II. Indications
III. Physiology
- Efficiency of gas exchange between alveolus and artery
- A-a Gradient is the "report card of the lungs"
- An abnormal A-a Gradient suggests the lungs as possible cause of Hypoxemia
- A normal A-a Gradient suggests causes external to the lung as hypoexemia cause
- Swadron (2019) Pulmonary, National Emergency Board Review, CCME, accessed 5/24/2019
IV. Calculation
V. Interpretation: Calculating a normal A-a Gradient
- A-a Gradient = (Age/4) + 4
- Young person at sea level
- A-a increases 5 to 7 mmHg for every 10% increase FIO2
- Room Air: 10 to 20 mmHg
- 100% oxygen: 60 to 70 mmHg
- Increased age affects A-a Gradient (at sea level)
- Age 20 years: 4 to 17 mmHg
- Age 40 years: 10 to 24 mmHg
- Age 60 years: 17 to 31 mmHg
- Age 80 years: 25 to 38 mmHg
VI. Interpretation: Hypoxemia causes differentiated by A-a Gradient
- Increased A-a Gradient
- Right to Left Intrapulmonary Shunt (due to fluid filled alveoli)
- V/Q Mismatch (due to lung dead space)
- Alveolar hypoventilation
- Normal A-a Gradient
- Hypoventilation
- Neuromuscular disorders
- Central Nervous System disorder
- Low inspired FIO2 (e.g. high altitude)
- Hypoventilation
VII. References
- Davies (1986) Acute Respiratory Failure, Cyberlog, Cardinal Health Systems, p. 22-3
