The prone position has come into focus recently. However, many may not under what the prone position is used for. This article will focus on explaining prone positioning and the reasons why prone ventilation is effective to improve oxygenation.
- Understanding Lung Anatomy and Perfusion
- Alveolar Collapse
- Alveolar Ventilation Visualized
- Creating a V/Q Mismatch
What is Prone Position?
To put it briefly, prone position is lying face down.
According to Hackensack Meridian Health, “Proning is the process of turning a patient with precise, safe motions from their back onto their abdomen (stomach) so the individual is lying face down.”
Proning position is used for patients in Acute Respiratory Distress Syndrom (ARDS) to “help them breath better.”
Prone position is used to prevent alveolar collapse.
Why Do We Prone?
The physiology of why we prone is key to its understanding. It also gives a greater appreciation of its efficacy.
There are three terms that play a key part in the understanding of why we prone:
These papers are the ones I learned most from and, whilst a little science-heavy in places, I think well worth a read:
- Efficacy of prone position in acute respiratory distress syndrome patients: A pathophysiology-based review
- A Comprehensive Review of Prone Position in ARDS
Understanding Lung Anatomy and Perfusion
The first part to understand is the shape matching of the lung and the effect that has on the patient’s ability to ventilate well.
Let us start with a simple diagram of the lung, which we will gradually build upon, to aid understanding.
Imagine that the patient is now lying on their back (supine position) and that we have taken a slice through their lungs. We are now looking at that slice from the position of the feet up.
We represent each lung in an enclosed box, which represents the pleura and the chest wall, giving the lung some limitations as to how it can expand.
You can see the individual alveoli represented by the circles within the box. In this diagram, they are all of equal shape and size.
However, our lungs are subject to the force of gravity, just like everything else and when we add that, you can see that the picture changes.
Now, the alveoli at the top of the image, or the ventral part of the lung are pressing down on the alveoli in the lower part of the lung or the dorsal part.
Due to this compression effect, we have larger alveoli higher up and more compressed alveoli lower down. This results in alveolar collapse.
Alveolar Ventilation Visualized
We now consider the true shape of the lung. This can be seen in the tear-shaped lung illustrated here. This is an exaggeration of the true shape but helps illustrate the principles we need to understand.
This shape means that, when the patient is on their back, or supine, there is slightly less room at the top then the bottom. Consequently, with the added gravity much of the lung tends to drop into the lower part where it is compressed by the lung above it.
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The key point here is that there is a lot of room for this compressed lung to fall into.
Add to this the fluid that will also be affected by gravity and you can see that now we have compressed alveoli surrounded by the fluid- making diffusion in these alveoli much harder.
Creating a V/Q Mismatch
The perfusion to the dorsal part of the lung is a little better than the perfusion to the ventral part- and the key here is that does not change significantly when the patient is proned.
So, you can see in the first illustration where the patient is supine, we have the better perfusion where there is the poorest ventilation and the better ventilation with the slightly less good perfusion- in other words, a V/Q mismatch.
In a V/Q mismatch, V stands for ventilation while Q stands for perfusion. Healthline states: “A V/Q mismatch happens when part of your lung receives oxygen without blood flow or blood flow without oxygen.”
What Happens to the Lungs in Prone Position
In the below illustration, you can see what the lungs are doing in a proned patient. Because of the shape matching, and the effects of gravity on the fluid in the patient’s lungs we now have better perfusion taking place where there is better ventilation. This improves the V/Q balance.
The Impact of Abdominal Pressure
The other effect to be considered is the way the abdominal contents can add pressure to the diaphragm when it is moving up and down.
Pressure in a Supine Patient
In this illustration, we are looking at the patient from the side. The arrows indicate the pressure from the abdomen on the diaphragm. Remember that there is already a V/Q mismatch in this region when the patient is supine, and this added pressure contributes to make it worse.
Side View of Prone Position Pressure
If we then turn our patient onto their front (prone positioning) the pressure remains the same but is now pressing on the front of the diaphragm and no longer the back. Again, remember that the back of the lung, when proned, is where we have the improved V/Q matching. In the prone position,we have also relieved some of that pressure also.
Elevating the Head Slightly
We can also help the patient by placing them slightly head up, which will drop the abdomen the other way as well as the benefit of effectively sitting the patient up, helping in the prevention of ventilator acquired pneumonia much like we do when we try to sit them up to thirty degrees when they are on their back.
A Heavy Heart, Indeed
The final point is about the position of the heart within the chest cavity. The heart is nearer to the front of the chest than the back. This means that, when the patient is lying on their back the weight of the heart, gravity in action again, is lying on top of much of the lung, thereby adding to the compression.
In the above illustration, you can see that when we prone the patient there is less lung for the heart to lie on top of, and much of it is supported by the sternum. This again reduces some of the compression on the alveoli and certainly takes some of the weight off that area of the lung with the best V/Q match.
So… What is the Prone Position Used For?
In summary, we prone patients to reduce pressure on the lungs. This changes the lung shapes in such a way that allows better ventilation and perfusion. Overall, this leads to improving oxygenation.
About the Author:
Jonathan a.k.a. The Critical Care Practitioner, is an Advanced Critical Care Practitioner in the UK. He is also an Associate Professor at the University of Warwick. He has been producing podcasts and teaching resources online for over 10 years to help others in Critical Care learn also. You can check out his website, his teaching page, The Critical Care Practioner, and also follow him on Twitter at @ccpractitioner.
- The Critical Care Practioner website
- Unveiling the Mysteries of Mechanical Ventilation from Nicole Kupchik – The course starts with a review of arterial blood gas (ABG) interpretation as well as uses of Capnography. The basic fundamentals of mechanical ventilation with a build-up to managing complex ARDS patients will be discussed. A total of 6.0 contact hours are provided with 6-month access.
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