inspired air. If alveolar ventilation is reduced, PAO2 will also be reduced. Whilst arterial PO2 (written PaO2) 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 PAO2 and PACO2 described by the oblique line which joins PAO2 20 kPa and PACO2 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 PaCO2 and PaO2 were those represented by point (c), this would imply that the fall in PaO2 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).
The carriage of CO2 and O2 by blood
Blood will carry different quantities of a gas when it is at different partial pressures, as described by a dissociation curve. The dissociation curves for oxygen and carbon dioxide are very different (they are shown together on the same scale in Fig. 1.9). The amount of carbon dioxide carried by the blood is roughly proportional to the PaCO2 over the whole range normally encountered, whereas the quantity of oxygen carried is only proportional to the PaO2 over a very limited range of about 3–7 kPa (22–52 mmHg). Above 13.3 kPa (100 mmHg), the haemoglobin is fully saturated. Further increases in partial pressure result in hardly any additional oxygen being carried.
Effect of local differences in V/Q
In the normal lung, the vast majority of alveoli receive ventilation and perfusion in about the correct proportion (Fig. 1.10a). In diffuse disease of the lung, however, it is usual for ventilation and perfusion to be irregularly distributed, so that a greater scatter of V/Q ratios is encountered (Fig. 1.10b). Even if the overall V/Q remains normal, there is wide local variation in V/Q. Looking at Fig. 1.10, it is tempting to suppose the effects of the alveoli with low V/Q might be nicely balanced by the alveoli with high V/Q. In fact, this is not the case: the increased range of V/Q within the lung affects the transport of CO2 and O2 differently.
Figure 1.9 Blood oxygen and carbon dioxide dissociation curves drawn to the same scale.
Figure 1.10 Distribution of V/Q relationships within the lungs. Although the overall V/Q ratio is the same in the two examples shown, the increased spread of V/Q ratios within the diseased lung (b) will result in a lower arterial oxygen tension and content than in the normal lung (a). Arterial PCO2 will be similar in both cases.
Fig. 1.11b and c show regions of low and high V/Q, respectively, while Fig. 1.11d shows the result of mixing blood from these two regions. Fig. 1.11a shows normal V/Q, for contrast.
Figure 1.11 Effect of V/Q imbalance. (a) Appropriate V/Q. The V/Q ratio is shown diagrammatically on the left. When ventilation is appropriately matched to perfusion in an alveolus or in the lung as a whole, the PCO2 is about 5.3 kPa (40 mmHg) and the PO2 is about 12.6 kPa (95 mmHg). The dissociation curves shown in the centre of the diagram describe the relationship between the blood gas tension and the amount of gas carried by the blood. The normal blood gas contents are represented very diagrammatically on the right. (b) Low V/Q. Reduced ventilation relative to blood flow results in a rise in arterial PCO2 and a fall in PO2. Reference to the dissociation curves shows that this produces a rise in arterial CO2 content and a fall in O2 content. (c) High V/Q. Increased ventilation relative to blood flow results in a fall in PCO2 and a rise in PO2. Reference to the dissociation curves shows that this results in a fall in CO2 content below the normal level but no increase in O2 content. In health, the vast majority of alveoli have an appropriate balance of ventilation and perfusion and the arterial blood has a normal CO2 and O2 content, as shown in (a). In many disease states, the V/Q ratio varies widely between areas. Such variation always results in a disturbance of blood gas content. The effects of areas of low V/Q are not corrected by areas of high V/Q. The result of mixing blood from areas of low and high V/Q is shown diagrammatically on the extreme right (d). It can be seen that, with respect to CO2 content, the high content of blood from underventilated areas is balanced by the low content from overventilated areas. However, in the case of O2, the low content of blood from underventilated areas cannot be compensated for by an equivalent increase in the O2 content of blood from overventilated areas. Arterial hypoxaemia is inevitable if there are areas of low V/Q (relative underventilation or overperfusion).