TESTS OF GAS EXCHANGE FUNCTION

Alveolar-Arterial Oxygen Tension Difference

Abnormally high alveolar-arterial oxygen tension gradients (PAO₂ -PaO₂ ) during room air breathing are common in asymptomatic smokers and in patients with minimal signs of chronic bronchitis. Although the PAO₂ -PaO₂ appears to be a sensitive means of detecting regional ventilation-perfusion (V̇/) inequalities, the test is not widely used for screening purposes because of the difficulty in measuring alveolar oxygen tension (PAO₂ ), which must be estimated from the alveolar air equation. The oxygen tension of the warmed and humidified inspired gas in the trachea is represented as PIO₂ . The gas in the alveoli also contains carbon dioxide. As a rule, less carbon dioxide is produced than oxygen consumed. The ratio of carbon dioxide production to oxygen consumption is the respiratory exchange ratio (R), which is usually assumed to be 0.8. A simple calculation of PAO₂ for bedside use may be derived by dividing the arterial carbon dioxide tension (PaCO₂ ) by R or by multiplying the carbon dioxide value by 1.25 and then subtracting the value from PIO₂ . The alveolar gas equation may be written as follows:

PAO₂ = PIO₂ − PaCO₂ /R

The normal PAO₂ -PaO₂ in subjects breathing room air averages 8 mm Hg in young persons and increases linearly with age. By the eighth decade, typical values may reach 25 mm Hg. This widening of the PAO₂ -PaO₂ with age and in disease states results solely from decreases of PaO₂ , not PAO₂ .

The simple bedside measurement of room air arterial O₂ tension (PaO₂ ) provides a useful estimate of lung function. A value of less than 60 mm Hg indicates significant if not advanced lung disease, unless drug-induced hypoventilation is responsible. The PaO₂ has also been used in conjunction with the peak expiratory flow rate (PEFR) for the differentiation of dyspnea of cardiac versus pulmonary origin. This dyspnea differentiation index (DDI) is PEFR × PaO₂ /1000. ^[20] Patients with pulmonary dyspnea typically have lower PEFR values and a lower DDI than cardiac patients with dyspnea.