“..and s/he is probably suffering from some element of a V/Q mismatch.”
-Brilliant medical student, AM Rounds, c. 2017
It sounds smart because it is, and there’s a very good chance that a ventilation perfusion (VQ) mismatch is why your patient is hypoxic. Let’s investigate what this really means so that when you drop knowledge bombs like this in front of your residents and attendings, you have some data and understanding to back it up.
What does the 'V' in "VQ Mismatch" stand for?
Ventilation. It’s what keeps us going. For our intents and purposes, this is air/oxygen moving in and out of the lungs.
What does the 'Q' in "VQ Mismatch" stand for?
Blood flow. Also keeps us going. This is blood flowing through the pulmonary arteries into pulmonary capillaries, and ultimately pouring out the pulmonary veins back into the left atrium.
Along the way, as long as it’s passing by alveoli that are full of oxygen, and that oxygen can diffuse across pneumocytes into the blood, this blood will pick up some oxygen in the pulmonary capillaries, and drop off some CO2.
Now that groundwork is laid, let us investigate the concept of a mismatch between these two forces that we strive to keep in balance.
There will always be some element of “mismatch” between blood flow and ventilation as one moves through different lung zones, but this is normal physiology and consistent with normal oxygenation. Upper (apical) zones of the have less ventilation, but far less blood flow, so V>Q, and the V/Q ratio is high. At the lung bases, there’s more ventilation, but much more blood flow, so V<Q and the V/Q ratio is less than 1.
Pathological conditions create extreme amounts of V/Q mismatching which are not consistent with proper oxygenation.
Let’s start with extremes which will best illustrate the example:
V/Q Mismatch vs. Shunt
V = 0. Therefore, V/Q = 0
There is no ventilation; no air is moving in and out of alveoli. Blood flow, Q, is AOK. This is referred to as a SHUNT. These can occur because of an obstruction in the airway, like a mucous plug, a peanut, or a tumor (cannot ventilate distal to the mucus plug); or more distal at the level of alveoli, from edema/blood/pus in an alveolus (cannot get air/oxygen to the alveoli/capillary interface), or a collapsed airway. Again, blood flow is maintained, ventilation is non-existent, and a SHUNT exists.
An important teaching point regarding shunts: They do not improve with administration of 100% oxygen. As you might imagine, this is because this oxygen is not reaching its effect site to diffuse across the pneumocytes.
Now, for our other extreme:
Q = 0. Therefore, V/Q = ∞
We have no issues with ventilation of an area, but blood flow is compromised. This creates DEAD SPACE. What causes blood flow to cease? Extremely low cardiac output, external compression of the lung vasculature (too much PEEP, tension PTX, overdistended lungs, surgical clamping of the pulmonary artery), or an internal blockage (massive pulmonary embolism).
Now, it’s rare to exist at either end of this spectrum. In a non-arrest, non-apneic state, for the lungs as a whole, some ventilation and some blood flow will always be maintained. However, there can always be some element of a “localized” V/Q mismatch in a particular lung segment.
A V/Q mismatch is usually a heterogenous problem, meaning that ventilation and perfusion are not properly matched across different areas of the lungs. While as a whole, there might be acceptable ventilation and blood flow, if areas of compromised blood flow are getting most of the ventilation, hypoxemia will ensue. This is because areas of normal blood flow are not getting enough ventilation.
As far as specific etiologies, consider the following: a lobar pneumonia fills a lobe with neutrophils and mucous - there will not be adequate ventilation in that lobe. Atelectasis collapses distal airways - there will not be adequate ventilation in these airways. Pulmonary edema fills alveoli with fluid - you get the picture. COPD, a disease of increased pulmonary secretions, hyperinflation, and easily collapsible airways, sets a patient up perfectly for lots of areas of V/Q mismatch.
When the natural balance of ventilation and perfusion to an area is compromised, a V/Q mismatch exists which will negatively impact systemic oxygenation.
It doesn't have to be as complicated as you want it to be. Just remember and understand the bolded sentence, and you’re there!
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