A a gradient fin

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. G enerally- 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 = P A O2 - P a O2 ( 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 = P A O2 - P a O2 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 – P aCO2 / 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_gradient.html https://www.openanesthesia.org/pulmonary_physiology_and_respiratory_failure / https://www.youtube.com/watch?v=hKACkc5aUTE&list= PLQ_IRFkDInv9INje6o21 http://gpnotebook.co.uk/simplepage.cfm?ID=436600836PxV1pIpHV_QrT&index= 1