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