Respiratory Medicine. Stephen J. Bourke. Читать онлайн. Newlib. NEWLIB.NET

Автор: Stephen J. Bourke
Издательство: John Wiley & Sons Limited
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Жанр произведения: Медицина
Год издания: 0
isbn: 9781119774235
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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).

Schematic illustration of oxygen–carbon dioxide diagram.

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

a l v e o l a r normal upper P o 2 plus left-parenthesis StartFraction a l v e o l a r normal upper P c o 2 Over 0.8 EndFraction right-parenthesis equals 20 upper K upper P a

      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

       Effect of local differences in V/Q

Schematic illustration of blood oxygen and carbon dioxide dissociation curves drawn to the same scale. Schematic illustration of distribution of V/Q relationships within the lungs. Schematic illustration of the effect of V/Q imbalance.