response. Tapping the sympathetic nervous system will cause increased blood flow to the three main organs in the body – the lungs, the heart, and the brain. Thereby decreasing the rest and digest phase and getting the required amount of oxygen where the body needs it. This is how sympathomimetic drugs are created – to imitate the parasympathetic response system.

The sympathomimetic drugs that are used in hospitals are:

  • Epinephrine
  • Isoproterenol
  • Ephedrine

Administration of either one of these drugs would immediately cause vasoconstriction, increasing perfusion by shunting blood to the brain, lungs, and heart.


Among these three kinds, epinephrine is the most common medication used in any healthcare setting, especially with emergency cases. Epinephrine is what’s given first during a cardiac arrest to jumpstart the heart that stopped beating. The body naturally creates epinephrine or what is also known as adrenaline; therefore, epinephrine is just adrenaline in a bottle.

How to Remember Epinephrine

Epinephrine or sympathomimetic drugs have several effects on the body, to quickly recall what these drugs to do the body, remember the acronym, NASCAR.

  • N – nervousness
  • A – anxiety
  • S – sugar in the blood is increased
  • C – cardiac arrest
  • A – allergic reaction
  • R – respiratory bronchodilator

A Summary

Sympathomimetic drugs stimulate the body’s oxygen and blood perfusion by imitating the sympathetic nervous system’s effects. Introduction of epinephrine to the body can cause adverse reactions like nervousness, anxiety, and increased blood sugar. Sympathomimetic drugs also cause headaches, palpitations, tremors, and dizziness.

Aside from cardiac arrest, epinephrine is also used for the anaphylactic reaction that can be caused by several factors like food, medication, latex, and insect stings. A secondary effect of sympathomimetic drugs is respiratory bronchodilation or lung expansion that is due to the shunting of blood to the respiratory tract.

So that’s sympathomimetic drugs in a nutshell. For those who want to brush up on the topic of sympathetic versus parasympathetic nervous system, we have a lecture dedicated entirely on that matter. Or if you just want to check out our other nursing-related videos and NCLEX® reviews, head on to our Simple Nursing website and YouTube channel.

Dopamine & Epinephrine: Emergency Meds

For the last two drugs included in the acronym, Sax Fifth Avenue when dealing with chronotropic, inotropic, and dromotropic drugs, we have dopamine and epinephrine.


Dopamine has a lot of effects on the body.

Dopamine directly affects blood pressure and is also considered as a vasopressor. Vasopressors put pressure on veins, specifically the vascular space, to bring back blood into the heart and lungs.

Dopamine is also potent in helping with renal perfusion by opening the renal tubules and allowing perfusion in the kidneys. Dopamine is extremely helpful for clients experiencing acute renal failure.

Dopamine is also an inotropic drug because it helps with forceful heart contractions.

Digoxin vs. Dopamine

Digoxin is also an inotropic drug; however, it is more focused on atrial fibrillation, specifically on targeted on the atrial kick. As for dopamine, it is more on perfusion. By being a vasopressor, dopamine presses on the veins to bring blood back to the heart and push all that oxygenated blood out to the rest of the system.

Dopamine Indications

Dopamine is usually given to clients who are extremely hypotensive or low blood pressure to facilitate proper flow of oxygen. Since oxygen is the money that makes the body go around, having extremely low blood pressure can be detrimental because oxygen is not appropriately distributed to different areas of the body. Dopamine helps in situations like this.

Dopamine as an Inotropic Drug

Not all drugs are negative chronotropic. Dopamine is an inotropic drug that increases the heart rate to increase blood pressure and oxygen perfusion. The goal is to increase the amount of oxygen in the red blood cells, specifically the hemoglobin, to carry them out to the different parts of the body.


Dopamine and epinephrine are somehow similar but different. How?

Epinephrine pretty much has the same qualities of dopamine with regards to having a positive inotropic effect, which helps the heart contract. Epinephrine also has a positive chronotropic effect in increasing the heart rate. Lastly, epinephrine is also known as adrenaline, which is a sympathomimetic drug that mimics the sympathetic nervous system.

Fight and Flight

A sympathomimetic response or a fight and flight response is when the body increases the heart rate to increase blood flow to the heart, lungs, and brain. However, with epinephrine, it is done differently; there is no pressure acted on the vessels to increase perfusion. Instead, epinephrine acts like a hormone that stimulates the fight and flight response.

You can say that this approach is just one of many different routes, and is useful and effective in bringing the heart rate up and contracting more forcefully.

Different Approach

Giving emergency drugs is like being locked out of the house; there are a couple of ways to go in, and you will still get the same result of being able to get in. Whether you’re trying to decrease blood pressure or heart rate, it can be done with the ABC drugs. On the other hand, if the goal is to increase the pressure, heart rate, and perfusion, you can choose from inotropic, chronotropic, and dromotropic drugs.