Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 32

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).