A-a Oxygen Gradient
Interpretation
- Alveolar-arterial (A-a) gradient
- Difference between the alveolar and arterial concentration of oxygen
- Can help identify the mechanism of hypoxemia especially when arterial O2 is measured before and after oxygen therapy
- Normal values:
- Young adult, non-smoker breathing room air: 5–10 mmHg
- Older adults: approximated by (age / 4) + 4. Example: a 40 year old should have an A–a gradient < 14.
- Increased A-a gradient can be caused by:
- Shunt (low arterial O2 not corrected by oxygen therapy)
- Alveolar collapse (atelectasis)
- Filing of alveoli (pneumonia, pulmonary edema)
- Intracardiac shunt (right-to-left shunt)
- Vascular shunt (in lungs)
- Ventilation/perfusion mismatching (low arterial O2 corrected by oxygen therapy)
- Airway disease (COPD, asthma)
- Interstitial lung disease
- Alveolar disease
- Pulmonary vascular disease
- Shunt (low arterial O2 not corrected by oxygen therapy)
Formula
A-a oxygen gradient = PAO2 - PaO2
where:
PAO2 = FiO2 × ( Patm - PH2O ) – ( PaCO2 ÷ 0.8 )
PaO2 = arterial PO2 (measured in arterial blood)
FiO2: inspired oxygen concentration OR
( oxygen administered in L/min × 3 ) + 21
Patm: atmospheric pressure, in mmHg (sea level = 760 mmHg)
PH2O: vapor pressure of water at body temperature (typically 47 mmHg)
References
- Deutsch AB et al: Assessment of the alveolar-arterial oxygen gradient as a screening test for pulmonary embolism in pregnancy. Am J Obstet Gynecol 203:373.e1, 2010 [PMID:20554265]
- Moammar MQ et al: Alveolar-arterial oxygen gradient, pneumonia severity index and outcomes in patients hospitalized with community acquired pneumonia. Clin Exp Pharmacol Physiol 35:1032, 2008 [PMID:18518885]
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