Alveolar Ventilation

The process of moving air into and out of the lungs is called ventilation. The amount of the inspired air which reaches the alveoli so that it is available for gas exchange is called the alveolar ventilation. The amount of oxygen reaching the lungs is dependent upon:

The partial pressure of oxygen in the inspired gas

Alveolar Gas Equation (AGE)

The O₂ level at the alveolus i.e. the partial pressure at the alveolus (P_A) can be calculated by using the AGE.

P_AO₂ = PiO₂ – P_ACO₂/R

Where:

• PiO₂ = partial pressure of oxygen in inspired gas
• P_ACO₂ = partial pressure of carbon dioxide in the alveoli
• R = respiratory quotient

The respiratory quotient is the ratio of CO2 production to O2 consumption and depends upon what macronutrients the body is using for fuel (carbohydrate/fat/protein). The respiratory quotient of a mixed diet is around 0.8.

When breathing atmospheric air:

• P_AO₂ = FiO₂ x (PB – PH₂O) – P_ACO₂/0.8
• PB = Barometric (atmospheric) pressure
• P_ACO₂ cannot be easily measured and so PaCO₂ (as measured by the blood gas machine) is used as the two values are virtually identical

P_AO₂ = 0.21 x (101-6.3) – PaCO2/0.8

P_AO₂ = 19.9 – (5.3/0.8) = 13.3kPa

The respiratory quotient may equal 1 in a carbohydrate enriched diet which would render the AGE: P_AO₂ = 95 x F_IO₂(%) – P_aCO_2.

The amount of gas reaching the alveoli

Alveolar partial pressure

Increasing or decreasing the amount of oxygen in the inspired gas causes the alveolar partial pressure to change in the same direction but not necessarily by the same amounts.

Example

Consider what happens to the PAO₂ when the inspired gas is changed from room air (21%) to 40% oxygen.

The PAO₂ when inspiring room air:

= [95 x 21%] – [1.25 x 5.3] = 13.3 kPa

And the PAO₂ with an inspired oxygen concentration of 40%:

= [95 x 40%] – [1.25 x 5.3] = 31.3 kPa

Compare this increase to the fall in PAO₂ which occurs when the inspired oxygen concentration is reduced to 15%:

= [95 x 15%] – [1.25 x 5.3] = 7.6 kPa

The fall in PAO₂ following a reduction in inspired oxygen concentration is greater than the rise in PAO₂ following the same increase in inspired oxygen concentration.

The second example demonstrates that by almost doubling the inspired oxygen percentage to 40%, we have more than doubled the normal PAO2.

In contrast, by reducing the inspired oxygen percentage by only a quarter to 15%, the PAO₂ has nearly halved. The reason for this disproportionate effect is because the partial pressure of carbon dioxide and saturated water vapour pressure in the alveoli remains the same.