Explaining the Aa gradient Using analogy of underground trains (for blood flow) and passengers (for oxygen). Peer reviewed on 26/07/17

1. A a Gradient
26/07/2017
Nicholas Parkinson

2. Definition and purpose
• Definition:
- Alveolar-arterial gradient (A-a gradient) is a
measurement of the difference between the
alveolar concentration of oxygen and the arterial
concentration of oxygen.
Purpose:
- Help differentiate between causes of hypoxemia.

3. Hypoxemia
• 5 causes:
– Hypoventilation
– High Altitude
– VQ mismatch (pneumonia, PE, COPD)
– Shunt
– Diffusion (infiltration, pulmonary fibrosis)
– Analogy of passengers as oxygen
– Analogy of blood flow as tube.

4. Hypoventillation / high altitude
• Low availability of Alveolar oxygen (few
passengers)
• Eg. Narcotics, alcohol, head injury.

5. VQ mismatch
• Normal and pathological
• Normal – apex VQ = 3. Bases 0.6.
• V (ventilation) and Q (blood flow) increase on
descent but Q mostly.

6. VQ mismatch
• Pathological
• Oxygen: blood flow to oxygen mismatch
• Eg 1. Pneumonia (reduced flow and oxygen,
2.PE (flow blockage), 3. COPD. (air trapping)

7. Diffusion
• Delayed transfer O2 to capillaries.
• E.g. interstitial disease – pulmonary fibrosis.

8. Shunt
• Flow but NO oxygen.
• E.g. Shunt (pulmonary oedema, ARDS, congenital
heart). Aa Gradient, oxygen reduced benefit.

9. Aa gradient increase causes
• ✔ VQ mismatch, Shunt and reduced diffusion
• ✗ Hypoventilation and altitude.

10. Use complications
• Age: Aa gradient increases with age 
diffusion.
- So need to calculate expected Aa for age and
compare with calculated value.
Assumes normal breathing rate:
- So need to account for very low respiration
rate. Low respiration rates correlate with higher
CO2.

11. Equations 5 steps
• 1. Establish what is normal for age.
• 2. Get PaO2 and PaCO2 (arterial O2 and CO2)
from ABG. (UK uses KPa, US prefers mmHg)
• 3. Calculate PAO2 – i.e. Alveolar O2 pressure.
• 4. Difference of 2 and 3 is Aa gradient.
• 5. Compare Aa gradient with age expected.

12. Normal for age
• Age worsens i.e increases Aa Gradient.
• Generally- 1-3 KPa.
– (Age /4 +4) / 7.5 = expected Aa gradient.
• So 60 years = 19/7.5 = 2.53

13. Get ABG
• Gives PaO2
• Gives PaCO2

14. Calculate PAO2 (Alveolar)
– PAO2 = (Patm - Pwater) FiO2 - PaCO2/.8
– (Downward atmospheric pressure – upwards
water pressure in alveoli) x fraction of oxygen in
air)
– (Patm - Pwater) FiO2 at sea level is because
(100-6.3) x 0.21 = 19.8KPa
– Then deduct the arterial CO2 which competes for
space crossing alveoli / constant 0.8

15. Calculate Aa gradient
• Gas equation:
– A-a gradient = PAO2 - PaO2
– (Alveolar Oxygen pressure – arterial oxygen
pressure)

16. Compare
• Expected Aa gradient with expected.
• Act!

17. Example part 1.
• 40 year old man. Hypoxic – expected Aa
gradient?
• (Age / 4 + 4)/ 7.5
• 40/4 +4 = 14/7.5= 2
• More than 2 implies more impedance of gas
exchange.

18. Example part 1 PA02
• We expect normal Aa gradient is 2 or less, next find
PAO2
• What is the O2 pressure in the Alveoli (PAO2) if a 40-
year-old man is breathing room air, at sea level, and his
ABG is: 7.48/4/13.3 (pH/PaCO2/PaO2)?
PAO2 = (Patm - Pwater) FiO2 - PaCO2/.8
• PAO2 = 20 KPa - PaCO2/0.8
• = 20 KPa – (4 mm KPa/0.8)
• = 20KPa – 5KPa = 15

19. Example 1
• What is the A-a gradient of our 40-year-old
with PAO2 of 15 KPa?
• His ABG is: 7.48/4/13.3 on room air
(pH/PaCO2/PaO2)
• A-a gradient = PAO2 - PaO2
• 15-13.3 = 1.7.
• 1.7 is better than expected Aa 2. So no VQ,
shunt or diffusion issues!

20. Example 2
• 20 year old girl @sea level– OD codeine.
(20/4 + 4) = 9/7.5= 1.2
• ABG 7.21/10/5.46 pH/CO2/O2
• PAO2 = 20KPa – PaCO2/ 0.8
• PAO2 = 20 KPa – PaCO2/0.8 = 12.5
• Aa = PA02 – PaO2. 12.5 – 10 = 2.5.
• Expected 1.2.
• So is this hypoventilation alone?

21. Application of Aa gradient
• So approach?
– Presentation? Aa Gradient normal. O2 benefit?
Bloods? Radiology?
– Aa Gradient, O2 beneficial?
– Yes yes – high altitude or hypoventilation.
– No No – Shunt
– No yes – VQ mismatch.

22. References
• http://lifeinthefastlane.com/ccc/a-a-gradient/
• http://missinglink.ucsf.edu/lm/abg/abg1/a_a_gra
dient.html
• https://www.openanesthesia.org/pulmonary_ph
ysiology_and_respiratory_failure/
• https://www.youtube.com/watch?v=hKACkc5aUT
E&list=PLQ_IRFkDInv9INje6o21
• http://gpnotebook.co.uk/simplepage.cfm?ID=436
600836PxV1pIpHV_QrT&index=1