VQ Relationships Flashcards
what is the alveolar gas equation
PAO2 = PIO2 - PaCO2/R
PAO2 is the alveolar partial pressure of oxygen
PIO2 is the inspired partial pressure of oxygen - the water vapor
-PIO2 = (760 - 47) x oxygen percentage (usually 21)
PaCO2 = arterial CO2
R = V(dot)CO2/V(dot)O2 = 0.8 (depends on the fuels but this average)
what is the alveolar gas equation used for
It is used to calculate what inspired O2 needs to be to produce a desired alveolar (and therefore arterial) O2 level
Often listed with a small correction factor but can be ignored since it is small
helps for obtaining information to determine the A-a O2 gradient
what is the equation for the A-a O2 gradient and how can it be useful
A-a O2 gradient = PAO2 - PaO2
normal is less than 20 mmHg
helps in determining the causes of Hypoxemia
What does it mean if their is an Increase of the A-a O2 gradient
this indicates that their is a diffusion impairment and something is wrong at the alveoli
this cold be a Right to left shunt
Ventilation/Perfusion mismatch
Diffusion Limitation
what does it mean if the paitent has Hypoxemia but the AA gradient is normal
Low inspired PO2 or could be Hypoventilation
what is the regional blood flow to the lungs
lowest is to the apex or zone 1 ( BP pressure is reduced because above the heart)
medium blood flow for zone 2 since at level of the heart
High blood volume for zone 3 (base) because it is below the heart, the Blood pressure is high too
how is the intrapleural pressure affected by gravity
at the apex there is less intrapleural fluid so the intrapleural pressure is more negative (up to -10 cm H20)
at the base there is more intraplera fluid, so the pressure is less negative than you expect (-2 - -3 cm H20)
what is the size of the alveoli based on the affect of gravity
since their is more negative intrapleural pressure at the apex of the lung due to gravity, the alveoli are large at rest
at the base of the lung there is less negative pressure so those alveoli are small at rest (not expanded as much)
middle is normal size
with inspiration where does the alveoli change its volume the most
since at rest the alveoli at the top are larger and the intrapleural pressure is more negative, the change in volume is very small
at the base of the lung the alveoli are much smaller and with less negative pressure they can change their volume much more
what is the Ventilation/Perfusion ratio and how is it often spoken of
V/Q is the mathematical ratio between ventilation and perfusion
over the entire lung, the average is 4L/min / 5L/min = 0.8
often spoken of a high or low V/Q
- high means ventilation is high in regards to perfusion
- low means ventilation is low in regards to perfusion
what happens to the alveoli V/Q if their is a blockage of the airway
V = 0
low V/Q
blood will continue to arrive to the alveolus, and in the short time the O2 in the alveolus decreases while the CO2 increases as the blood delivers CO2
-and these same changes occur in the blood
eventually the blood and alveolus will have the same pO2 and pCO2 as the venous blood since no inhaled oxygenated air can enter the alveolus
this represents a right to left shunt where the blood goes pass the lungs but doesn’t get air (as if it never went to the lungs)
in a Low V/Q region: what is the PaO2, PaCO2, pHa, and volume of blood
PaO2 = low
PaCO2= High
pHa = low
VOlume of blood= High relative to V
what happens to the alveoli if their is no blood coming to the alveoli
considered alveolar dead space
High V/Q ratio
with less blood removing the oxygen and adding CO2 the alveolar O2 will increase and CO2 decrease, this brings the alveolus closer to atmospheric pressure
eventually the alveolus will be similar to the air outside the body
however the small amount of blood that is here will follow suit and have high oxygen and very low CO2
or the blood goes somewhere else and causes a low V/Q for other alveolus
in a High V/Q region: what is the PaO2, PaCO2, pHa, and volume of blood
PaO2 = High
PaCO2 = low
pHa = high
Volume of blood = low
how does gravity affect the V/Q in zone 1, 2 and 3. and then what happens to that blood
since already the gravity affects the blood flow to certain spots of the lung it causes V/Q inequality of the lung
apex = high V/Q PaO2=130 PaCO2 = 28
middle = normal PaO2 = 100 PaCO2 = 40
base = low V/Q PaO2 = 89 PaCO2 = 42
but will all mix in the pulmonary vein to get that average number V/Q = 0.8
