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

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Presentation on theme: "Why is hypoxemia more common than hypercarbia? Tom Archer, MD, MBA UCSD Anesthesia August 20, 2012."— Presentation transcript:

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. ml

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

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

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

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

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 0 20 ETCO2 = 20 mm Hg With 50% alveolar dead space Alveolar dead space gas (with no CO2) dilutes other alveolar gas.

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

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 The low V/Q alveolusThe high V/Q alveolus 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. pO2 = 50 mm Hg SaO2 = 75% pO2 = 50 mm Hg SaO2 = 80% SaO2 = 75% SaO2 = 100% pO2 = 130 mm Hg pO2 = 40 mm HgpO2 = 130 mm Hg pO2 = 40 mm Hg

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

21 The low V/Q alveolusThe high V/Q alveolus 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 pCO2 = 44 mm Hg pCO2 = 36 mm Hg pCO2 = 46 mm HgpCO2 = 36 mm Hg pCO2 = 46 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 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: He3 MR showing ventilation defects in a normal subject and in increasingly severe asthmatics.

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

25

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 % = 75% Sat % = 97%

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

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

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

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

32 The End


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