A-a (O2) = (FiO2%/100) * (Patm
- 47 mmHg) - (PaCO2/0.8) - PaO2
Where:
FiO2
Room Air = 21 %
Atmospheric Pressure= 760 mm Hg at sea level
Water vapor pressure pH2O
(mmHg) = 47 mm Hg at 37 degrees Celsius
Respiratory quotient RQ (VCO2/VO2) = 0.8 (usual)
Normal range increases with age. 5 to 20 is normal up to middle age
Hypoxemia with a normal gradient suggests:
- Hypoventilation (decreased respiratory
drive or neuromuscular disease)
- Low FiO2
Hypoxemia with an increased gradient suggests:
- Ventilation-perfusion imbalance -also known as V/Q mismatch (asthma, COPD)
- Shunt : Cardiac right to left shunt such as patent foramen ovale,
alveolar collapse, (atelectasis), intraalveolar filling (pneumonia, pulmonary
edema), or intrapulmonary shunt.
Supplemental O2 will help to correct the hypoxemia in hypoventilation
and V/Q mismatch but not hypoxemia
resulting from a shunt.
All calculations must be confirmed before use. The suggested results are not a substitute for clinical
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