The Ventilation-Perfusion (V/Q) ratio

The ventilation-perfusion ratio is exactly what you think it should be - the ratio between the amount of air getting to the alveoli (the alveolar ventilation, V, in ml/min) and the amount of blood being sent to the lungs (the cardiac output or Q - also in ml/min). Calculating the V/Q ratio is quite easy -

V/Q = alveolar ventilation/cardiac output

V/Q = (4 l/min)/(5 l/min)

(here I've just used the 'average' resting values for each of our parameters)

V/Q = 0.8

Sadly, this number is not really that useful to us (why is it that the easy to calculate things tell us very little????) and I will never ask you to calculate the V/Q ratio on a test. What is more useful to us is the consequences of differences in the V/Q ratio that exist in different parts of the lung. Therefore, we usually speak of high or low V/Q ratios without ever assigning them a numerical value.

First thing to consider: Why is the V/Q ratio important?

As we've seen above, the V/Q ratio is the balance between the ventilation (bringing oxygen in to /removing CO2 from the alveoli) and the perfusion (removing O2 from the alveoli and adding CO2). The V/Q ratio is important because the ratio between the ventilation and the perfusion is one of the major factors affecting the alveolar (and therefore arterial) levels of oxygen and carbon dioxide.

Under normal conditions, 4 liters of ventilation each minute enter the respiratory tract while 5 liters of blood go through the pulmonary capillaries. This ratio (the 0.8 we calculated above) gives us our normal blood gases:

Variable
Normal Value
PAO2
~ 100 mm Hg
PACO2
40 mm Hg
PaO2
95 - 100 mm Hg
PaCO2
40 mm Hg

Notice that I'm assuming that the lungs are normal here so diffusion is happening normally (the fact that the arterial and alveolar values are the same or close is what gives that away). We don't have to make that assumption as we talk about V/Q ratios, but I did so in these normal calculations.

There are two ways to change the V/Q ratio (prepare yourselves for a shock): You can change the ventilation and/or the perfusion (I'm sure you're schocked). I'm going to discuss what happens when we make a single change first - physiologically, you see compensatory changes to maintain homeostasis but we'll discuss those later.

The first thing I can do is to decrease the V/Q ratio. A decrease in the V/Q ratio is produced by either decreasing ventilation or increasing blood flow (without altering the other variable). These will both have the same effect - the alveolar (and therefore arterial) levels of oxygen will decrease and the CO2 will increase. The reason for each of these changes is simple:

  • A decrease in ventilation (without a compensatory change in perfusion) means we are not bringing in enough oxygen to meet our metabolic need for oxygen (the oxygen consumption) as well as not blowing enough CO2 to get rid of the CO2 we produced. It is easy for us to figure out why the alveolar and arterial blood gases change the way they do with a decrease in ventilation.
  • An increase in perfusion wil have the same effect on the blood gases because an increase in perfusion (without a compensatory change in ventilation) means more blood cells are coming to remove oxygen from the alveolus as they deliver more CO2 than will be exhaled.

When you consider a decrease in the V/Q ratio, all you need to remember is:

    • Ventilation is not keeping pace with perfusion.
    • The alveolar oxygen levels will decrease, which will lead to a decrease in arterial oxygen levels (PaO2)
    • The alveolar CO2 levels will increase (we're not getting rid of it as fast), also leading to an increase in arterial CO2.

I can also increase the ventilation-perfusion ratio. The good news is that, for our purposes, increasing the V/Q ratio does exactly the opposite of a decrease...

To produce an increase in the ventilation-perfusion ratio, I can do one of two things:

  • Increase ventilation (bring in more oxygen to the alveoli, blow off more CO2 from the lungs)
  • Decrease the perfusion (so the blood takes away less oxygen, delivers less CO2).
  • This will lead to an increase in the PAO2 (and therefore PaO2)
  • and a decrease in PACO2 and PaCO2

Summarizing, an incease in the V/Q ratio means that ventilation is in excess of the metabolic needs being met by perfusion, so we blow off CO2 (lower PACO2) and increase our PAO2 (and PaO2).

Changing the V/Q ratio physiologically

Everytime you stand up, the blood flow to the different parts of the lung (apex vs. base) changes due to gravity. More blood flows to the base of the lung than goes to the apex. This creates a V/Q mismatch (or inequality) and changes the blood gas values of the arterialized blood leaving each region of the lungs. You already know what a V/Q mismatch or inequality is, even though I hadn't written out that term before - it is when one of the two variables changes with a matching change in the other variable (just what we were talking about!).

In the case of standing up, more blood goes to the base of the lung, while relatively less air gets there. That means we see a LOW V/Q ratio and LOW PAO2 and PaO2s.. (along with high PCO2's). Blood leaving the base of the lungs is estimated to have a PaO2 of 89 mm Hg and a PaCO2 of 42 mm Hg.

