V/Q Relationship & Hypoxemia Flashcards
The Alveolar Gas Equation for PAO2
[FiO2(barometric-47)]-(PACO2/R)
Causes of hypoxemia
Low PiO2 (high altitude) Shunts Hypoventilation Diffusion impairment (infiltrates) V/Q mismatch
If the atmospheric pressure is 486 mmHg, patient PACO2 is 40 mmHg, what is their PAO2 with no supplemental oxygen?
~42
0.21*(486-47)-(40/0.8)
What does the human body do to acclimatize to higher altitudes (hypoxic conditions)
Hyperventilation
Polycythemia
Disphosphoglycerate
Can supplemental oxygen help in hypoventilation?
Yes, rectifies hypoxemia but can still have high CO2 and low pH that needs attention
Hemoglobin is almost saturated even at what PaO2?
70-75
How does diffusion impairment across the alveoli cause an inability to saturate completely?
Reduced equilibration reserve with thickened diffusion barrier (edema or fibrosis), so doesn’t completely saturate in time
Can supplemental oxygen help in a case of diffusion impairment (edema or fibrosis)?
Yes, increases pressure gradient and aids equilibration reserve
Single most important cause of hypoxemia
V/Q mismatch
Is supplemental oxygen helpful in a case of an anatomic shunt?
No, won’t respond (Hb from good alveoli is already saturated and “extra” oxygen dissolved in blood is not enough to compensate)
If the V/Q value lies closer to 0, then that means you have what abnormality?
Poor ventilation (Bronchitis, CF, etc.)
If the V/Q value lies closer to infinity, then that means you have what abnormality?
Alveolar dead space that creates physiologic dead space (poorly perfused: emphysema, PE)
Why does V/Q rise dramatically during exercise?
Ventilation and perfusion both increase, but ventilation much more so
Which cause of hypoxemia does not result in an increase in the A-a gradient?
Hypoventilation (and also carbon monoxide poisoning)
Is supplemental oxygen helpful in a case of V/Q mismatch (bronchitis, asthma, cystic fibrosis)?
Yes
Anatomic vs. physiologic dead space
Anatomic dead space: portion of lung that doesn’t exchange gases (is fixed)
Physiologic dead space: includes anatomic dead space and any alveolar dead space (variable)
What is alveolar/pathologic dead space?
Alveoli are being ventilated, but there is no perfusion (so it is wasted); seen in the apex of the lung and with PE
Alveolar dead space (increases/decreases) with exercise
Decreases: upper lobes who usually receive minimal perfusion (alveolar dead space) are now more heavily perfused
Why are minute ventilation and alveolar ventilation different?
Minute ventilation is RR x TV
Alveolar ventilation is RR x (TV-anatomic dead space)
The normal movement tendency of the lung is (inward/outward)
Inward
The normal movement tendency of the chest wall is (inward/outward)
Outward
If you see an A-a gradient of >15 in the presence of hypoxemia, you should be thinking of what pathologies?
Right-to-left shunt
V/Q mismatch
Diffusion limitation
Relationship of pressures for alveoli/arterioles/venules in the apex of the lung
PA>Pa>Pv (blood flow is least, so pinched off)
Relationship of pressures for alveoli/arterioles/venules in the base of the lung
Pa>Pv>PA (highest blood flow)
V/Q= 0 means what?
Complete shunt (perfusion but no ventilation) (asthma or obstruction) (inc. oxygen won’t help)
V/Q= infinity means what?
Dead space ventilation (ventilation but no perfusion) (PE) (oxygen will help)
A physiologist divides the lung into three zone (apex, middle, and base). In an experiment, he applies a small amount of positive pressure ventilation while studying the blood flow in the different zones. What will he note in the apex (assuming the patient is standing)?
Blood flow will be reduced in the apex. Blood flow is already the least in this area (V/Q close to infinity) and inc. pressure ventilation will inc. alveolar pressure and compress capillaries even further
