Cremo 5: Gas exchange Flashcards
PaO2 depends upon two things
- PAO2
2. Architecture of the lungs (lungs do not perform in a perfectly efficient manner)
The causes of hypoxemia can be divided into 2 main categories
- Not enough O2 getting into the alveoli
2. Not enough O2 transferred into the capillary blood
Two things that can cause not enough O2 to get into the alveoli
- low atmospheric pressure
2. pure hypoventilation
Three things that can cause not enough O2 to be transferred into the capillary blood
- ventilation-perfusion mismatch
- right-to-left shunting
- diffusion defects
Why do we even calculate PAO2?
So that we can determine PaO2 using the P(A-a)O2
Which will always be greater: PAO2 or PaO2
PAO2 will always be greater.
In hypoventilation, a decrease in PaO2 occurs in response to (blank). This means that the A-a difference does not change.
a decrease in PAO2.
What does “pure” hypoventilation refer to?
Hypoventilation (low O2) when NO gas exchange problem present. PaO2 will decrease in response to drop in PAO2.
Two main problems during hypoventilation
- acidemia
2. hypercarbia (high CO2)
What kinds of things can cause hypoventilation IF there is no intrinsic problem with the lungs?
- depression of the respiratory center (morphine)
- diseases of the respiratory muscles (muscular dystrophy)
- extreme obesity
Two reasons for “wasted blood.” This refers to any fraction of the venous blood that does not get fully oxygenated.
- anatomic shunt
2. low regional V/Q ratios
What percentage of cardiac output is not oxygenated in a normal person?
2-4%
Compare PO2 in end-pulmonary capillary blood to the PO2 systemic arterial blood when an anatomic shunt is present.
Small decrease in PO2 in the arterial blood. Ex: PO2 = 102mmHg in end pulmonary capillary blood.
PO2 = 95mmHg in arterial blood
What is the average alveolar ventilation rate?
5.25L/min
What is the average pulmonary blood flow perfusion rate?
5.00L/min
What does a V/Q = 0 suggest?
Shunt; ventilation is totally absent
What does a V/Q = infinity suggest?
Dead space; perfusion is totally absent
What is the alveolar gas equation?
PAO2 = PIO2 - 1.2(PaCO2)
Is the V/Q ratio higher at the apex or the base?
V/Q is higher at the apices
Is it normal to have anatomic shunts and anatomic dead space?
Is it normal to have low V/Q regions and alveolar dead space?
Yes and yes. Trouble starts if these are not at normal levels.
What is the normal A-a difference that results from gravitational effects on V/Q and on anatomic shunting?
10-15
Is V/Q mismatch among different alveoli clinically relevant?
Yes.
What would happen if the V/Q ratio was high in the apex and really low at the base? In other words, what would happen if there was a mismatch?
What happens in this case is the top unit creates blood that is hyperoxemic. Blood leaving the lower unit will be hypoxemic. When the blood mixes, the hypoxemic blood “wins” and can ultimately decrease the SaO2. The flow-weighted average of SaO2sat will be on the lower end!!
What is a key variable in understanding V/Q mismatch?
SaO2
Say you mix two equal volumes of blood with different PO2’s. Can you just take the mean of the two PO2’s? Can you take the mean of the two CaO2’s?
No!! Due to the flat shape of the oxygen binding curve the resulting PO2 after mixing is lower than the mean PO2.
You CAN use the mean of the two CaO2s.
Can supplemental O2 rescue effects of perfused but not ventilated region?
No!
Can supplemental O2 rescue effects of V/Q mismatch?
Yes!
If there is a V/Q mismatch between two alveoli, which one will have a greater effect?
Hypoxemic one always wins. The one that has received less O2.
Main reason for an A-a difference larger than normal?
Intrinsic problem with the lungs
THE most important cause of gas exchange abnormalities in most lung diseases
V/Q mismatch
Although V/Q mismatching can influence PaCO2, this effect is often overcome by an increase in (blank)
minute ventilation
Four mechanisms for hypoxemia
- Decrease in PIO2 (inspired O2 is low due to high altitude)
- Pure hypoventilation (no intrinsic problem with the lungs)
- V/Q mismatching
- Shunt
