From Dr. John B. Downs, University of Florida Department of Anesthesiology (added Aug 4, 2008).
There are 6 main factors that will lower the arterial partial pressure of oxygen (PaO2):
a. Decreased alveolar ventilation
b. Reduced ventilation/perfusion ratio
c. Increased shunt fraction
d. Diffusion defect
e. Decreased barometric pressure
f. Decreased inspired fraction of oxygen (FiO2)
An increased FiO2 will increase PaO2 in the face of all 6 physiologic derangements that lower PaO2.
Therefore, increased FiO2 may delay or prevent diagnosis of these derangements. This is especially true of decreased PaO2 secondary to hypoventilation.
Although clinicians aggressively respond to even mild arterial hypoxemia, it has been noted that acute hypoventilation can be detected with 100% accuracy when supplemental O2 is avoided. Thus, breathing room air will allow accurate detection by pulse oximetry of acute hypoventilation long before detrimental levels of PaCO2 occur.
For example, click on this link to view an interactive model of the alveolar gas equation that depicts hypoventilation on room air (PaCO2 = 70 mm Hg; FiO2 = 0.21). The SpO2 value derived from the pulse oximeter is 92 and would alert the clinician of the hypoxemia.
On the other hand, click on this link for the same hypoventilation situation with supplemental oxygen (PaCO2 = 70 mm Hg; FiO2 = 0.3). The SpO2 reported by the pulse oximeter is 100. Effectively, the supplemental O2 has masked the hypoventilation.
Click this link for instructions on using the interactive model of the alveolar gas equation.
References
Fu ES, Downs JB, Schweiger JW, Miguel RV, Smith RA: Supplemental oxygen impairs detection of hypoventilation by pulse oximetry. Chest 2004 Nov;126(5):1552-8 - Download PDF of paper