Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 35

An understanding of the relationship between P_aCO₂ and P_aO₂ is critical to the interpretation of blood gases (see Chapter 3). The relationship can be summarised in an equation known as the alveolar gas equation.

 Pure underventilation leads to an increase in PaCO2 and a ‘proportionate’ fall in PaO2. This is known as type 2 respiratory failure.

 A disturbance in V/Q matching leads to impaired gas exchange with a fall in PaO2 but no change in PaCO2. This is known as a type 1 respiratory failure.

 Because these two problems can occur simultaneously, the alveolar gas equation is needed to determine whether an observed fall in PaO2 can be accounted for by underventilation alone or whether there is also an intrinsic problem with the lungs (impairing gas exchange).

Rather than merely memorise the alveolar gas equation, spend just a moment here understanding its derivation (this is not a rigorous mathematical derivation, merely an attempt to impart some insight into its meaning).

Imagine a lung, disconnected from the circulation, being ventilated. Clearly, in a short space of time, P_AO₂ will come to equal the partial pressure of oxygen in the inspired air (P_IO₂):

In real life, the pulmonary circulation is in intimate contact with the lungs and is continuously removing O₂ from the alveoli. The alveolar partial pressure of O₂ is therefore equal to the partial pressure in the inspired air minus the amount removed.

If the exchange of oxygen for carbon dioxide were a 1:1 swap then the amount of O₂ removed would equal the amount of CO₂ added to the alveoli and the equation would become:

The CO₂:O₂ exchange, as already discussed, is, however, not usually 1:1. The RQ is usually taken to be 0.8.

Thus:

As CO₂ is a very soluble gas, P_ACO₂ is virtually the same as P_aCO₂. P_aCO₂ (available from the blood gas measurement) can therefore be used in the equation in place of P_ACO₂:

This is (the simplified version of ) the alveolar gas equation. If P_IO₂ is known then P_AO₂ can be calculated.

But, so what? What do we do with the P_AO₂?

Unlike in the case of CO₂, there is normally a difference between alveolar and arterial PO₂ (which should be the greater?). The difference P_AO₂ − P_aO₂ is often written P_A–aO₂ and is known as the alveolar–arterial (A–a) gradient. In healthy young adults, breathing air, this gradient is small; it would be expected to be comfortably less than 2 kPa. If the gradient is greater than this then the abnormality in the blood gas result cannot be accounted for by a change in ventilation alone; there must be an abnormality intrinsic to the lung or its vasculature causing a disturbance of V/Q matching. For examples, see the multiple choice questions at the end of the chapter.