The AaDO2 is the difference between the calculated PAO2 and the measured PaO2. In individuals with normal lungs breathing room air, the AaDO2 is usually less than 15 mm Hg. It tends to increase by about 3 mm Hg per decade of life after the age of 30. An AaDO2 below 25 mm Hg is considered within the normal range.

Let's consider a case of an individual with pneumonia who is receiving 30% supplemental oxygen through a face mask. The arterial blood gas analysis shows a pH of 7.40, PaCO2 of 44 mm Hg, and PaO2 of 70 mm Hg. To calculate the patient's AaDO2, we can use the alveolar air equation (Eq. 23.13):

R 44PAO2 = [0.3 × (760 − 47)] − 0.8 = 159 mm Hg

Therefore, the patient's AaDO2 is high, indicating lung disease (in this case, pneumonia).

Abnormalities in PaO2 can occur with or without an elevation in AaDO2. Therefore, the relationship between PaO2 and AaDO2 is useful for determining the cause of an abnormal PaO2 and predicting the response to therapy, particularly supplemental oxygen administration. The causes of a reduction in PaO2 (arterial hypoxemia) and their effect on AaDO2 are listed in Table 23.2. Each of these causes is discussed in more detail in the following sections.

Arterial hypoxemia is defined as a PaO2 below 80 mm Hg in an adult breathing room air at sea level. Hypoxia, on the other hand, occurs when there is insufficient oxygen to carry out normal metabolic functions and is often associated with a PaO2 less than 60 mm Hg. There are four major categories of hypoxia.

The most common category is hypoxic hypoxia, which includes anatomical shunt, physiological shunt, decreased FiO2, V̇ /Q̇ mismatching, diffusion abnormalities, and hypoventilation. Anemic hypoxia is the second category and is caused by a decrease in functioning hemoglobin due to insufficient hemoglobin, abnormal hemoglobin, or interference with the chemical combination of oxygen and hemoglobin (e.g., carbon monoxide poisoning).

Hypoperfusion hypoxia is the third category and occurs due to low blood flow (e.g., decreased cardiac output) and reduced oxygen delivery to the tissues. The fourth category is histotoxic hypoxia, which happens when the cellular machinery that utilizes oxygen to produce energy is poisoned, as in cyanide poisoning. In this case, arterial and venous PO2 may be normal or increased because oxygen is not being utilized.
Eq. 23.13 ). The difference between the calculated PAO2 and the measured PaO2 is the AaDO2. In individuals with normal lungs who are breathing room air, the AaDO2 is less than 15 mm Hg. The mean value rises approximately 3 mm Hg per decade of life after 30 years of age. Hence, an AaDO2 lower than 25 mm Hg is considered the upper limit of normal.
Assume that an individual with pneumonia is receiving 30% supplemental O2 by face mask. Arterial blood gas pH is 7.40, PaCO2 is 44 mm Hg, and PaO2 is 70 mm Hg. What is the patient’s AaDO2? (Assume that the patient is at sea level and the patient’s respiratory quotient is 0.8.) According to the alveolar air equation (
Eq. 23.13
R 44PAO2 = [0 3. × (760 − 47)] − 0 8. = 159 mm Hg
Therefore,
This high AaDO2 suggests that the patient has lung disease (in this case, pneumonia).
Abnormalities in PaO2 can occur with or without an elevation in AaDO2. Hence, the relationship between PaO2 and AaDO2 is useful in determining the cause of an abnormal PaO2 and in predicting the response to therapy (particularly to supplemental O2 administration). Causes of a reduction in PaO2 (arterial hypoxemia) and their effect on AaDO2 are listed in
Table 23.2 . Each of these causes is discussed in greater detail in the following sections.
Arterial Blood Hypoxemia, Hypoxia, and Hypercarbia
Arterial hypoxemia is defined as a PaO2 lower than 80 mm Hg in an adult who is breathing room air at sea level. Hypoxia is defined as insufficient O2 to carry out normal metabolic functions; hypoxia often occurs when the PaO2 is less than 60 mm Hg. There are four major categories of hypoxia. The first, hypoxic hypoxia, is the most common. The six main pulmonary conditions associated with hypoxic hypoxia—anatomical shunt, physiological shunt, decreased FiO2, V̇ /Q̇ mismatching, diffusion abnormalities, and hypoventilation—are described in the following sections and in
Table 23.2 . A second category is anemic hypoxia, which is caused by a decrease in the amount of functioning hemoglobin as a result of too little hemoglobin, abnormal hemoglobin, or interference with the chemical combination of oxygen and hemoglobin (e.g., carbon monoxide poisoning; see the following “
In the Clinic” box). The third category is hypoperfusion hypoxia, which results from low
CHAPTER 23 Ventilation, Perfusion, and Ventilation/Perfusion Relationships
AaDO2, alveolar-arterial difference for oxygen; FiO2, fraction of inspired oxygen; PaO2, partial pressure of arterial oxygen.
blood flow (e.g., decreased cardiac output) and reduced oxygen delivery to the tissues. Histotoxic hypoxia, the fourth category of hypoxia, occurs when the cellular machinery that uses oxygen to produce energy is poisoned, as in cyanide poisoning. In this situation, arterial and venous PO2 are normal or increased because oxygen is not being utilized.