Ventilation-Perfusion Pt 1 Flashcards
What is the alveolar gas equation?
[Oxygen]alveolar = [Oxygen in lungs] - [Oxygen used by tissue]
PAo2 = PIo2 - PaCO2/R
PAo2 = alveolar partial pressure of oxygen
PIo2 - inspired partial pressure of oxygen
PaCO2 = arterial CO@
R= VdotCO2/VdotO2 = 0.8
PiO2 = (760-47) x oxygen % (to account for water vapor)
What does the alveolar gas exchange equation calculate?
It calculates what inspired O2 needs to be to produce a desired alveolar and therefore arterial O2 level
How is Inspired Partial Pressure of Oxygen (PIO2) calculated?
= (Barometric P - Water vapor P) x FiO2
BP = 760 mm Hg
WVP= 46 mm Hg
FiO2 (fraction of inspired oxygen) = 21% (0.21)
What does the PAO2 allow us to calculate?
It is not very useful on it’s own
But can be used to calculate to A-a O2 Gradient - which is useful in determining the health of the alveoli
What is the equation for A-a O2 gradient?
= PAO2 - PaO2
Normally < 20 mm Hg
What does an increase in the A-a O2 gradient indicate?
Diffusion impairment
something is wrong with the alveoli
What is the intrapleural fluid and pressure like in the apex of the lung?
Intrapleural fluid is acted on by gravity and affects the pressure
At the apex, there is less intrapleural fluid, so the intrapleural pressure is more negative (up to -10 cm H2O)
What is the intrapleural fluid and pressure like in the base of the lung?
Intrapleural fluid is acted on by gravity and affects the pressure
At the apex, there is more intrapleural fluid, so the intrapleural pressure is less negative (-2 - 3 cm H2O)
How does the intrapleural pressure affect the alveoli?
At the apex, the pressure is more negative so the alveoli are LARGE at rest
At the base, the pressure is less negative, so alveoli are SMALL at rest
What is pressure and fluid like in the middle of the lung (Zone 2)?
It is “normal”
Just like with blood flow
How do the alveoli in the apex of the lungs react to inspiration?
They can only get a little larger since already so large
How do the alveoli in the base of the lungs react to inspiration?
The alveoli get much larger
What is V/Q?
The mathematical ratio between ventilation and perfusion
What is the average V/Q?
4/5 = 0.8
What is High V/Q?
Ventilation is high relative to perfusion
What is Low V/Q?
Ventilation is low compared to perfusion
What can cause Low V/Q?
What happens?
What is this called?
Extreme condition V=0
The airway is blocked by something
But the blood continues to arrive at the alveolus
In a short time the O2 in the alveolus decreases because the blood has removed it all
While the CO2 increases (the blood continues to deliver CO2)
The same changed occur in the arterial blood
With time the alveolus will equilibrate with the non-arterialized (venous) blood entering the pulmonary capillary and the blood leaving that alveolus will leave without exchanged oxygen or carbon dioxide
*This represents a shunt - based on O2 and CO2 - the blood gained nothing by going to the lungs = as if the blood never went to the lungs
What are the arterial blood gases that are attained by regions with low V/Q ratio?
PaO2: Low
PaCO2: High
pHa: Low
Volume of Blood: High relative to V.
What is a high V/Q ratio mean?
Extreme: No blood coming to the alveolus (alveolar dead space)
With less blood removing oxygen and adding CO2, the alveolar O2 will increase and CO2 decrease, bringing the alveolar closer to atmospheric conditions
Eventually the alveolar air in these areas will be most similar to the air outside of the body
Not much blood benefits from this since not substantial blood flow
Even worse, the blood that couldn’t get to this alveolus has to go somewhere else. It goes to the normal alveolus and turns it into a low V/Q alveolus. Because even though the airflow is normal, the blood flow is increased, so the V/Q ratio decreases.
Overall PaO2 and PaCO2 decreases
What are the arterial blood gases attained by regions with high V/Q ratio?
PaO2: High
PaCO2: Low
pHa: High
Volume of blood: Low
What is the V/Q ratio at the apex of the lung?
HIGH
Leading to higher PaO2 and lower PaCO2
What is the V/Q ratio at the middle of the lung? Zone 2
Normal
What is the V/Q ratio at the base of the lung?
LOW
Blood gases are lower
How can we minimize differences in V/Q ratios in different zones of the lung?
Hypoxic Vasocontriction
- contraction of pulmonary vessels to redirect blood away from hypoxic regions
