Ventilation Perfusion Relationships Flashcards
What are the 4 mainc auses of hypoxemia?
- hypoventilation
- diffusion limitation
- left to right shunts
- ventilation-perfusion mismatch
What is the oxygen level in air? in the alveoli? in tissues?
150 mmHg in air
100 in alveoli (since some has gone to the blood)
1-100 in tissues
How do you maintain the 100 mmHg in the alveoli if O2 is being taken into the blood?
it’s constantly being replenished by inhalation of fresh air - if it’s not replenished fast enough in the alveoli, the alveolar pO2 will decline and you get hypoxia
What are some causes of hypoventilation?
opiates, chest wall damage, paralysis
pO2 will decrease in hypoventilation, but what happens fo pCO2?
will increase
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What is the respiratory quotient?
it’s produced CO2/consumed O2
it depends on substrate - 1 with carbohydrates as substrate, 0.7 with fatty acid substrate
How do you impair diffusion?
reduce the area available for gas exchange or increase the diffusion distance
Describe how a shunt can naturally occur.
Mixing of O2-depleted blood from the bronchial circulation with a small amount of blood from the thebesian veins of the heart.
What are ways a shunt can occur abnormally?
congenital cardiac abnormalities and PDA
How do you calculate shunt flow?
Qs/Qt = (CcO2 - CaO2 )/ (CcO2 - CVo2)
Can 100% supplemental O2 correct the hypoxemia resulting from a shunt?
nope
What’s the major cause of hypoxia in lung diseases?
mismatch of ventilation and perfusion
If there is no perfusion of a unit but the alveoli are still ventilation, what will the pO2 and pCO2 of the alveoli be?
they will approach that of inspired gas = 100 mmHg O2 and 0 mmHg of CO2
If there is perfusion but no ventilation, what will the pO2 and pCO2 of the alveoli be?
they will approach that of venous blood = 40 mmHg O2 and 46 mmHg CO2
Describe how shunts are represented by oxygen/carbon dioxide curves
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Describe the gradation of ventilation-perfusion differences thorugh the lung. Why do these occur?
The aprex has a high ventilation-perfusion ratio (pO2 132 pCO2 28) while the base has a low ventilation-perfusion ratio (PO2 89, pCO2 42)
this occurs because the apex has more oxygen, but lower perfusion because of gravity while the base has higher perfusion but less O2
Why does pH dramatically change from apex to base?
because pH is largely influenced by CO2 and CO2 is higher at the base than the apex. More CO2, more buffering to increase [H], so the base has a lower pH than the apex
Normally ventilation perfusion mismatches produce only a slight lowering of arterial pO2. WHen is it dramatic?
in lung disease
Which can rapid breathing correct - the hypoxia or hypercapcnia?
can’t fix the hypoxia completely but it can fix the hypercapnia by flowing off the CO2 since CO2 elimination is a function of breathing rate
Why are O2 and CO2 responses to rapid breathing different?
it’s a function of the different shapes of their dissociation curves:
CO2 dissodiation is linear in the physiological range
O2 dissociation is sigmoidal and almost flat at the top of the range
How do you assess ventilation-perfusion abnormalities by calculation?
Use the alveolar gas equation to calculate what arterial pO2 should be and compare it to what is observed…
What is the alveolar gas equation?
PaO2 = PiO2 - (PaCO2/R)
O2 in the alveolus = pressure of O2 in inspired air minus the pressure oc CO2 in the alveolus divided by the respiratory coefficiency
