Shock is a topic you’re bound to come across on the USMLEs, so it’s important to be familiar with it. The good news is the pathophysiology of shock is an immensely high-yield topic. Studying it will help you understand the cardiovascular system and the impact changes in the system have on the whole body.
Of course, the most important reason to understand shock is it’s an important pathology you need to recognize and treat in the real world. Your patient’s life depends on having a rapid and precise response to it!
In this post, we’ll cover what you need to know about shock for the USMLEs and beyond. To make sure we have the basics covered before diving into more detail, let’s begin by defining what precisely we mean when we say a patient is in shock.
What is shock?
In general, shock is the condition when the body cannot adequately perfuse end organs, leading to cellular damage. There are different kinds of shock a patient can be in. Each has different underlying mechanisms.
How do we determine what category of shock a patient is in?
There are several measures used to diagnose which category of shock a patient is experiencing.
These include:
1. The Central Venous Pressure
The central venous pressure (CVP) is a measure of pressure within the vena cava and is a surrogate for blood returning to the heart.
2. The Pulmonary Capillary Wedge Pressure
The pulmonary capillary wedge pressure (PCWP) helps determine the pressure within the left atria.
3. Cardiac Output (CO)
Cardiac output is the volume of blood leaving the heart in a given unit of time. This value is affected by heart rate and stroke volume, which is affected by preload (aka how much blood is in the heart prior to systole).
4. The Systemic Venous Resistance (SVR)
The systemic venous resistance is how much resistance the blood vessels in the periphery are creating. This can be thought of as how constricted the blood vessels may be.
5. The Mixed Venous Oxygen
Finally, the mixed venous oxygen is a measure of the amount of oxygen in the blood returning to the heart. If tissues extract an increased percentage of oxygen from the blood, the mixed venous oxygen will be decreased, and if the tissues are extracting a decreased percentage of oxygen, it’ll be increased.
When presented with a patient in shock, take note of the numbers above and you’ll likely know what kind they’re in without needing many other details.
An additional piece of information that’ll help guide your diagnosis is whether the patient is “warm” or “cold.” We’ll have more on that shortly.
Now that we’ve covered the basic shock measurements, let’s take a look at the different kinds of shock and how these measurements are used to diagnose them.
What are the 4 kinds of shock?
1. Hemorrhagic/Hypovolemic Shock
Hemorrhagic/hypovolemic shock is the most common subtype of shock in the traumatized patient and should always be at the top of your mind when a patient may be bleeding. Burns may also lead to hypovolemic shock due to evaporation from the burned surfaces.
“Cup of Water” Exercise
To conceptualize this type of shock, I want you to sit for a second and imagine that the venous system is a cup of water that you suddenly dump half of the volume out of.
What happens to the pressure on the walls (the CVP)? It decreases.
What would happen to the heart’s preload? It decreases.
This would lead to decreased stroke volume and decreased cardiac output even in the setting of tachycardia as the hemorrhage progresses.
Since the entire volume of the circulatory system is decreased, what would happen to the pressures inside the atria (PCWP)? They also decrease.
Now, since the body is experiencing decreased blood pressure due to less volume inside the vessels, how may the vessels attempt to compensate to maintain blood pressure? That’s right, they vasoconstrict, which increases the SVR.
For the mixed venous oxygen in this scenario, tissues need the same amount of oxygen as they did before the hemorrhage, but now there’s less blood to deliver it. This means that each tissue is extracting a larger percentage of the oxygen from the blood that does reach it, which decreases the mixed venous oxygen.
Given that the body attempts to preserve vital organs in states of hemorrhagic shock, the skin is often left to the wayside and the patient is described as “cold and clammy.”
The treatment for hemorrhagic shock is simple. Replace the volume! Giving packed red blood cells, fresh frozen plasma, and platelets in a 1:1:1 fashion or whole blood replaces what has been lost.
Stopping the bleeding with surgery or interventional radiology will prevent further losses.
These actions may be supplemented with medications such as pressors or crystalloid fluids to maintain blood pressure for adequate tissue perfusion.
2. Cardiogenic Shock
Cardiogenic shock is lack of perfusion from left heart dysfunction due to MI, heart failure, or arrhythmia resulting in decreased cardiac output, or valvular disorders.
The main derangement in this scenario is a decrease in cardiac output. The lack of throughput in the heart results in the backup of blood in the venous circuit and a lack of blood in the arterial system. That buildup in the veins results in increased CVP and PCWP.
On the arterial side, this condition acts similarly to hemorrhagic shock because it appears there’s a decrease in blood volume because of a decreased cardiac output. This results, again, in increased SVR and decreased mixed venous oxygen content by the same thought process as hemorrhagic shock.
These patients are also described as “cold and clammy” because the warm blood is staying in the heart rather than warming the skin.
Treatment of cardiogenic shock entails treating the underlying cause and using medications to increase cardiac output.
3. Obstructive Shock
Thus far, we’ve discussed a decrease in blood volume and a broken pump. Obstructive shock, however, is an impediment of blood flow for a reason external to the heart itself.
