14.1: Calculating alveolar PO₂
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The difference in PO2 in the arterial system and the alveoli of the normal lung is minimal (i.e., there is usually no substantial alveolar–arterial PO2 difference). A fall in arterial PO2 is indicative of a problem arising with gas exchange, but knowing whether this fall is accompanied by a growing alveolar–arterial PO2 difference or not is a quick, cheap, and powerful diagnostic tool that can hone you in on the source of the arterial desaturation.
This chapter will describe how the alveolar–arterial PO2 difference is calculated and what assumptions can be made from it.
Before you start, a quick reminder that an uppercase A refers to alveolar and lowercase to arterial.
Calculating Alveolar PO₂
Obviously to measure the alveolar–arterial PO2 difference, we need to know both the alveolar and arterial PO2s. The arterial PO2 is routinely measured as part of a blood gas panel, along with arterial PCO2. However, from your understanding of V/Q distribution across the lung, you might appreciate that the measurement of a "typical" alveolar PO2 is difficult, and it must be calculated as an estimate of the whole lung. This is the role of the alveolar gas equation, and we will look at it now, not just because it may appear on your board exams, but primarily because of its clinical importance. As there are several forms of the equation, we will take the easy way out and use the simplest one (figure 14.1), which is accurate for the vast majority of cases you will ever see.
Figure 14.1: The alveolar gas equation.
The alveolar gas equation estimates whole lung alveolar PO2 as the inspired PO2 minus the arterial PO2 divided by the respiratory exchange ratio. For those interested in the derivation of the equation, more detailed sources are available. But here, we will just look at the factors involved and try and make this simpler to commit to memory (which I suggest you do).
First let us look at arterial PCO2; this measurement is included in a blood gas panel so will be readily available to you. The alveolar gas equation really needs the alveolar PCO2, but since CO2 is so soluble then we assume that equilibration has taken place and PaCO2 and PACO2 are the same, and we use the number we have at the bedside.
Now let us look at R, or the respiratory exchange ratio. The respiratory exchange ratio describes how much CO2 is produced per unit of oxygen consumed. (Perhaps you can see why we are using this in conjunction with the arterial PCO2; we are relating CO2 production as a proxy measurement of oxygen consumption.) When utilizing carbohydrate as a fuel (the most common situation) there are eight CO2 molecules produced for every ten oxygen molecules burnt, so R is generally 0.8. Lastly, there is the inspired PO2. Generally, breathing room air at sea level this will be ~150 mmHg. But it is important to note that this might change in the clinic if the patient is given oxygen therapy.
So this simple form of the alveolar gas equation really has two basic halves: the amount of oxygen taken into the alveoli (PIO2), and a reflection of the amount that is taken out (PaCO2/R) to supply metabolism.
You will find more complex and accurate forms of this equation, but for the vast majority of situations this one is perfectly adequate and is considerably easier to remember, particularly when some of the numbers we plug in are frequently the same. If we look at normal values (equation 14.1.1), we see that our equation gets us close to what we have learned to be a normal alveolar PO2. Inspired PO2 at sea level and room air is 150 mmHg, we will assume R is 0.8, and a normal arterial PCO2 is 40 mmHg. Here is the alveolar gas equation with normal values:
\[P_AO_2 = 150 - \frac{40}{0.8} = 100 \nonumber \]
Now let us see the clinical use of being able to determine alveolar PO2 and thus calculate any alveolar–arterial PO2 difference.