Pulmonary Physiology: Matching Ventilation and Perfusion Flashcards
What is the difference between hypoxia and hypoxemia?
Hypoxia is a general term for low PO2 levels, whereas hypoxemia refers specifically to low PaO2 (specifically, PaO2 < 85 mmHg)
What are the three general causes of hypoxemia that do NOT affect A-a DO2?
- Hypoventilation from CNS depression, myasthenia gravis, etc.
- Reduced PIO2 (partial pressure of inspired oxygen) - high altitude, etc. Note that the fraction of oxygen is not changed because at higher altitudes the partial pressures of all atmospheric gases are reduced.
- Reduced FIO2 - such as in being caught in a fire; fire burns up all the oxygen.
What are the three general causes of hypoxemia that DO affect A-a DO2?
- Right to left shunts.
- Diffusion abnormalities in alveoli.
- V/Q mismatch (major cause)
Define hypercapnia.
PaCO2 > 45 mmHg
What are four general causes of hypercapnia?
- Decreased alveolar ventilation.
- Increased dead space.
- Increased CO2 production by the body in the setting of fixed ventilation.
- Severe V/Q mismatch - too much blood going to poorly ventilated alveoli.
What is the ideal V/Q value? What is normally seen in health?
Ideal is 1.0. Normally, V/Q = 0.84. It should be between 0.8 and 1.0
When V/Q approaches 0, what does the composition of the gas in the alveoli start to resemble?
The gas in the alveoli in the setting of increased flow and decreased perfusion will resemble the composition of venous blood (PAO2 of 40 mmHg and PACO2 of 46 mmHg)
When V/Q approaches infinity, what does the composition of the gas in the alveoli start to resemble?
The composition will resemble atmospheric air (PAO2 of 150 mmHg and PACO2 of nearly 0 mmHg)
Compare the effect of gravity on alveoli at the apex of the lungs vs the base of the lungs.
The weight of the lungs pulls on the apex, generating a higher negative pressure near the apex, pulling the alveoli open. At the base, the opposite occurs. Therefore, the alveoli at the base of the lungs are able to expand more than those at the apex during inspiration. There is also more blood flow at the base of the lungs (low pressure system).
Describe the consequences of a high V/Q.
Blocked pulmonary blood is shunted to unblocked vessels, increasing flow through other capillaries. The increased flow adds more CO2 and takes out more O2 of the alveoli. The lung compensates via hypocapnic constriction of the lung leading to the blocked alveoli, shunting air to other alveoli to try to correct the gas exchange in the remaining perfused alveoli.
Describe the compensatory mechanism that occurs in the setting of low V/Q.
After PAO2 in the alveoli falls due to airway blockage, pulmonary vessels will undergo hypoxic vasoconstriction to re-route blood to unblocked alveoli.
What is it called when an airway is completely blocked?
A shunt.
How is a shunt diagnosed? Explain how this works.
The patient is given 100% O2 to breathe. If there is a complete shunt, little to no increase will be seen in O2 content of arterial blood because the working alveoli will already have been saturating Hb. If the shunt is incomplete, blood flowing through the affected alveoli will benefit greatly, Hb will bind O2, and a significant increase in arterial O2 content will be seen.
Will a high V/Q in one part of the lung compensate for a low V/Q in another? Explain why or why not.
It will not compensate because lung units with a high V/Q ratio add little oxygen to the blood compared to the decreased oxygen added in alveoli with low V/Q.