Pulmonary Embolism & Tension Pneumothorax
Pulmonary embolism and tension pneumothorax are the first causes of obstructive shock we’ll discuss. These pathologies prevent blood circulation through the pulmonary circuit. When this happens, blood again backs up causing increased central venous pressure.
However, instead of the blood emptying into the left side of the heart, it remains in the pulmonary vasculature, causing a decrease in PCWP. The cardiac output subsequently decreases because of the lack of left ventricular preload.
This lack of cardiac output causes increased SVR as before. Interestingly, in this case the mixed venous oxygen is increased. The way that I think about this is that the blood sits in the lungs and continues to be oxygenated and backs up to increase the mixed venous oxygen.
Cardiac Tamponade
The second case of obstructive shock is cardiac tamponade. For a thorough explanation of this, I’ll refer you to the blog post regarding high-yield cardiology topics. As a quick reminder, cardiac tamponade is when blood fills the sac surrounding the heart and impedes its ability to fill and pump.
This will increase the CVP (due to lack of flow through the heart) as well as increase the PCWP because blood does make it to the heart (as opposed to PE). The SVR is increased due to lack of CO and the mixed venous oxygen is decreased because the blood does not sit in the lungs.
4. Distributive Shock
The final category of shock involves the distribution system of the blood itself, the arteries. This is in the form of systemic vasodilation resulting in low blood pressure that isn’t high enough to perfuse tissues.
This category is also split into two subcategories: sepsis/anaphylaxis and neurogenic shock.
Sepsis/Anaphylaxis
Septic shock is the result of an infection where the anti-inflammatory and pro-inflammatory factors result in systemic vasodilation. Anaphylaxis, a type 1 hypersensitivity reaction, is the result of an allergen triggering a significant IgE response and histamine release which causes vasodilation. This vasodilation decreases the CVP and PCWP.
When the vasodilation is detected, the heart increases its rate to increase cardiac output. The SVR is necessarily decreased, as this is the main derangement in these conditions. Because vasodilation is the cause of lack of perfusion, the blood doesn’t reach the tissues for oxygen extraction and, therefore, the mixed venous oxygen content is increased. Since the blood flow itself is not impaired, these patients are described as “warm and dry.”
Neurogenic Shock
Finally, neurogenic shock is most often encountered after a traumatic brain injury in which the sympathetic tone is lost. This lack of tone decreases the CVP, PCWP, cardiac output, and SVR. Mixed venous oxygen content will usually be normal but occasionally increased.
Note how neurogenic shock differs from spinal shock, which is often a result of trauma to the spine but the process is physiological rather than anatomic. The condition results in flaccid paralysis and the loss of sensation, reflexes, and autonomic function below the level of injury. This condition often resolves spontaneously over days to weeks.
Shock Cheat Sheet
| Shock Type | CVP | PCWP | CO | SVR | Mixed Venous O₂ | Clinical Features | Treatment Focus |
|---|---|---|---|---|---|---|---|
| Hemorrhagic / Hypovolemic | ↓ | ↓ | ↓ | ↑ | ↓ | Volume loss from bleeding or burns; patient is cold and clammy | Replace volume (PRBCs/FFP/platelets or whole blood); stop bleeding; support BP with pressors/crystalloids |
| Cardiogenic | ↑ | ↑ | ↓ | ↑ | ↓ | Left heart dysfunction (MI, HF, arrhythmia, valvular disease); patient is cold and clammy | Treat underlying cause; use medications to increase cardiac output |
| Obstructive – PE / Tension Pneumothorax | ↑ | ↓ | ↓ | ↑ | ↑ | Impediment to blood flow through pulmonary circuit (PE, tension pneumothorax) | Relieve obstruction (e.g., thrombolysis, chest decompression) |
| Obstructive – Cardiac Tamponade | ↑ | ↑ | ↓ | ↑ | ↓ | Blood in pericardial sac impeding filling and pumping of the heart | Relieve tamponade (pericardiocentesis) and treat underlying cause |
| Distributive – Sepsis / Anaphylaxis | ↓ | ↓ | ↑ or normal | ↓ | ↑ | Systemic vasodilation; blood doesn’t reach tissues; patient is warm and dry | Fluids, vasopressors, and treatment of cause (antibiotics, epinephrine) |
| Neurogenic | ↓ | ↓ | ↓ | ↓ | Normal or ↑ | Loss of sympathetic tone after traumatic brain injury | Fluids, vasopressors, stabilization of CNS injury |
Final Thoughts
As you can see, each type of shock has its own distinct characteristics that’ll help you identify it in a question prompt. I encourage you to review the table above and then create your own table by hand!
Think about the cardiovascular circuit in its entirety, and where each type of shock interrupts that process. This exercise is an important tool for every pathology you encounter throughout your medical training, as it will help you retain the information and apply it to patients in real-world scenarios.
Happy studying and good luck on the USMLEs!
For more (free!) high-yield topics to know for your USMLE exams, check out these other posts:
