Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1108 page)

   Alveolar hypoxia (e.g., high altitude, gaseous inhalation).

   Pulmonary diffusion abnormalities (e.g., interstitial lung disease): Supplemental oxygen usually improves pO
₂
.

   Right-to-left shunt: Supplemental oxygen has no effect; requires positive end-expiratory pressure.

   Congenital anomalies of the heart and great vessels

   Acquired (e.g., ARDS)

   Ventilation—perfusion mismatch: Supplemental O
₂
usually improves pO
₂
.

   Airflow obstruction (e.g., COPD, asthma)

   Interstitial inflammation (e.g., pneumonia, sarcoidosis)

   Vascular obstruction (e.g., PE)

   Decreased venous oxygenation (e.g., anemia).

   Cyanosis is clearly visible at pO
₂
<40 mm Hg; may be seen at 50 mm Hg depending on skin pigmentation.

   Limitations

   Capillary blood is not suitable for estimation of high arterial pO
₂
values.