1. Why is hypoxemia more common than hypercarbia?
Tom Archer, MD, MBA
UCSD Anesthesia
August 20, 2012

2. The dance of pulmonary physiology— Blood and oxygen coming together.
The dance of pulmonary physiology—
Blood and oxygen coming together.

3. But sometimes the match between blood and oxygen isn’t perfect!

4. Failures of gas exchange
Shunt
Low V/Q
Alveolar dead space
Diffusion barrier
High V/Q

5. Failures of gas exchange
Shunt
Low V/Q
Alveolar dead space
Diffusion barrier
Don’t cause hypoxemia. Do cause increased PaCO2 – ETCO2 gradient.
High V/Q

6. Failures of gas exchange
Cause hypoxemia. Cause increased “A-a gradient for oxygen.”
Shunt
Low V/Q
Alveolar dead space
Diffusion barrier
High V/Q

7. Alveolar dead space and high V/Q alveoli
“Wasted ventilation”
Does not cause hypoxemia
Hallmark is ETCO2 << PaCO2
Alveolar gas without any CO2 dilutes expired alveolar gas which contains CO2, thereby decreasing (mixed) ETCO2.

8. Alveolar dead space gas (with no CO2) dilutes other alveolar gas.
ETCO2 = 40 mm Hg
With no alveolar dead space
ETCO2 = 20 mm Hg
With 50% alveolar dead space 20 40 40 20
Alveolar dead space gas (with no CO2) dilutes other alveolar gas. 40 40 40 46 46 46

9. Alveolar dead space
In normal, non-pregnant adult, PaCO2- ETCO2 = 3-5 mm Hg.
In normal pregnancy PaCO2 – ETCO2 < 3, because of increased blood volume and pulmonary perfusion.

10. Shunt and low V/Q alveoli do cause hypoxemia

11. Failures of gas exchange
Cause hypoxemia. Cause increased “A-a gradient for oxygen.”
Shunt
Low V/Q
Alveolar dead space
Diffusion barrier
High V/Q

12. Hypoxemia Always think of mechanical problems first:
Mainstem intubation
Partially plugged (blood, mucus) or kinked ETT.
Disconnect or other hypoventilation
Low FIO2
Pneumothorax

13. For hypoxemia: Hand ventilate and feel the bag! Examine the patient!
Look for JVD.
Do not Rx R mainstem intubation with albuterol!
Do not Rx narrowed ETT lumen with furosemide!
Consider FOB and / or suctioning ETT with NS.

14. Hypoxemia from shunt or low V/Q alveoli:
Mainstem intubation / mucus plugs
External compression of lung causing atelectasis and shunt.
Obesity, Trendelenburg, ascites, surgical packs, pleural effusion
Parenchymal disease (V/Q mismatch and shunt)
Asthma, COPD, pulmonary edema, ARDS, pneumonia,
Tumor, fibrosis, cirrhosis
Intra-cardiac RL shunts (ASD, VSD, PDA)

15. Hypoxia occurs more easily than hypercarbia.
Why?

16. The strong alveolus (high V/Q)

17. The weak alveolus (low V/Q).

18. A key question:
Can the high V/Q alveolus make up for the low V/Q alveolus?
No, for O2.
Yes, for CO2.

19. Can the high V/Q alveolus compensate for the low V/Q alveolus?
pO2 = 50 mm Hg
pO2 = 130 mm Hg
SaO2 = 80%
SaO2 = 100%
SaO2 = 75%
SaO2 = 75%
pO2 = 50 mm Hg
pO2 = 40 mm Hg
pO2 = 130 mm Hg
pO2 = 40 mm Hg
Can the high V/Q alveolus compensate for the low V/Q alveolus?
Not for oxygen! The high V/Q alveolus can’t saturate hemoglobin more than 100%.
SaO2 of equal admixture of high and low V/Q alveolar blood = 90%. PaO2 = 60.

20. Low V/Q) alveolus
SaO2 = 75%
Normal alveolus
SaO2 = 96%
High V/Q alveolus
SaO2 = 99%
Equal admixture of blood from low and high V/Q alveoli has SaO2 = ( )/ 2 = 87%.
Modified by Archer TL 2007

21. Can the high V/Q alveolus compensate for the low V/Q alveolus?
pCO2 = 44 mm Hg
pCO2 = 36 mm Hg
pCO2 = 44 mm Hg
pCO2 = 46 mm Hg
pCO2 = 36 mm Hg
pCO2 = 46 mm Hg
Can the high V/Q alveolus compensate for the low V/Q alveolus?
Yes, for CO2! The high V/Q alveolus can blow off extra CO2.
PaCO2 = 40 mm Hg

22. Hypoxemia is more common than hypercarbia
High V/Q alveoli compensate for low V/Q alveoli for CO2– but cannot compensate with respect to O2!
Hence, when there is V/Q mismatch, hypoxemia will occur long before hypercarbia occurs.

23. He3 MR showing ventilation defects in a normal subject and in increasingly severe asthmatics.
Author Samee, S ; Altes T ; Powers P ; de Lange EE ; Knight-Scott J ; Rakes G Title Imaging the lungs in asthmatic patients by using hyperpolarized helium-3 magnetic resonance: assessment of response to methacholine and exercise challenge Journal Title Journal of Allergy & Clinical Immunology Volume 111   Issue 6   Date 2003   Pages:

24. Baseline
Methacholine
Albuterol
He3 MR scans – methacholine produces ventilation defects, corrected by albuterol.
Modified by Archer TL 2007

26. 100% O2 corrects hypoxemia due to low V/Q
100% O2 corrects hypoxemia due to low V/Q. 100% O2 does not correct hypoxemia due to shunt.

27. Normal gas exchange, V/Q = 1, FIO2 = 0.21
Inspired PO2 = 140 mm Hg
PAO2 = 100 mm Hg
Sat % = 97%
Sat % = 75%

28. FIO2 = 0.21 does not allow saturation of hemoglobin in low V/Q alveoli.
Inspired PO2 = 140 mm Hg
Low V/Q
PAO2 = 50 mm Hg
Saturation = 80%
Saturation = 75%

29. 100% O2 allows saturation of hemoglobin in low V/Q alveoli.
Inspired PO2 = 600 mm Hg
Low V/Q
PAO2 = 100 mm Hg
Sat % = 97%
Sat % = 75%

30. 100% O2 will not correct hypoxemia due to shunt.

31. Shunt prevents saturation of hemoglobin regardless of inspired FIO2.
Inspired PO2 = 600 mm Hg
Shunt, V/Q = 0
PAO2 = 40 mm Hg
Saturation = 75%
Saturation = 75%

32. The End