What’s a V/Q Mismatch?
In your advanced Med/Surg course you will be introduced to the concept of a V/Q mismatch. Once you “get it” you totally GET IT…but it can be a tricky concept to understand. Well, it used to be. I’m going to break it down for you Straight A style. Let’s do this!
What is the V and what is the Q?
The first thing to understand is that the V stands for “Ventilation” and the Q stands for “Perfusion”. Don’t ask me why it’s not a V/P mismatch…I’m sure there’s a logical reason out there somewhere. Let’s put that mystery aside for a moment and ask…what is the difference between ventilation and perfusion?
Ventilation refers to the air infusing the alveoli (from bringing air into the body). Perfusion, on the other hand, refers to oxygen being delivered to the tissues (via blood circulation).
Right now, as you are reading this…you are ventilating AND perfusing just fine. If things get out of whack, then you have a “mismatch” and you’d start to feel pretty cruddy.
Two main types of V/Q mismatch:
DEAD SPACE occurs when you have ventilation, but no perfusion…the air is getting to the alveoli, but the alveoli is not perfused properly, so the oxygen can’t travel around to the tissues as it should…it is also not participating in gas exchange, as there are no RBCs coming by to “exchange” gasses with. The most obvious version of this is due to pulmonary embolism.
INTRAPULMONARY SHUNTING occurs when you have perfusion but no ventilation. The circulation is fine, but air is not reaching the alveoli like it should and the alveoli is therefor unable to participate in gas exchange. The blood then goes to the left side of the heart unoxygenated and around in the circulation without the oxygen it is supposed to carry. A shunting situation is the most extreme form of V/Q mismatch…think about blood traveling around the body that is NOT getting fresh loads of oxygen each time it passes through the lungs. The patient is going to become systemically hypoxic to a severe degree pretty quickly! The most common causes are due to alveolar collapse or the alveoli being full of fluids, blood or even pus from an infection.
Let’s look at this visually, shall we?
This first example above shows an alveolus (blue) and the blood vessel (red). In a normal, healthy person the alveolus fills with air (which is blue) and the blood vessel is full of oxygenated blood. Ventilation and perfusion are matched…perfection!
In this example of INTRAPULMONARY SHUNTING, something is keeping the alveolus from filling with air. Maybe the alveolus is completely full of fluid, or maybe something is blocking a whole section of lung (like a big ol’ chunk of fried chicken, perhaps…,shown in green though obviously a chunk of fried chicken would not block a single alveolus, it would block a whole bronchiole but we’re just illustrating the concept here). Whatever the cause, the alveolus does not have any oxygen in it, and the blood vessel can’t pick up any oxygen molecules that aren’t there so the blood is unoxgenated. When this occurs, we say that the alveolus is not participating in gas exchange. We have perfusion, but no ventilation.
Here we have an alveolus that is partially full of fluid, also leading to a shunt situation. I included this so you could see that the alveoli don’t have to be COMPLETELY full of fluid or completely blocked in order for shunt to occur. The alveolus could also just be collapsed due to atelectasis, but how in the world do you draw a collapsed alveolus? I wish I knew!
And here we have an example of DEAD SPACE. The blood vessel is blocked and even though the alveolus is being ventilated beautifully, there is no perfusion due to the blockage. This is what occurs in pulmonary embolism.
How do you measure the V/Q ratio?
The most accurate way to measure the V/Q ratio is by using the classic shunt equation, but this is an invasive and complicated procedure that isn’t all that accurate in critically ill patients. So, we often go by the A:a gradient. The uppercase “A” refers to the alveolar oxygen concentration, and the lowercase “a” refers to the arterial oxygen concentration. In the equation PAO2 – PaO2, these are typically about equal. In cases where the venous blood is just cruising on past the malfunctioning alveoli and returning to the heart without picking up any oxygen, the A:a gradient is going to be off.
Normal A:a gradient value is typically around 5-10mmHg for patients on room air and younger than 61 years of age…the gradient value will increase for older patients (typically about 1mmHg for every decade). If your A:a gradient is elevated, think V/Q mismatch due to shunting. The good news is, the A:a gradient should be automatically calculated on your ABG slip and taking into account how much oxygen the patient is receiving.
So, what are you going to do about it?
Both types of V/Q mismatch are essentially acute respiratory failure, so the baseline treatment is essentially to oxygenate and, in some cases, ventilate the patient. If the problem is that the alveoli are hypoventilated, tossing on an oxygen mask is a great first move. If the problem is due to intrapulmonary shunting due to something like collapsed alveoli, you’re going to have to be more aggressive and look at positive-pressure ventilation to open up those alveoli…start with BIPAP if you can, but in extreme cases you may need to intubate the patient. If it’s a PE, and depending on the size of the embolism, they may need to be intubated and the clot may need to be surgically removed if fibrinolytic agents don’t do the trick. It the alveoli are filled with fluid, then get the fluid off. If they’ve choked on a piece of chicken, get the chicken out of there…you get the idea.
Next time someone mentions A:a gradient, dead space, pulmonary shunting or V/Q mismatch…you’ll have an idea of what they’re talking about. There is much much more to this topic, but for the most part these are the basics that you should know. If you move on to become an advanced practice nurse such as an NP, then you can get into the super-duper nitty gritty. But for your day-to-day nursing care, this should get you started in the right direction. Thanks for visiting my blog…have fun and be safe out there!
Get this on audio in Episode 46 of the Straight A Nursing podcast.
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