Why is hypoxemia more
common than hypercarbia?
Tom Archer, MD, MBA
UCSD Anesthesia
August 20, 2012
www.argentou
r.com/tangoi.ht
ml
The dance of pulmonary physiology—
Blood and oxygen coming together.
But sometimes the match between blood and
oxygen isn’t perfect!
http://www.bookmakersltd.com/art/edwards_art/3PrincessFrog.jpg
Failures of gas exchange
Shunt
Low V/Q
Alveolar dead space
Diffusion barrier
High V/Q
Failures of gas exchange
Shunt
Low V/Q
Alveolar dead space
Diffusion barrier
High V/Q
Don’t cause hypoxemia. Do cause increased
PaCO2 – ETCO2 gradient.
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
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.
40
ETCO2 =
40 mm Hg
ETCO2 =
20 mm Hg
With no
alveolar
dead space
With 50%
alveolar
dead space
20
40
20
Alveolar dead
space gas
(with no CO2)
dilutes other
alveolar gas.
40
46
40
46
0
0
40
46
Alveolar dead space
• In normal, non-pregnant adult, PaCO2ETCO2 = 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
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
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 RL 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
pO2 = 50 mm Hg
pO2 = 130 mm Hg
SaO2 = 80%
SaO2 = 75%
SaO2 = 100%
SaO2 = 75%
pO2 = 50 mm Hg
pO2 = 40 mm Hg
pO2 = 40 mm Hg
pO2 = 130 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.
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
The low V/Q alveolus
pCO2 = 44 mm Hg
The high V/Q alveolus
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
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.
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: 1205-11
Baseline
Methacholine
Albuterol
He3 MR scans – methacholine produces
ventilation defects, corrected by albuterol.
Modified by Archer TL 2007
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 % = 97%
Sat % = 75%
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%
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%
100% O2 will not correct
hypoxemia due to shunt.
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%
The End