What are 2 mechanisms that help correct V/Q mismatch?
1. Bronchodilation due to high PCO2 in bronchiole
2. Vasoconstriction due to low PO2 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.
Why does a small V/Q ratio result in high CO2 in blood?
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).
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?
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.
___ and ____ are two opposite extremes that can both reduce oxygenation by causing local V/Q mismatch.
Does pneumonia causes shunts or dead-space?
Alveoli fill up with leukocytes and V/Q goes to 0.
What are 3 modes of CO2 carriage? Which one carries most of the CO2?
1. Freely-dissolved = 1.2 mM
2. HCO3- = 24 mM
3. Carbamino compounds (mainly Hb) = 1.2 mM
HCO3- carries the most by far.
What does CO2 and H2O turn into and what enzyme catalyzes this transition?
This reaction is highly favorable which is why HCO3- is the mode of CO2 carriage with the highest mM.
What is the Bohr Effect?
CO2 binding reduces O2 affinity for Hb.
Increased Temperature reduces O2 affinity for Hb.
Increased [H+] reduces O2 affinity for Hb.
What is the Haldane effect?
O2 binding reduces CO2 affinity for Hb
Do muscles take up Hb bound O2 or freely-dissolved O2?
Muscle cells can only take up freely-dissolved O2
When looking at the oxygen dissociation curve, is a right or left shift corresponding to a decreased O2 affinity?
What does a right shift in the oxygen dissociation curve do for oxygen "off-loading"?
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.
How does the Bohr effect help muscles to get more oxygen during exercise?
The Bohr effect is when there is a reduced binding affinity of O2 to Hb when there is an increased PCO2, 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.
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?
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.
What is DO2?
This is the volume of O2 delivered in one minute
How do you calculate for DO2?
DO2 = Q x CaO2
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.
What is a typical DO2 value?
1000 ml (1 L)
When calculating CaO2, 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?
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.
What is O2 carrying capacity and how is it calculated?
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.
What is a typical value for [Hb]?
15 gm/100 ml blood
What is VO2? how is it calculated? what is a typical VO2 value?
What is hypoxia?
Low O2 at the level of the tissue (PO2 is less than 1-2 torr in the mitochondria)
Name the 3 causes of hypoxia
1. Low Cardiac Output (Q)
2. Low SaO2 associated w/ a low PaO2 (hypoxemia)
3. Delivery problems (e.g. Anemia/low[Hb] , carbon monoxide poisoning)
What is hypoxemia? Name 5 causes of hypoxemia
Hypoxemia is low SaO2 associated with low PaO2
1. Low PIO2
2. Low PAO2
3. Diffusion Problems
5. V/Q Mismatch
What value PaO2 is considered hypoxemia? (at sea level vs in denver)
Sea-level hypoxemia is less than 80 torr
Denver hypoxemia is less than 65 torr
What are the 3 problems caused by CO binding to Hb?
1. Competition with O2 reduces SaO2
2. CO binds extremely tightly
3. CO increases O2 affinity, which reduces off-loading
What is the A-a gradient? What is a normal value for this?
The difference in oxygen level between alveoli (A) and arterial circulation (a)
A typical gradient is about 5-10 torr
How do you calculate A-a gradient?
A-a gradient = PAO2 - PaO2
PAO2 is estimated through PaCO2 through two relationships
PACO2 = PaCO2
PAO2 = (PIO2 - PACO2)/R
PAO2 = ((Patm - Pwater) FiO2) - (PACO2/R)
Patm = the atmospheric pressure (760 mm Hg, 630 in denver)
Pwater = the vapor pressure of water at body temp (47 mm Hg)
FiO2 = the fraction of O2 in the inspired gas (21% on room air)
PaCO2 = the partial pressure of CO2 in arterial blood (from ABG)
R = respiratory quotient = 0.8
How can you tell the difference between hypoxemia from high altitude vs. hypoventilation?
Measure the PaCO2.
The PaO2 and SaO2 for both high altitude and hypoventilation are going to be decreased. However, in high altitude, PaCO2 will be decreased but with hypoventilation, PaCO2 will be increased. This is because in high altitude, you are breathing more to try to compensate for the low PaO2 and this will eliminate CO2. With hypoventilation, you're eliminating less CO2 so it increases.