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The partial pressure of alveolar O₂ (P_AO₂) also varies with alveolar ventilation. If alveolar ventilation increases greatly then P_AO₂ will rise and begin to approach the PO₂ of the inspired air. If alveolar ventilation is reduced, P_AO₂ will also be reduced. Whilst arterial PO₂ (written P_aO₂) also varies with alveolar ventilation (in the same direction as alveolar PO₂), it is not a reliable index of alveolar ventilation, as it is also profoundly affected by regional changes in V/Q matching (see later in this chapter).

The possible combinations of PCO₂ and PO₂ in alveolar gas are shown in Fig. 1.8. Moist atmospheric air at 37 °C has a PO₂ is between 20 and 21 kPa. In this model, oxygen can be exchanged with carbon dioxide in the alveoli to produce any combination of P_AO₂ and P_ACO₂ described by the oblique line which joins P_AO₂ 20 kPa and P_ACO₂ 20 kPa. The position of the cross on this line represents the composition of a hypothetical sample of alveolar air. A fall in alveolar ventilation will result in an upward movement of this point along the line; conversely, an increase in alveolar ventilation will result in a downward movement of the point.

Figure 1.8 Oxygen–carbon dioxide diagram. The continuous and interrupted lines describe the possible combinations of PCO₂ and PO₂ in alveolar air when the RQ is 1 versus 0.8. When the alveolar gas composition is represented by ‘+’ then (a) represents the partial pressures in arterial blood. With progressive underventilation the arterial blood pressures would change to (b). At (c) the PO₂ is lower than can be accounted for by underventilation alone.

In practice, RQ is not 1.0 but closer to 0.8. In other words:

This is represented by the dotted line in Fig. 1.8.

Point (a) represents the PCO₂ and PO₂ of arterial blood (it lies a little to the left of the RQ 0.8 line because of the small normal alveolar–arterial oxygen tension difference). Point (b) represents the arterial gas tension following a period of underventilation. If the P_aCO₂ and P_aO₂ were those represented by point (c), this would imply that the fall in P_aO₂ was more than could be accounted for by reduced alveolar ventilation alone. This would indicate a problem with V/Q matching and thus gas exchange (see below and Chapter 3).