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A-a Gradient, Alveolar-Arterial Oxygen Tension Difference, (A-a) PO2 difference, (A-a) DO2, Alveolar Oxygen Equation

• Indications
• Physiology
1. Efficiency of gas exchange between alveolus and artery
1. Reflects the status of the oxygen exchange in the distal lungs (alveoli) and blood
2. A-a Gradient is the "report card of the lungs"
1. An abnormal A-a Gradient suggests the lungs as possible cause of Hypoxemia
2. A normal A-a Gradient suggests causes external to the lungs for hypoexemia (e.g. Bellows Failure, CNS depression)
3. Swadron (2019) Pulmonary, National Emergency Board Review, CCME, accessed 5/24/2019
• Calculation
1. Alveolar Oxygen Equation
1. PAO2 = FIO2 x (Pb - 47) - PaCO2/0.8
1. where PAO2 is the predicted alveolar oxygen based on the arterial CO2
2. where FIO2 on room air = 0.21
3. where Pb is barometric pressure (760 mmHg at sea level)
4. where vapor pressure = 47
5. where PaCO2 is the measured arterial CO2 (nearly identical to alveolar CO2 at end-expiration)
6. where respiratory quotient = 0.8 (based on relative rates of CO2 production and Oxygen Consumption)
2. A-a Gradient (at sea level)
1. A-a Gradient = PAO2 - PaO2
1. where PAO2 is predicted alveolar oxygen pressure (based on Alveolar Oxygen Equation above)
2. where PaO2 is measured arterial oxygen pressure
2. A-a Gradient = FIO2 x (760 - 47) - (PaCO2/0.8) - PaO2
1. where FIO2 on room air = 0.21
2. where atmospheric pressure = 760
3. where water vapor pressure = 47
4. where PaO2 is the measured arterial pressure of oxygen
3. A-a Gradient on room air (FIO2 0.21)
1. A-a Gradient = 150 - (PaCO2/0.8) - PaO2
• Interpretation
• Calculating a normal A-a Gradient
1. A-a Gradient = (Age/4) + 4
2. Young person at sea level
1. A-a increases 5 to 7 mmHg for every 10% increase FIO2
2. Room Air: 10 to 20 mmHg
3. 100% oxygen: 60 to 70 mmHg
3. Increased age affects A-a Gradient (at sea level, on room air)
1. Age 20 years: 4 to 17 mmHg
2. Age 40 years: 10 to 24 mmHg
3. Age 60 years: 17 to 31 mmHg
4. Age 80 years: 25 to 38 mmHg
• Interpretation
• Hypoxemia causes differentiated by A-a Gradient