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"Stressed and Unstressed Volume" - A Scientific Reflection on Hemodynamics and Shock

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Imagine you are blowing up a beach ball. As you begin to exhale into the port you notice very little to no external changes in the progress of your efforts. For the first few breaths you are creating what is referred to in physiology as “unstressed volume.” This is a linear volume that is proportional to the compliance of the container. As you keep blowing, you will break that linear unstressed volume line and begin to ramp up pressure against the walls of the beach ball. 

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When we look at our venous system we can separate the total hydrostatic pressure into two partial pressures, stressed and unstressed. The unstressed volume just keeps the vessels at their minimally open position, while the stressed volume exerts pressure against the walls of the vessel. In order to increase the hydrostatic pressure you can either add volume (fluid bolus), or decrease the fraction of unstressed volume (pressors). 

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In a hypotensive patient, how do we know which action to take? 

This has been the topic of conversation for some time now. The question continues to be “Is my patient fluid responsive?”  The question really is, if I give my patient fluid will it increase the stroke volume (SV)?

To understand how we can increase pre-load to in-turn increase SV, we will take a closer look at the dynamics of flow. In order to create movement or flow in a certain direction, you need a gradient of pressures.

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If we relate this to the venous system, we can appreciate the importance of upstream vs downstream pressures in relation to flow. Our predominant downstream pressure is the right atrium (pRA). If the pRA is above the mean systemic filling pressure, we will not have forward flow or adequate preload. This is why a full IVC is a pathological condition and demonstrates venous congestion.

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What Is The Mean Systemic Filling Pressure?

If you were to stop the heart and allow the vessels to equilibrate in pressure, that pressure would be your mean systemic feeling pressure (MSFP). This pressure amounts to the pressure of venous return when cardiac activity is removed from the equation. While we obviously don’t put patients in arrest to measure this value, it represents the blue line seen in the graph above in which demonstrates that a lower pRA will amount to a higher venous return. 

As mentioned earlier both fluid and pressors can increase the hydrostatic pressure of the venous system. One by filling the stressed volume, and the other by decreasing the fraction of unstressed volume. Depending on the fluid we utilize to fill the system will determine how long it stays intravascular. If we say a patient is volume depleted, we are saying that they have exhausted their capacitance system, or are we?

Because of the increased compliance of the venous system, it holds about 70-80% of your total blood volume at any given time. This means that venous return to the heart can maintain somewhat consistent given significant fluctuations in circulating volume. 

With that being said, unless a patient has been hung upside down and bled for an extended period of time, they should have a significant amount of volume within their venous reservoir. The catecholamine surge that patients experience in shock will start off by increasing heart rate and shunting blood to the core by an endogenous adrenal response. This is where I believe the age and health of the patient will modify how long they hold down the gas pedal given the same level of fuel. Which is why I commonly ask myself “Did they let off the gas first, or run out of fuel.” We see kids compensate for a very long time, until almost a third of their volume is depleted. Whereas with sick adults we see a more acute compensation that fluctuates given underlying co-morbidities. 

I believe this to be the reason we are seeing an era of vasopressors response challenges and push dose vasopressors. These (vasopressors) are not without  negative sequela, but neither is iatrogenic salt water drowning.

References:

1. http://www.heart-lung.org/chapter-six.html

2. P.E Marik, Evidence Based Critical Care, DOI 10.1007/978-3-319-11020-2_9

3. http://ceemjournal.org/m/journal/view.php?number=77

Peer Review #1:

This commentary offers an interesting perspective that reminds us about our venous system, one of the most overlooked components of our circulation, and makes several critical points.  First, the vast majority of our blood volume is harbored within the veins.  Second, cardiac filling pressures – one of the main determinants of stroke volume and cardiac output – are influenced not only by the volume of fluid in the venous system but also by the tone of these veins.  Third, venous tone can be influenced by various pharmacological agents, representing a legitimate therapeutic target during resuscitative efforts.  Other than the ability of nitroglycerin to reduce venous tone and decrease preload, there is little awareness of how various medications might be used to manipulate venous pressures to achieve our therapeutic objectives.  Instead, our singular focus seems to be intravenous fluid therapy, which appears to have resulted in an epidemic of “iatrogenic salt water drowning”…

Dr. Dan Davis, MD

Peer Review #2:

This is a great topic that highlights the physiology behind our volume status during normal physiology and compares it to the patient’s ability to compensate using their stressed volume. This is truly a hot topic and one that’s highlighted in many different treatment guidelines.  The question is often centered around how much fluid is necessary to fulfill the volume-depleted state, with a focus on not overloading them with the dreaded “salt water”!  One aspect to resuscitation that we often forget that’s so elegantly highlighted by many of Dr. Paul Marik's talks, focus on passive leg raising. This is an attempt to identify true volume status instead of just filling each patient with fluids, fluids and more fluids.

We know the average adult carries 15-20% of their blood volume in their legs. That gives the provider an easy way of identifying volume status and illustrates how responsive or unresponsive the patient is to this “internal fluid challenge”.  Once you lift the legs and reassess the blood pressure, the provider should either identify that the patients’ hemodynamic status improved with passive leg raising, thus being volume responsive. In contrast, if the hemodynamic status goes unchanged then the provider shouldn’t focus on volume, but instead vasopressors.

Eric Bauer, MBA, FP-C, CCP-C, C-NPT

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