Arterial Blood Gases-O2 Carriage/CO2 Carriage

Question 1

What are 2 mechanisms that help correct V/Q mismatch?

Answer 1

1. Bronchodilation due to high P_CO2 in bronchiole
2. Vasoconstriction due to low P_O2 in blood

This makes sense because bronchodilation increases ventilation and vasoconstriction decreases flow. So a low V/Q ratio would be normalized by these mechanisms.

Question 2

Why does a small V/Q ratio result in high CO₂ in blood?

Answer 2

A small V/Q ratio means that ventilation to the area is less than the perfusion. Ventilation is how you get rid of CO₂ and perfusion is the bringing of CO₂ to the area. So, with a small V/Q ratio, you are bringing more CO₂ to the area than you can remove (higher Q than V).

Question 3

In pulm, what is a shunt? What is the V/Q ratio of a shunt?

What is dead-space? What is the V/Q ratio of dead-space?

Answer 3

A shunt is volume of blood that does not engage in gas exchange. V/Q ratio of a shunt is 0. There is blood flow (perfusion) but no ventilation. This would happen in an occluded bronchiole.

Dead-space is volume of lung that does not engage in gas exchange. V/Q ratio of dead-space approaches infinity. There is air flow (ventilation) but no perfusion. This would happen in an occluded capillary.

Question 4

___ and ____ are two opposite extremes that can both reduce oxygenation by causing local V/Q mismatch.

Answer 4

Shunts, dead-space

Question 5

Does pneumonia causes shunts or dead-space?

Answer 5

Shunts.

Alveoli fill up with leukocytes and V/Q goes to 0.

Question 6

What are 3 modes of CO₂ carriage? Which one carries most of the CO₂?

Answer 6

1. Freely-dissolved = 1.2 mM
2. HCO₃^- = 24 mM
3. Carbamino compounds (mainly Hb) = 1.2 mM

HCO₃^- carries the most by far.

Question 7

What does CO₂ and H₂O turn into and what enzyme catalyzes this transition?

Answer 7

This reaction is highly favorable which is why HCO₃^- is the mode of CO₂ carriage with the highest mM.

Question 8

What is the Bohr Effect?

Answer 8

CO₂ binding reduces O₂ affinity for Hb.

Increased Temperature reduces O₂ affinity for Hb.

Increased [H⁺] reduces O₂ affinity for Hb.

Question 9

What is the Haldane effect?

Answer 9

O₂ binding reduces CO₂ affinity for Hb

Question 10

Do muscles take up Hb bound O₂ or freely-dissolved O₂?

Answer 10

Muscle cells can only take up freely-dissolved O₂

Question 11

When looking at the oxygen dissociation curve, is a right or left shift corresponding to a decreased O₂ affinity?

Answer 11

Right shift

Question 12

What does a right shift in the oxygen dissociation curve do for oxygen “off-loading”?

Answer 12

A right shift in the oxygen dissociation curve is a decrease in binding affinity of O₂ to Hb which results in an increase of oxygen “off-loading”. This increase in oxygen “off-loading” increases the freely-dissociated oxygen for muscles to pick up.

Question 13

How does the Bohr effect help muscles to get more oxygen during exercise?

Answer 13

The Bohr effect is when there is a reduced binding affinity of O₂ to Hb when there is an increased P_CO2, increased temperature, or increased [H⁺]. During exercise, there is an increase of all of these factors which causes O₂ to dissociate from Hb so that it can be picked up by muscles as freely-dissolved O₂.

Question 14

What is an agent that can cause a right shift in the oxygen dissociation curve that’s not included in the Bohr effect? When is this factor created?

Answer 14

2,3 diphosphoglycerate (2,3-DPG).

2,3-DPG is over-produced in any situation that causes Hypoxia (e.g. high altitude).

During times of hypoxia, this will cause a decrease in O₂ Hb binding affinity (right shift) which will increase the freely-dissolved O₂ to provide more O₂ to tissues in need.

