Ventilation-perfusion ratios and V/Q mismatch

Definitions & Key takeaways

Ventilation-perfusion (V/Q) ratio is a measure of the relationship between the amount of air entering the alveoli (V) and the amount of blood flowing through the capillaries surrounding the alveoli in the lungs (Q). V/Q mismatch is a condition that occurs when this ratio is not properly matched resulting in ventilation-perfusion mismatch or dead space ventilation. V/Q mismatch can be caused by various factors, such as lung conditions like pulmonary embolism, asthma, COPD, and interstitial lung diseases.

Chapters:

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Introduction0:00–0:33

Alveolar ventilation (V) is the amount of air that reaches alveoli in the lungs, measured in liters/minute (L/min); and perfusion (Q) is the pulmonary blood flow, or cardiac output, that reaches the arteries, and specifically the capillaries, surrounding the alveoli, also measured in L/min.
When the lungs are upright and at rest, ventilation is about 4 L/min and perfusion is about 5 L/min, giving a ratio of 0.8.

Lung zones0:33–1:41

Now, the lungs can be divided into three distinct zones. Zone 1 is the top of the lungs, or the apexes; zone 2 is the middle of the lungs; and zone 3 is the bottom, or bases, of the lungs.
In an upright position, gravity dramatically affects both ventilation and perfusion across all three zones, and overall the V/Q ratio progressively decreases from zone 1 to zone 2 and finally to zone 3.
In zone 1, the flow of air and blood is the lowest with ventilation of around 0.25 L/min, and perfusion of around 0.07 L/min; generating a V/Q ratio of 3.6.
In zone 2, ventilation is equal to perfusion; generating a V/Q ratio of about 1. In zone 3, the flow of air and blood is the highest with ventilation of around 0.8 L/min, and perfusion of around 1.3 L/min; generating a V/Q ratio of 0.6.
So the V/Q ratio varies depending on which part of the lung is involved, but the overall ratio is an average of the three zones and works out to be 0.8.

V/Q ratio1:41–3:09

Now, the ratio of V to Q influences how efficiently gases, specifically O2 and CO2 , are exchanged in the lungs. In healthy lungs with a V/Q ratio of 0.8, the alveolar partial pressure of O2 (PAO2), is about 100 mmHg or millimeters of mercury; and the alveolar partial pressure of CO2 (PACO2) is about 40 mmHg.
Meanwhile, the arterial partial pressure of O2 (PaO2) is around 95 mmHg - slightly lower than what’s on the alveolar side; and the arterial partial pressure of CO2 (PaCO2) is about 40 mmHg - the same as what’s on the alveolar side.
But these partial pressures are also an average over the three lung zones. In zone 1 the arterial partial pressure of O2 (PaO2) is 130 mmHg and arterial partial pressure of CO2 (PaCO2) is 28 mmHg.
In Zone 2 the arterial partial pressure of O2 (PaO2) is about 108 mmHg and arterial partial pressure of CO2 (PaCO2) is about 39 mmHg.
And in zone 3, the arterial partial pressure of O2 (PaO2) is 88 mmHg and arterial partial pressure of CO2 (PaCO2) is 42 mmHg.
So, a drop of blood in zone 1 gets more oxygen diffused into it than a drop of blood in zone 3, but because zone 3 has about 19 times more blood flowing through per minute, it ends up accounting for more of the overall gas exchange.

V/Q mismatch3:09–3:35

Now, any change to alveolar ventilation or perfusion in any of these zones will change the V/Q ratio. This is called a V/Q mismatch.
When this happens, either ventilation or perfusion limits the availability of O2 and CO2 to exchange. This causes changes in the partial pressures of O2 and CO2 and can make gas exchange less efficient.

Reduced blood flow3:35–4:34

So, let’s say there’s a pulmonary embolism, which is where a blood clot might block off blood flow going into the lung. In that situation, blood flow is reduced to the alveoli, but alveoli may be still getting good ventilation.
So a normal V, but a low Q, which means that V/Q is high, and in the extreme case, if there’s absolutely no blood flowing through an artery, then V/Q can equal infinity.
In that situation, the unperfused tissue can become ischemic, and the alveoli that are ventilated but not perfused are considered dead space.
There are no PaO2 and PaCO2 values because there’s no blood in the tissue that’s not perfused. And without blood, gas exchange can’t happen.
So PACO2 becomes 0 mmHg because CO2 is not diffusing from the blood into the alveoli; and PAO2 becomes 150 mmHg, which is what you find in inspired air, because O2 isn’t diffusing into the blood.

Reduced ventilation4:34–5:41

On the flip side, let’s say there’s an obstruction in the airway, which can happen with obstructive lung diseases like emphysema.
In that situation, ventilation is reduced to the alveoli, but the alveoli may still be getting good blood flow. So a normal Q, but a low V, which means that V/Q is low.
Less CO2 diffuses out of the blood and into the alveoli, meaning that PaCO2 increases; and less O2 diffuses into the blood from the alveoli, so PaO2 decreases.
In an extreme case, if there’s absolutely no ventilation of the alveoli, then V/Q can equal zero. In that situation, the blood can be diverted away from the non-ventilated alveoli to better ventilated alveoli where gas exchange may occur.
This is called a shunt. There are no PAO2 or PACO2 values because air is not reaching the alveoli at all.
And without ventilation, gas exchange can’t happen. So, the partial pressures in arterial blood can change to match those of venous blood: PaO2 decreased to 40 mmHg and PaCO2 increases to 46 mmHg.

Review5:41–6:09

So, to recap: The average ratio of alveolar ventilation (V) to alveolar perfusion (Q) is 0.8. A V/Q mismatch, or V/Q defect, can occur when there are changes to ventilation or perfusion that diminish gas exchange.
Extremes occur when: there is dead space and perfusion is cut off, V/Q goes to infinity; or there is a shunt and ventilation is obstructed, V/Q goes to zero.