31 mechanisms of hypoxia

Question 1

what determines PaO2?

Answer 1

ventilation and perfusion

Question 2

What is FiO2?

Answer 2

the fraction of inspired air that is O2. (21%)

Question 3

How do you calculate PAO2?

Answer 3

PAO2=FiO2 x (barometric pressure - H2O pressure) - PCO2 pressure/R

P(baro)=751 in Iowa city
P(H2O)=47
P(CO2)=40

Question 4

What is PAO2 in Iowa city?

Answer 4

98mmHg

This is only important because he uses it all the time and its easier to remember it than calculate it

Question 5

In the lungs, is oxygen perfusion limited or diffusion limited?

Answer 5

Perfusion limited. It is not diffusion limited because any O2 that can get into the alveoli can easily pass through the thin membranes to get into the blood.

Question 6

What is the A-a gradient?

Answer 6

The difference between PAo2 and PaO2.
A-a gradient=(PAO2 - PaO2)
This number should be near zero because it is measuring diffusion and this is not normally a problem.

Question 7

All Alveoli are not ideal (the same) in the lungs. Why?

Answer 7

Blood flow is not homogeneous

Ventilation is not homogeneous

Question 8

What is the west three-zone model?

Answer 8

A model describing the blood flow patterns in the lungs. It shows that the bases receive the majority of the blood while the apex receive the least. This is mainly due to gravity.

LOOK AT THIS IN THE NOTES!

Question 9

what happens when (Palveolar>Ppulmonary artery>Ppulmonary vein)?

Answer 9

There is no flow because the capillaries are collapsed. This is what happens in the apex of the lung. [Zone 1]

Question 10

what happens when (Ppulmonary artery>Ppulmonary vein>Palveolar)?

Answer 10

capillaries are fully open and perfusing well because they are not collapsed like in the apex.
[Zone 3]

Question 11

Transpulmonary pressure under normal physiological conditions is always positive/negative?

Answer 11

Positive.

Transpulmonary pressure= alveolar pressure - pleural pressure.

Question 12

How would you describe the PO2 and PCO2 at the apices or the lungs?

Answer 12

At the apices, the alveoli are usually overinflated and have more ventilation than perfusion can keep up with. Therefore, you would see a higher than average PO2, and a lower than average PCO2.

Question 13

why is V/Q mismatch so bad?

Answer 13

1) Low V/Q has low PAO2, leaving the blood with a low PaO2.
2) High V/Q has high PAO2, leaving the blood with a high PaO2
3) When you mix these 2 (High and low) PaO2, the result is not an average of PaO2 but a value closer to the lower PaO2.

[Key: High V/Q units cant compensate]

Question 14

What is one of the major reasons why your oxygen concentration in you blood (SpO2) is not 100%?

Answer 14

The blood that has dumped oxygen to the bronchioles is released into the pulmonary veins which are already oxygenated. (This also makes it so the A-a gradient is larger then 0)

Question 15

What percent of cardiac output goes to bronchial circulation?

Answer 15

about 1%

Question 16

what are the normal BP values for systemic and pulmonary circulation?

Answer 16

systemic= 120/80

pulmonary=24/9

Question 17

Under normal conditions, what percent of blood flows to normal V/Q matched areas? Low V/Q areas? High V/Q areas?

Answer 17

90% of blood flows to normal V/Q areas
5% of blood flows to Low V/Q areas
5% of blood flows to High V/q areas

Question 18

what does the body do to the low V/q areas since they do so much harm?

Answer 18

The body chokes them off. This happens when the pulmonary arteries are exposed to low PAO2 areas.
[this is opposite to other areas of the body]

Question 19

How does the body deal with over ventilated alveoli?

Answer 19

Over ventilated alveoli have lower PACO2 so the body can constrict the bronchial smooth muscle sending the oxygen to other areas that need it.

Question 20

What would a V/Q ratio of 50 mean? When would you see this?

Answer 20

It would mean that there is 50x more ventilation than perfusion going on.
You could see it with severely diseased lungs that cause diffusion to be significantly reduced.

Question 21

What has a worse effect of blood oxygen, Low V/Q or a shunt?

Answer 21

Shunt. A shunt is like an ultra low V/Q mismatch because nothing is being exchanged, but be careful because this is not exactly true.

They tend to occur in different areas of the lung, and their response to oxygen is different.

Question 22

Why is the shunt worse than the V/Q mismatch?

Answer 22

A shunt will not respond to higher oxygen levels very well because PaO2 will only rise minimally.

V/Q mismatch will respond to higher oxygen levels pretty well and many times bring oxygen saturation up to 100%

Question 23

what would be a reason for a shunt in the lungs?

Answer 23

1) Congenital heart disease
2) alveolar filling with blood/fluid
3) alveolar collapse