Why is hypoxemia more common than hypercarbia?

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

2. www.argentour.com/tangoi.html The dance of pulmonary physiology— Blood and oxygen coming together.

3. But sometimes the match between blood and oxygen isn’t perfect! http://www.bookmakersltd.com/art/edwards_art/3PrincessFrog.jpg

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. 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. 0 40 40 40 46 0 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/Qalveolus make up for the low V/Q alveolus? • No, for O2. • Yes, for CO2.

19. The low V/Q alveolus The high 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 = (75 + 99)/ 2 = 87%. http://www.biotech.um.edu.mt/home_pages/chris/Respiration/oxygen4.htm Modified by Archer TL 2007

21. The low V/Q alveolus The high 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 challengeJournal Title Journal of Allergy & Clinical ImmunologyVolume 111   Issue 6   Date 2003   Pages: 1205-11

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 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