What Is Hemodynamic Instability?

Understanding How Blood Flow Affects Medical Stability

senior woman in ambulance
 leaf/istock

When determining the course of treatment for many medical ailments—especially cardiac conditions—healthcare providers are often counseled to take into account whether the patient is suffering from "hemodynamic instability" or not. It's a very common term, but it can have different meanings to different healthcare professionals, depending on their level of education and, in some cases, where they were trained. Understanding what it means and how it looks is important to fully understand the intent of an author who uses it in a medical guideline or diagnosis.

On the surface, the term is self-explanatory. Hemo- refers to blood while -dynamic refers to movement of the blood around the body. Movement of blood is pretty much the sole responsibility of the circulatory system. Mentioning instability is also self-explanatory. It means that unless we healthcare providers do something, the process is unstable and in danger of failing.

Some common synonyms for hemodynamic instability include circulatory collapseshock (especially decompensating shock), hypoperfusion, and cardiovascular failure. In addition, some terms describe causes of hemodynamic instability while also acting as synonyms, such as hypovolemia and hypotension.

Hemodynamically unstable patients don't have enough pressure in the circulatory system to keep blood flowing reliably to all the various parts of the body where it needs to be at the same time. It's important to understand that not generating enough pressure is not the same thing as being able to measure a low blood pressure.

Signs of Hemodynamic Instability

This doesn't happen alone. Patients don't just become hemodynamically unstable for no reason. This is a syndrome that always accompanies (or heralds) another problem, such as a heart attack or hemorrhagic shock. In the face of other conditions, here are the telltale signs of a hemodynamically unstable patient (it only takes a couple of these to be concerned):

Physicians have typically had a broader definition of hemodynamic instability than many other healthcare providers. Understanding the nuances of the body's reactions to a failing circulatory system are very important when trying to identify a hemodynamically unstable patient. Physicians, as a group, tend to have a pretty strong grasp on those nuances.

Hypotension (low blood pressure) is very much a sign of hemodynamic instability, but it is by no means the first thing one will see in an unstable patient. Too often, low blood pressure has been used as a stand-in for hemodynamic instability by some healthcare providers, but the body will have reactions to fluctuations of blood pressure long before that particular vital sign can be clearly identified.

Fight or Flight Syndrome

As the circulatory system fails, there are several signs of the body's attempt to fix the problem that indicate a problem. Almost all of these come from the sympathetic nervous system, the body's acceleration device and our response to stress. The sympathetic nervous system is driven by, among other things, epinephrine (also known as adrenaline) and is often referred to as the "fight or flight syndrome." This is the physiological mechanism that the human body invokes when it feels as if it is threatened. The idea is that the body is preparing to either fight or run from a predator.

The fight or flight syndrome is mostly about circulation. As the sympathetic nervous system is stimulated, it causes blood flow to be concentrated in large muscle groups in the arms and legs, so the fighting or running can begin. Blood flow and activity in the gastrointestinal system is reduced temporarily (you can digest that cheeseburger if you live through this encounter with the mountain lion). Blood flow to the eyes, nose, and brain increases, all the better to hear and see you with, my dear. The airways in the lungs dilate to let more air flow and the rapid heart rate ensures that blood flows faster to the lungs to stay fresh and oxygenated. Lastly, just in case that saber-tooth tiger gets a few shots in, blood flow is shunted away from the surface of the skin to reduce bleeding and the sweat glands kick in to help cool things down as the situation heats up.

Much of that sympathetic response to stress, which is exactly the same whether the stress comes from decreased perfusion (filling with blood) in the brain or because the brain thinks it's about to be eaten by a zombie, is just as helpful during a medical condition as it is in a zombie apocalypse.

The skin signs—pale and cool because the blood is not flowing at the surface and wet because of the sweating (diaphoresis)—are giving away the fact that there is a stress response happening. Notice that nothing in here requires a low blood pressure. Indeed, in a typical fight or flight response, the blood pressure goes up, which makes the fight or flight syndrome a great response to medical conditions that result in a loss of blood pressure.

When to Use Hemodynamic Instability

In the treatment of cardiac-related medical conditions, hemodynamic stability is the primary indicator of when it's time to intervene. A patient with bradycardia or tachycardia should be treated much more aggressively in the presence of hemodynamic instability than if she appears to be stable.

Many guidelines use this as the litmus test, which makes it all that much more important to recognize it clearly. There are ways to assess hemodynamic stability with invasive monitoring, but it is not a difficult syndrome to identify with a proper clinical assessment.

Shock vs. Hemodynamic Instability

Hemodynamic instability is easy to confuse with shock, but shock is a term that has several meanings and only one of them has anything to do with whether or not a patient is able to adequately move blood around his body. The other two have to do with being emotionally overwhelmed or with electricity.

Shock is a medical condition with defined types and levels, unlike hemodynamic instability. There are four types of shock: cardiogenic, distributive (includes neurogenic, anaphylactic, and septic), hypovolemic (includes hemorrhagic), and respiratory (which some healthcare providers do not agree is a shock syndrome). On the scale of bad to worse, shock can be either compensated (blood pressure is still within normal range) or decompensated (blood pressure is low). It could be said that decompensated shock is a hemodynamically unstable shock patient.

As far as identifying the underlying cause of a patient's medical condition, understanding the types of shock and being able to identify them will go a lot further than simply understanding hemodynamic instability. 

A Word About Hypoperfusion

Perfusion is one of those old terms that holds on in the medical profession after having lost its grip on the rest of modern lexicon. It means to fill with blood and is often used specifically to refer to blood filling the capillaries. Capillaries are basically passive channels where red blood cells can enter only in single file. Capillary beds are like sponges and will fill with blood only if there is adequate pressure to push the blood into the tiny vessels.

Hypo- (too little) perfusion (filling) refers to not enough blood filling the capillary beds in the body. This is a result of hemodynamic instability but is synonymous with shock. In fact, the terms shock and hypoperfusion are completely interchangeable. 

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Article Sources
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  • Sevransky, J. (2009). Clinical assessment of hemodynamically unstable patients. Current Opinion In Critical Care15(3), 234-238. doi:10.1097/mcc.0b013e32832b70e5