PULMONARY 06: VENTILATION/PERFUSION Flashcards
In an ideal ventilation/perfusion ratio, we transport what?
What we use in the tissues
How do we measure V/Q? What units?
mL O2/mL blood
If the V/Q ratio is greater than 1, this means____
Ventilation has exceeded perfusion
If V/Q ratio is less than 1 this means ____
Perfusion has exceeded ventilation
A normal V/Q ratio is __
near 1 (0.8) This indicates slightly more perfusion than ventilation
What are the two extremes of O2/CO2 that we experience?
- Inspired air, where there is a VERY high pO2 and very low PCO2
- Mixed-venous blood, where there is a lower PO2 and pCO2 that exceeds the pO2 (40 for O, 46 for CO2)
What happens to the v/q ratio in high ventilation?
We shift the VQ ratio toward a higher number, alveolar O2 and CO2 approach atmospheric
If we increase V/Q ratio too much, this is an indication of what, clinically speaking?
Dead space alveolus; this means there are places with ventilation but no perfusion. Perfusion may be blocked due to something like a pulmonary embolism.
If we see that there is a very very low V/Q ratio, what is this an indication of, clinically speaking?
A shunt alveolus; this is a bypass and would indicate there are areas where we have no ventilation in the system. We do have perfusion through the system, however. Therefore, alveolar content is approaching that of mixed venous blood
Slide 9
Ventilation and perfusion are both elevated in the (base/apex) of lung and why
Base of lung due to gravity
Which is more skewed toward the base of the lung, ventilation or perfusion?
Perfusion, although not by much
About where on the lung do ventilation and perfusion match each other
2/3 of the way up
Average V/Q over the entire system is what
0.8
Slide 13
The V/Q ratio determines WHAT for any single lung unit
Gas exchange
V/Q is high in what part of the lung
apex
V/Q is low in what part of the lung
Base
V/Q mismatch and venous drainage (shunting) results in PaO2 slightly above or below the PAO2
Slightly below
Is some level of V/Q mismatch normal in a normal physiological state
Yes
Hypoxia
Decreased O2 delivery to tissue
Hypoxemia
Decreased O2 in arterial blood (<60mmHg clinically, technically <80mmHg)
Ischemia
Loss of blood flow
What is AaDO2
Alveolar-arterial difference of O2
What is hypoxemia with normal AaDO2 an indication of?
High altitude (decreased PiO2) Hypoventilation
Basically, not enough oxygen is getting into the airways, so therefore blood oxygen is also low
What is hypoxemia with INCREASED AaDO2 an indication of?
Diffusion limitations, V/Q mismatch, right-to-left shunts.
Basically, there is oxygen in the airways, but it’s not getting to the blood
How do you determine alveolar-arterial difference of PO2
Patient breathes in 150mmHg oxygen, you get what thei rblood gas is, and you solve for the difference
What does alveolar PO2 depend on
Rate of removal from blood (metabolic demand)
Rate of replenishment of O2 by alveolar ventilation
If you stop breathing, what is the effect on alveolar O2 and CO2
Po2 decreases
PCO2 increases
In hypoventilation, will supplemental oxygen be sufficient to help raise PaO2?
Yes
Between high altitude, hypoventilation, diffusion limitations, right-left shunts, and V/Q mismatches, which is the one situation that supplemental oxygen will NOT result in an improved blood-oxygen level
R-to-L shunt
Why does supplemental oxygen help if you have a diffusion limitation?
You’re increasing the driving force by increasing P1
You might use this in the case of lung edema, fibrosis, etc
Why will supplemental oxygen NOT help blood oxygen level in the case of a shunt?
A shunt is a bypass/alley / connection. This means that the blood is going from R heart to L heart without being oxygenated.
In these examples, blood does not see alveolar capillary beds. Therefore, you can increase oxygen but that blood is never going to see it.
What are normal anatomical shunts?
Thebesian vessels (myocardium drainage into heart chamber) Bronchial circulation drainage
What are pathologic anatomic shunts
Vascular lung tumors
pulmonary arteriovenous malformations
What is a capillary/Physiological shunt
Blood does go through pulmonary capillaries, but does not fully equilibrate
This can be caused by tumors that block airflow, pneumothorax, severe pneumonia and edema, foregin bodies
What is the most common reason for hypoxemia in patients with respiratory disorders
V/Q mismatch
What are some reasons for a V/Q mismatch
Broadly speaking, anything that perterubs ventilation
Asthma
COPD
fibrosis
Cystic fibrosis
Why does adding oxygen help in the case of a V/Q mismatch
If you take a single inhale of oxygen it won’t necessarily go into the space very well particularly if there’s an issue of poor ventilation. But we consider supplemental oxygen to not be transient, but over hours/days. Over time, because we have more access to this spce, it will eventually get to raising the PiO2 and rely on diffusion of oxygen from that increased driving pressure. It’ll get into the blood plasma space eventually nd into RBC’s, and O2 can get to a normal level. Increasing O2 to the entire system isn’t great either, but it can at the very least rescue the V/Q ratio
Two main mechanisms of compensation with regard to V/Q mismatch and hypoxemic conditions
Hypoxic vasoconstriction
Bronchiolar constriction
What is hypoxic vasoconstriction? What instigates it? During what is this very important?
It is instigated by a low PAO2 (indicating high perfusion)
It redirects perfusion to ventilated parts of the lung
Arterial O2 level does not affect this at all
Baby’s first breath! tere is a dramatic decrease in PVR and increase in pulmonary blood flow (15% CO before birth to 100%)
What is bronchiolar constriction? What instigates it?
Instigated by high PAO2, low CO2, and a high pH - indicative of the airways sensing CO2 changes and shuttling itself , rather than circulatory side like in hypoxic vasoconstriction
It is when the bronchioles themselves constrict to redirect ventilation to perfused parts of the lung.
