Ventilation Perfusion Relationships Pt. 1 Flashcards
Use the alveolar gas equation to determine alveolar-arterial O2 difference and be able to determine if deficits are present
PAO2=PIO2-PaCO2/R
PAO2 is alveolar partial pressure of O2
PIO2 is inspired partial pressure of oxygen accounting for water vapor (47)
PaCO2 is arterial CO2
R=VdotCO2/VdotO2=0.8
What is the alveolar gas exchange used for?
calculate what inspired O2 needs to be to produce a desired alveolar (and arterial) O2 level
often listed with a small correction factor. This is small and can be ignored
PIO2 is calculated as
Barometric pressure minus the water vapor P times the FiO2
760-47 x .21
barometric pressure will change with altitude and be given if needed
FiO2 is normally 21% but changes with altitude and will be given if needed
What is the Alv-art O2 gradient?
useful for determining the health of the alveoli
PalvO2-PartO2=
ex. 121.8-43
=78.8
normal is <20mmHg
An increase in the A-A O2 gradient indicates a
diffusion impairment (something is wrong at the alveoli)
RtL shunt
COPD/PE
Intstl. lung dz , pulmonary arterial HTN
Summary of alveolar gas equation
the gas exchange allows us to predict the O2 concentration in the alveoli (non invasive)
once you have the alveolar concentration of oxuygen you can calculate the A-A O2 gradient
If the A-A gradient is freater than normal, you know there is an alveoli problem
if the A-A gradient is normal, but PaO2 and PAO2 are noth low, the problem is elsewhere
what is the intrapleural pressure in the apex?
What is the intrapleural pressure at the base?
there is less intrapleural fluid, so the intrapleural pressure is more negative (up to -10)
alveoli are large at rest, can only get a little bigger with inspiration
there is more intrapleural fluid, so the intrapleural pressure is less negative (-2 to -3)
alveoli are small at rest, can get much bigger with inspiration
the middle apex has normal blood flow and pressure. What is the pressure?
-5
What is the V/Q ratio?
It is the ratio between ventilation and perfusion
over the entire lung it is 4l/min/5l/min=0.8 (avg)
Often spoke of as high or low V/Q
High-high ventilation relative to perfusion
Low: low ventilation compared to perfusion
What are the following at high V/Q regions
PaO2
PaCO2
pHa
Volume of blood
PaO2: high
PaCO2: low
pHa: high
volume: low
What are the following values for the low V/Q regions
PaO2
PaCO2
pHa
Volume of blood
PaO2: low
PaCO2: high
pHa: low
Volume of blood: high
What is the V/Q ratio of the apex?
What is the V/Q ratio in the mid-lung?
What is the V/Q ratio at the base?
At the apex, V/Q is high leading to a higher PaO2 and a lower PaCO2 (PaO2=130, PaCO2=28)
Mid-lung, V/Q is 0.8 and blood gases are as expected (PaO2=90-100, PaCO2=40)
At the base, V/Q is low, blood gases aren’t as good(PaO2=98, PaCO2=42)
V/Q parameters at Apex
PaO2
PaCO2
pHa
Volume
high
low
high
low
V/Q parameters at the base of the lung
PaO2
PaCO2
pHa
Volume of blood
Low
high
low
high
What can we do to minimize the differences in the V/Q ratios in the different zones of the lung
hypoxic vasoconstriction
constriction of pulmonary vessels to redirect blood away from hypoxic regions
