Respiration Lecture 10: Ventilation-Perfusion Relationships Flashcards
5 causes of hypoxemia
1)Hypoxic hypoxemia
2)alveolar hypoventilation
3) diffusion limitation
4)shunt
[5)VA/Q mismatch]
Shunt
flow of blood from R to L heart bypassing the gas exchange area
hypoxemia
below normal PaO2 (low BLOOD O2)
“hypoxic” means
low O2
Hypoxic hypoxemia –> PaO2, PaCO2, PiO2?
Low PaO2, LOW PaCO2, low PiO2. Low PaCO2 because of low PiO2
Tx for hypoxic hypoxemia
increase PiO2 (hyperventilate) to lower PaCO2
Alveolar hypoventilation –> PaO2, PaCO2?
Low PaO2, High PaCO2
Alveolar hypoventilation tx
Increase PiO2, increase Alveolar ventilation (open airway)
Diffusion limitation –> PaO2, PaCO2?
Low PaO2, unchanged PaCO2 (still released normally)
Diffusion limitation tx
Increase PiO2 to increase conc. gradient for O2 to flow in
Shunt –> PaO2, PaCO2?
Low PaO2, slight increase in PaCO2
Shunt tx
sx to fix shunt. INCREASING PiO2 HAS NO EFFECT
VA/Q Mismatch –> PaO2, PaCO2?
low PaO2, High PaCO2
Alveolar hypoventilation
decreased ventilation that doesn’t provide sufficient refreshening of the alveolar gas to maintain PACO2
Diffusion limitation
increasing the difficulty for O2 to diffuse from alveoli to blood by thickening the diffusion barrier such as pulmonary edema. Rarely causes clinical hypoxemia
Low Pb (barometric pressure) –> PiO2
decreases
What should VA/Q be normally?
1
Normal PAO2
100mmHg
Normal PACO2
40mmHg
Normal PaCO2
40mmHg
Normal PaO2
functionally 100mmHg (actually 95mmHg in a normal shunt)
Normal PvO2
40mmHg
Normal PvCO2
45mmHg
High perfusion with low ventilation –> V/Q?
Decreases
Low perfusion with high ventilation –> V/Q?
Increases
Restriction of blood flow to alveolar space –> physiological dead space
increases
Asthma –> V/Q?
decreases
why will oxygen therapy not help hypoxemia with shunts?
Hb is already saturated on functional side, and oxygen can’t get to non-functional side to saturate the Hb, so at most there is a slight improvement in Hb saturation with oxygen therapy
Why will increased PiO2 help when airway is restricted but not blocked off?
Some O2 can still get down to alveoli and participate in gas exchange, which will raise PO2
Reduced blood flow to alveoli –> PAO2, PACO2?
Increased PAO2 because there isn’t enough blood going by to draw out the O2 from the alveoli. Decreased PACO2 because less blood is flowing by to drop off its CO2
Increased blood flow to alveoli –> PAO2, PaO2?
PAO2 is normal, PaO2 is decreased because there is too much blood flowing by to oxygenate all of it and same amount of O2 is pulled from the alveoli
Increased physiological dead space –> PO2, PCO2?
High ventilation, low perfusion. PO2 increases because there is more O2 coming into alveoli than leaving. PCO2 drops because low perfusion allows less CO2 to be deposited back in alveoli
respiratory quotient (R) =
oxygen metabolism = VCO2/VO2
A fx of diet: carbs are most efficient, proteins least efficient. fat moderately efficient
test for VA/Q shunt
discrepancy between O2 and CO2
flow limited diffusion
when flow rate becomes high enough to pass blood through the gas exchange are before equilibrium is reached, resulting in lower PaO2
calculation of expected PAO2 with alveolar gas equation
PAO2 = PiO2 - [PACO2/R]
