Mechanical Ventilation- Ventilation/Perfusion

The main objective of the lung is to match the ventilation of the air with the perfusion of blood through the lungs.

V = Ventilation- air going into the lung. Approx 4 litres.

Q = Perfusion- blood going around the lung. Approx 5 litres.

Ventilation/perfusion should be approx 0.9. This is otherwise known as the Ventilation Quotient.

Model of 3 Zones of the Lungs

A useful concept to understand. The lung is divided into three zones in this model.

Zone 1 is the apex of the lung or the highest most part in the upright patient.

  • As this area is above the heart the blood pressure will be lower relative to the vessels below. That means that the blood vessels will be narrower than those below.
  • The reduced venous pressure will mean that the alveoli are bigger than those below.
  • As a consequence the ventilation will be much greater than the perfusion.

Zone 2 is the middle part of the lung.

  • Zone two tends to be the zone where the blood vessels and the alveoli are at their maximal sizes.
  • As a consequence the ventilation and perfusion will be balanced.

Zone 3 is the base of the lung, or the lowest part in the upright patient.

  • The blood supply is very distended in zone 3 because of the effects of gravity. They are below the heart, so are subject to a slightly higher pressure than those above them.
  • This increased venous pressure will also make the alveoli smaller as it acts on them, relative to those above.
  • As a consequence the perfusion will be greater than the ventilation.

A very important point to know here is that at any given time over half of our lung is not being utilised. Potentially our lungs can hold up to 6 litres of air, but normally is only holding just under 3 litres.

This means that a large proportion of out alveoli are collapsed. If we were perfusing all of those collapsed alveoli we would have a lot of our lung blood volume not being effectively ventilated. So how does the body compensate for this?

The body has a process called Hypoxic Vasoconstriction. Anytime a region is poorly ventilated and has low oxygen levels the muscles around the blood vessels in the area concerned constrict, thereby limiting the blood supply to the region.

So the blood flow to Zone 3 is actually reduced compared to Zone 2. This means that Zone 3 is neither perfused or ventilated well under normal circumstances. When we take a deep breath then we will utilise Zone 3 more fully.


Guidelines for the management of tracheal intubation in critically ill adults

Having read the guidelines I made these infographics. They are FREE. Just let me know your email address and they will be sent to you.