Question 15

What is D_O2?

Answer 15

This is the volume of O₂ delivered in one minute

Question 16

How do you calculate for D_O2?

Answer 16

D_O2 = Q x C_aO2

The volume of O₂ delivered in one minute is equal to the amount of cardiac output in one minute multiplied by the arterial oxygen content.

Question 17

What is a typical D_O2 value?

Answer 17

1000 ml (1 L)

Question 18

When calculating C_aO2, we ignore freely dissolved O₂ for the sake of simplicity as the freely dissolved O₂ is such a small proportion of the total O₂ arterial content. What is the 1.39 ml O₂/gm Hb constant stand for?

Answer 18

The 1.39 ml O₂/gm Hb constant represents the maximum possible O₂ content of Hb when it’s fully bound with oxygen at all sites.

Question 19

What is O₂ carrying capacity and how is it calculated?

Answer 19

O₂ carrying capacity is the maximal O₂ that can be carried by a particular amount of Hb. This is calculated as follows:

[Hb] x 1.39 ml O₂/gm Hb

This calculation makes sense because you are multiplying the concentration of Hb with the maximum O₂ that can be bound on a single Hb if all sites are occupied.

Question 20

What is a typical value for [Hb]?

Answer 20

15 gm/100 ml blood

Question 21

What is V_O2? how is it calculated? what is a typical V_O2 value?

Answer 21

Question 22

What is hypoxia?

Answer 22

Low O₂ at the level of the tissue (P_O2 is less than 1-2 torr in the mitochondria)

Question 23

Name the 3 causes of hypoxia

Answer 23

1. Low Cardiac Output (Q)
2. Low S_aO2 associated w/ a low P_aO2 (hypoxemia)
3. Delivery problems (e.g. Anemia/low[Hb] , carbon monoxide poisoning)

Question 24

What is hypoxemia? Name 5 causes of hypoxemia

Answer 24

Hypoxemia is low S_aO2 associated with low P_aO2

1. Low P_IO2
2. Low P_AO2
3. Diffusion Problems
4. Shunt
5. V/Q Mismatch

Question 25

What value P_aO2 is considered hypoxemia? (at sea level vs in denver)

Answer 25

Sea-level hypoxemia is less than 80 torr

Denver hypoxemia is less than 65 torr

Question 26

What are the 3 problems caused by CO binding to Hb?

Answer 26

1. Competition with O₂ reduces S_aO2
2. CO binds extremely tightly
3. CO increases O₂ affinity, which reduces off-loading

Question 27

What is the A-a gradient? What is a normal value for this?

Answer 27

The difference in oxygen level between alveoli (A) and arterial circulation (a)

A typical gradient is about 5-10 torr

Question 28

How do you calculate A-a gradient?

Answer 28

A-a gradient = P_AO2 - P_aO2

P_AO2 is estimated through P_aCO2 through two relationships

P_ACO2 = P_aCO2

P_AO2 = (P_IO2 - P_ACO2)/R

P_AO2 = ((P_atm - P_water) F_iO2) - (P_ACO2/R)

P_atm = the atmospheric pressure (760 mm Hg, 630 in denver)

P_water = the vapor pressure of water at body temp (47 mm Hg)

F_iO2 = the fraction of O2 in the inspired gas (21% on room air)

P_aCO2 = the partial pressure of CO2 in arterial blood (from ABG)

R = respiratory quotient = 0.8

Question 29

How can you tell the difference between hypoxemia from high altitude vs. hypoventilation?

Answer 29

Measure the P_aCO2.

The P_aO2 and S_aO2 for both high altitude and hypoventilation are going to be decreased. However, in high altitude, P_aCO2 will be decreased but with hypoventilation, P_aCO2 will be increased. This is because in high altitude, you are breathing more to try to compensate for the low P_aO2 and this will eliminate CO₂. With hypoventilation, you’re eliminating less CO₂ so it increases.