At the apex of the lung, we get relatively less blood (gravity pulls it down, not up) and relatively high ventilation, so we have a high V/Q ratio. Shockingly, this leads to an increase in alveolar and arterial oxygen levels while decreasing the carbon dioxide. The blood leaving the apex of each lung in a standing person is estimated to have a PaO2 of 130 mm Hg and a PaCO2 of 28 mm Hg.

The middle of the lungs have a good match of blood to ventilation - the arterial blood leaving this area of the lungs is generally thought of to have our standard blood gas values: PaO2 = 100 mm Hg and PaCO2 of 40 mm Hg.

The arterial blood gases you measure from the periphery are the result of blood from all three areas of the lung mixing together. The well-oxygenated blood from the apex of the lung has a relatively small effect because the volume is relatively small (that would be the low perfusion). On the other hand, the base of the lung gets a lot of blood, so it has a big effect on the mixture.

Changing the V/Q ratio pathologically

Just as standing changes the V/Q ratio in a norma person, various pathologies will change blood delivery and/or ventilation to alter the V/Q ratio. This is crucial because it can add to the alterations in blood gases produced directly by the pathology. We'll start with two extreme examples and then move to more subtle alterations.

Increasing the V/Q ratio to infinity: Mathematically, dividing by zero produces the answer of infinity - so an increase in V/Q to infinity is produced when perfusion goes to zero. In a patient, regions of zero blood flow will result from a pulmonary embolism that blocks the blood flow. For the sake of argument, let's assume that a very little bit of blood can get through. This blood will be very well oxygenated (lots of ventilation, little perfusion) and have a very low CO2. In fact, the arterial blood gases in this situation will approach (but not become) atmospheric (PaO2 ~ 140 mmHg; PaCO2 ~ 0 mmHg). This sounds very good EXCEPT for two things:

  1. Not much blood gets through to these alveoli, so the volume of blood in this condition is very low. However, 5 liters of blood is still coming to the lungs every minute - the blood that can't get to the area of lung affected by the embolism gets shunted to other parts of the lung (leading to a low V/Q ratio in those parts of the lung).

  2. We wasted energy by bringing ventilation to this area - in fact, this is alveolar dead space.

 

Decreasing theV/Q ratio to zero: In the human body, the easiest way to produce a V/Q ratio of zero is to stop ventilation to a part of the lung (e.g. inhale a peanut or a small toy). This will produce a V/Q ratio of zero and lead to blood being sent to alveoli that don't have fresh air coming to them. Therefore, the arterial blood will leave the alveolus looking exactly like it did when it was venous blood. Therefore, our arterial blood gases will be the same as our venous blood (PaO2 = 40 mm Hg; PaCO2 = 45 mm Hg). In this case, we wasted cardiac effort to send the blood to the lungs even though nothing happened to it as far as oxygen and carbon dioxide go. We call this a physiological shunt - although the blood travelled to the lungs, it didn't get any oxygen. In contrast, an anatomical shunt occurs when the blood physically doesn't enter the lungs (e.g. a right-to-left shunt - the blood jumps straight from the right ventricle to the left ventricle without going to the lungs). The end result is the same - some of the arterial blood has very low oxygen and high CO2.

More subtle changes in the V/Q ratio: Many lung diseases produce changes in the V/Q ratio that are not consistent throughout the lung. One easy to visualize example of this is what occurs in COPD/emphysema. As we discussed in class, this disease causes the destruction of the alveoli, leading the creation of large air spaces and loss of capillaries in the lungs. The large air spaces mean that some of the air breathed in gets nowhere close to a blood cell, while the loss of capillaries means that some areas of the lung are not getting much blood, while others are getting too much blood. This means that some areas of the lung have a high V/Q ratio (good news: relatively good arterial blood gases, bad news: too little blood going there to make a real difference) and others have a low V/Q ratio (lots of blood going there, but arterial blood has low oxygen and high CO2. These V/Q mismatches are important in contributing to the hypoxia and hypercapnia seen.

Steps taken by the body to normalize the V/Q ratio: The body has a couple of mechanisms that tend to normalize the V/Q ratio as long as the mismatches are confined to restricted areas of the lung. These include:

  • Hypoxic vasoconstriction: In cases where the V/Q ratio is low (lots of blood or too little ventilation), hypoxic vasoconstriction can occur and cause the blood coming into the area to be directed to other parts of the lung. Decreasing the perfusion of the hypoxic region will raise the V/Q ratio and bring the arterial blood gases closer to what we expect.
  • Bronchoconstriction: In cases of high V/Q ratio, the bronchi will constrict slightly to increase the resistance and decrease the amount of ventilation coming into an area that is not well perfused (although it won't shut it down entirely). This limits the amount of alveoalr dead space that occurs and minimizes the 'wasted' work that occurs with alveolar dead space.

Continue to forms of hypoxia