Ventilation-Perfusion (V/Q) Ratio

Increasing the V/Q Ratio

-produces the opposite of decreasing it

Steps to normalize V/Q ratio

1. Hypoxic Vasoconstriction 2. Bronchoconstriction

To produce an increase in V/Q ratio

1. increase ventilation (bring in more oxygen to the alveoli, blow off more CO2 from the lungs) 2. decrease the perfusion (so the blood takes away less oxygen, delivers less CO2) 3. This will lead to an increase in the PAO2 (and therefore PaO2) 4. and a decrease in PACO2 and PaCO2 *ventilation is in excess of the metablolic needs being met by perfusion, so we blow off CO2 (lower PACO2) and increase our PAO2 (and PaO2).

What is more useful to us than calculating the V/Q ratio?

The consequences of differences in the V/Q ratio that exist in different parts of the lung. V/Q ratio is typically thought of high or low and not specific values.

V/Q Ratio definition

balance between the ventilation and the perfusion

anatomical shunt

blood physically doesn't enter the lungs

Physiological shunt

blood traveled to the lungs but didn't get any oxygen

Importance of V/Q ratio

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.

perfusion

removing O2 from the alveoli and adding CO@

ABGs

*measured at the periphery are the result of blood from all three areas of the lung mixing together.

Key points for a decrease in V/Q ratio

1. Ventilation is not keeping pace with perfusion. 2. the alveolar oxygen levels will decrease, which will lead to a decrease in arterial oxygen levels (PaO2) 3. The alveolar CO2 levels will increase (we're not getting rid of it as fast), also leading to an increase in arterial CO2

ventilation

bringing oxygen in to/removing CO@ from the alveoli

Two ways to change the V/Q ratio

change ventilation and/or change perfusion

Decrease the V/Q ratio

produced by either decreasing ventilation or increasing blood flow (without altering the other variable) 1. a decrease in ventilation means we are not bringing in enough oxygen to meet our metablolic need for oxygen 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 2. an increase in perfusion will 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