Presentation on theme: "Why is hypoxemia more common than hypercarbia?"— Presentation transcript:
1 Why is hypoxemia more common than hypercarbia? Tom Archer, MD, MBAUCSD AnesthesiaAugust 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 ShuntLow V/QAlveolar dead spaceDiffusion barrierHigh V/Q
5 Failures of gas exchange ShuntLow V/QAlveolar dead spaceDiffusion barrierDon’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.”ShuntLow V/QAlveolar dead spaceDiffusion barrierHigh V/Q
7 Alveolar dead space and high V/Q alveoli “Wasted ventilation”Does not cause hypoxemiaHallmark is ETCO2 << PaCO2Alveolar 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 HgWith no alveolar dead spaceETCO2 = 20 mm HgWith 50% alveolar dead space20404020Alveolar dead space gas (with no CO2) dilutes other alveolar gas.404040464646
9 Alveolar dead spaceIn normal, non-pregnant adult, PaCO2- ETCO2 = 3-5 mm Hg.In normal pregnancy PaCO2 – ETCO2 < 3, because of increased blood volume and pulmonary perfusion.
11 Failures of gas exchange Cause hypoxemia. Cause increased “A-a gradient for oxygen.”ShuntLow V/QAlveolar dead spaceDiffusion barrierHigh V/Q
12 Hypoxemia Always think of mechanical problems first: Mainstem intubationPartially plugged (blood, mucus) or kinked ETT.Disconnect or other hypoventilationLow FIO2Pneumothorax
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 plugsExternal compression of lung causing atelectasis and shunt.Obesity, Trendelenburg, ascites, surgical packs, pleural effusionParenchymal disease (V/Q mismatch and shunt)Asthma, COPD, pulmonary edema, ARDS, pneumonia,Tumor, fibrosis, cirrhosisIntra-cardiac RL shunts (ASD, VSD, PDA)
15 Hypoxia occurs more easily than hypercarbia. Why?
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 HgpO2 = 130 mm HgSaO2 = 80%SaO2 = 100%SaO2 = 75%SaO2 = 75%pO2 = 50 mm HgpO2 = 40 mm HgpO2 = 130 mm HgpO2 = 40 mm HgCan 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) alveolusSaO2 = 75%Normal alveolusSaO2 = 96%High V/Q alveolusSaO2 = 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 HgpCO2 = 36 mm HgpCO2 = 44 mm HgpCO2 = 46 mm HgpCO2 = 36 mm HgpCO2 = 46 mm HgCan 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 BaselineMethacholineAlbuterolHe3 MR scans – methacholine produces ventilation defects, corrected by albuterol.Modified by Archer TL 2007