Presentation on theme: "Why is hypoxemia more common than hypercarbia?"— Presentation transcript:
1Why is hypoxemia more common than hypercarbia? Tom Archer, MD, MBAUCSD AnesthesiaAugust 20, 2012
2The dance of pulmonary physiology— Blood and oxygen coming together. The dance of pulmonary physiology—Blood and oxygen coming together.
3But sometimes the match between blood and oxygen isn’t perfect!
4Failures of gas exchange ShuntLow V/QAlveolar dead spaceDiffusion barrierHigh V/Q
5Failures of gas exchange ShuntLow V/QAlveolar dead spaceDiffusion barrierDon’t cause hypoxemia. Do cause increased PaCO2 – ETCO2 gradient.High V/Q
6Failures of gas exchange Cause hypoxemia. Cause increased “A-a gradient for oxygen.”ShuntLow V/QAlveolar dead spaceDiffusion barrierHigh V/Q
7Alveolar 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.
8Alveolar 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
9Alveolar 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.
11Failures of gas exchange Cause hypoxemia. Cause increased “A-a gradient for oxygen.”ShuntLow V/QAlveolar dead spaceDiffusion barrierHigh V/Q
12Hypoxemia Always think of mechanical problems first: Mainstem intubationPartially plugged (blood, mucus) or kinked ETT.Disconnect or other hypoventilationLow FIO2Pneumothorax
13For 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.
14Hypoxemia 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)
15Hypoxia occurs more easily than hypercarbia. Why?
18A key question:Can the high V/Q alveolus make up for the low V/Q alveolus?No, for O2.Yes, for CO2.
19Can 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.
20Low 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
21Can 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
22Hypoxemia 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.
23He3 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:
24BaselineMethacholineAlbuterolHe3 MR scans – methacholine produces ventilation defects, corrected by albuterol.Modified by Archer TL 2007