Ventilation Perfusion Flashcards
With respect to terminology, what is the difference between something that is “anatomical” and “physiological”?
Anatomical - born with it that way
Physiological - developed it after birth
With respect to terminology, a subscript that denotes “A” indicates […] and “a” indicates […]
A - alveolus
a - arterial
What is the definition of a V/Q ratio?
The ratio of the amount of air reaching the alveoli per minute to the amount of blood reaching the alveoli per minute
What is a normal value for:
- VA
- Q
- V/Q
- 4 - 5 L/min
- 5L / min
- 0.8 - 1
In general, what is the difference between a shunt and a dead space?
Shunt - problem with ventilation
Dead space - problem with perfusion
What is an anatomical shunt?
Shunts are places where there is decreased ventilation. An anatomical shunt is a place where there was never supposed to be any gas exchange so the blood supply has no contact with structures that participate in gas exchange so ventilation is 0.
Blood bypasses the gas exchange portion of the lungs and as such the V for that blood is 0 so V/Q is 0.
Examples: bronchial arteries, atrial septal defect, ventricular septal defect, etc. In the case of the bronchial arteries, the blood bypasses ventilation in order to supply oxygen to the lung tissue itself.
What is a physiological shunt?
A non-normal shunt that develops after a person is born. Perfusion is normal, but there is a connection that develops within the cardiovascular system that prevents a portion of blood flow from participating in ventilation. As such, V for that blood is 0 and the V/Q is 0.
What is an anatomic dead space?
Dead spaces are places where there is under or no perfusion. An anatomical dead space is one where there was never supposed to be any gas exchange so there is no perfusion of that structure for gas exchange.
A place within the respiratory tract where there is no respiration occuring but that is not pathological. For example, most of the respiratory tract does not pariticipate in respiration (nose to terminal bronchioles) and this is considered anatomical dead space.
What is physiological dead space?
When there is a problem getting blood to flow through the capillaries that participate in gas exchange so the pulmonary unit is unperfused or underperfused, despite normal ventilation. V/Q would be infinite due to Q being 0.
In shunting, V is […] and Q is […] and V/Q is […]
0
normal
0
In dead space, V is […], Q is […] and V/Q is […]
Normal
0
Infinite
What is this image showing?
Physiologic dead space
In case 1, there is underperfusion of the alveolus due to a narrowing of the blood supply that it interacts with. Ventilation would be normal.
In case 2, there is no perfusion of the alveolus due to a loss of blood supply. Ventilation would be normal.
What is this image showing?
An anatomical shunt - there are no alveoli for that blood supply to interact with so there is no gas exchange occuring. This is not pathological in this case.
What is shown in this image?
The left shows a normal alveolus. The right shows two cases of physiological shunting.
In case 1, there is a severe collapse of the alveolus, leading to severely decreased gas exchange and thus decreased ventilation despite normal perfusion.
In case 2, there is a moderate decrease in size of the alveolus leading to decreased gas exchange and thus decreased ventilation despite normal perfusion.
What are the alveolar gas equations we should know?
This image shows an example of an anatomical shunt. What will be the final PaO2 and PaCO2?
We don’t know the proportion of blood that is bypassing gas exchange in the shunt, so we can’t determine exact numbers. But we can reason that PaO2 will be less than 102 and PaCO2 will be greater than 40.
What is happening in this image?
Physiological shunt
There is obstruction of one of the alveoli so there is no ventilation occuring in the blood that would normally come in contact with that alveolus. Instead, other alveoli that share its airway increase their amount of ventilation to compensate. In this case, since there are just 2 alveoli, the other alveolus doubles its ventilation. Doubling its ventilation would lead to a decreased PACO2 because as V goes up PACO2 goes down. Because the PACO2 has changed, the PAO2 will also change and can be calculated using the alveolar gas eqn.
he new blood that enters the pulmonary vein is now a mixture of unchanged blood from the shunted side and blood from the side undergoing double ventilation.
If you have a sample of shunted blood that has a PaCO2 of 46mmHg and a sample of blood that has undergone gas exchange with a PaCO2 of 20mmHg, what will be the value of the PaCO2 when these blood samples mix in the pulmonary vein?
33mmHg
- If you have a sample of shunted blood with PaO2 of 28mmHg and a sample of oxygenated blood with PaO2 of 120mmHg, why can’t you average the PaO2 values?
- What is the best way to determine the PaO2 of these 2 blood samples when mixed?
- Because PaO2 vs. O2 sat % is sigmoidal not linear.
- The best way to determine would be to average te oxygen content (not O2 sat % or PaO2 )
Because there is only poor ventilation, it can be corrected by giving 100% oxygen because given enough time the PAO2 in the poorly ventilated alveoli will increase and be able to exchange more O2. Additionally, the healthy alveoli can make up some of the difference also.
Low V/Q due to poor ventilation that results in hypoxemia can be corrected with 100% O2.
