Why is hypoxemia more common than hypercarbia?
Tom Archer, MD, MBAUCSD AnesthesiaAugust 20, 2012
The dance of pulmonary physiology—
Blood and oxygen coming together.
www.argentour.com/tangoi.html
http://www.bookmakersltd.com/art/edwards_art/3PrincessFrog.jpg
But sometimes the match between blood and oxygen isn’t perfect!
Alveolar dead space
High V/Q
Shunt
Low V/Q
Diffusion barrier
Failures of gas exchange
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.
Alveolar dead space
High V/Q
Shunt
Low V/Q
Diffusion barrier
Failures of gas exchangeCause hypoxemia. Cause increased “A-a gradient for oxygen.”
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.
46046
4640
40
4040
40
ETCO2 = 40 mm HgWith no alveolar dead space
0
20
20
ETCO2 = 20 mm HgWith 50% alveolar dead space
Alveolar dead space gas (with no CO2) dilutes other alveolar gas.
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.
Shunt and low V/Q alveoli do cause hypoxemia
Alveolar dead space
High V/Q
Shunt
Low V/Q
Diffusion barrier
Failures of gas exchangeCause hypoxemia. Cause increased “A-a gradient for oxygen.”
Hypoxemia
• Always think of mechanical problems first:
– Mainstem intubation– Partially plugged (blood, mucus) or kinked ETT.– Disconnect or other hypoventilation– Low FIO2– Pneumothorax
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.
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 RL shunts (ASD, VSD, PDA)
Hypoxia occurs more easily than hypercarbia.
Why?
The strong alveolus (high V/Q)
The weak alveolus (low V/Q).
A key question:
• Can the high V/Q alveolus make up for the low V/Q alveolus?
• No, for O2.
• Yes, for CO2.
The low V/Q alveolus The 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 Hg pO2 = 130 mm Hg pO2 = 40 mm Hg
http://www.biotech.um.edu.mt/home_pages/chris/Respiration/oxygen4.htmModified by Archer TL 2007
Low V/Q) alveolus SaO2 = 75%
High V/Q alveolus SaO2 = 99%
Normal alveolusSaO2 = 96%
Equal admixture of blood from low and high V/Q alveoli has SaO2 = (75 + 99)/ 2 = 87%.
The low V/Q alveolus The 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 = 44 mm Hg
pCO2 = 36 mm Hg
pCO2 = 46 mm Hg pCO2 = 36 mm Hg pCO2 = 46 mm Hg
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.
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
He3 MR showing ventilation defects in a normal subject and in increasingly severe asthmatics.
Baseline Methacholine Albuterol
Modified by Archer TL 2007
He3 MR scans – methacholine produces ventilation defects, corrected by albuterol.
100% O2 corrects hypoxemia due to low V/Q.
100% O2 does not correct hypoxemia due to shunt.
Normal gas exchange, V/Q = 1, FIO2 = 0.21
Inspired PO2 = 140 mm Hg
PAO2 = 100 mm Hg
Sat % = 75%Sat % = 97%
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%
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%
100% O2 will not correct hypoxemia due to shunt.
Shunt, V/Q = 0
Shunt prevents saturation of hemoglobin regardless of inspired FIO2.
Inspired PO2 = 600 mm Hg
PAO2 = 40 mm Hg
Saturation = 75%Saturation = 75%
The End