Ventilation-Perfusion Relationships Flashcards
What occurs when the V/Q ratio is less than 1?
V/Q ratio < 1 - perfusion exceeds ventilation
What occurs when the V/Q ratio is greater than 1?
V/Q ratio > 1 - ventilation exceeds perfusion
What is the normal V/Q value?
0.8
What is the V/Q ratio in the perfect lung?
1
What does the V/Q ratio represent at the level of the alveoli?
It is the ratio of alveolar ventilation to capillary flow
What does the V/Q ratio represent at the level of the whole lung?
It is the ratio of total alveolar ventilation to cardiac output
What happens to the V/Q ratio when ventilation is obstructed?
The V/Q will decrease.
PO2 levels in the lung will decrease and PCO2 will increase.
What happens to the V/Q ratio when perfusion is obstructed?
The V/Q will increase.
PO2 levels in the lung will increase and the PCO2 will decrease.
Which part of the lung receives the best ventilation??
Bottom
Which part of the lung receives the best perfusion?
Bottom
What happens to the V/Q ratio as you go from the bottom of the lung to the top of the lung?
The V/Q ratio will increase towards the top of the lung as the ventilation decreases more slowly that the perfusion from the base to the apex.
Why is the PO2 of blood leaving the lung lower than that of inspired air?
Most of it comes from the base of the lungs which has a lower PO2 than the apex of the lungs.
What is the normal arterial-alveolar PO2 difference (AaDO2)?
<15 mmHg
What are some of the reasons for the AaDO2?
V/Q Inequality Anatomic Shunt (Thebesian Vessels)
What is the AaDO2 equation?
AaDO2 (alveolar) = [PiO2 (inspired) - (PCO2/R)] - PaO2 (measured)
What is the PO2 level in hypoxemia?
<80 mmHg PO2
What are the causes of hypoxemia?
Hypoventilation
Diffusion Limitation
Shunted Blood
Ventilation-Perfusion Inequality
How does hypoventilation result in hypoxemia?
It will decrease PO2 and increase PCO2 which will result in a decrease of PAO2 (alveolar).
How does hypoventilation affect the AaDO2?
It should not affect it as the gas is diffusing properly
How does diffusion limitation result in hypoxemia?
It will increase the AaDO2 and make it more difficult for O2 to diffuse into the blood
How does an anatomical or physiological shunt result in hypoxemia?
Blood can enter the arterial system being mixed with “dirty blood” or without going through ventilated areas.
Can increasing the O2 help to increase the PO2 in shunt hypoxemia?
No because it will not affect the shunted area. This is the ONLY hypoxemia that cannot be helped with increased O2.
What is the most frequent cause for hypoxemia?
Ventilation-Perfusion Inequality
How does ventilation-perfusion inequality result in hypoxemia?
It causes either a lack of adequate ventilation for the blood flow, or a lack of adequate blood flow for the ventilation and not enough O2 reaches the blood.
Can hypoxemia from ventilation-perfusion inequality be eliminated by increasing ventilation?
No.
How are PCO2 levels affected by VP inequality?
CO2 levels can be fixed with increased ventilation from chemoreceptors, whereas O2 cannot. Therefore CO2 levels should be normal
O2 dissociation curve is flat so only low V/Q inequality ratios will benefit from increased ventilation.
How are PO2 levels affected by VP inequality?
It will have decreased levels that cannot be fixed with hyperventilation. This is because O2 is near 100% saturation at normal breathing and increasing ventilation would not increase the O2 to the areas of inequality very much.
This is similar to the shunt problem.
What are the dead space units?
They are regions at the top of the lung where it is over-ventilated
What are the physiological shunt units?
They are regions at the base of the lung where turnover of alveolar gas is less frequent than normal
