PATHOPHYS: Gas Exchange II Flashcards
What is the equation for partial pressure of a gas?
Partial Pressure = Atmospheric Pressure X Fraction of gas
What is the fractional percent of oxygen in air?
21%
What is the partial pressure of oxygen in air?
160 mmHg
Why is does the air we breathe in not have a partial pressure of oxygen at 160 mmHg?
Because air is inspired, rapidly warmed, and saturated with H2O (so you must account for water vapor)
What is the partial pressure of water vapor at body temp?**
47 mmHg
this will be on test
What is total gas pressure (accounting for water vapor)?
760-47= 713
What is the partial pressure of oxygen that we breathe into the lungs?
150 mmHg (760-46) X 0.21= 150
True or False: water vapor varies greatly with altitude.
FALSE: water vapor does NOT vary with altitude
What is the definition of minute ventilation?
the volume exhaled per minute, calculated as:
Tidal Volume X Respiratory Rate
What is the definition of anatomic dead space?
the air which remains int he conducting airways at the end of inspiration and DOES NOT participate in gas exchange
What is alveolar dead space?
alveoli that are ventilated but not perfused (so they cannot participate in gas exchange)
How do alveoli with reduced but not absent ventilation behave?
They act as if a portion was normal and a portion was dead space
What is physiologic dead space?
the sum of anatomic and alveolar dead space
What is the equation for alveolar ventilation?
Minute Ventilation - Dead Space = Alveolar ventilation (effective ventilation)
How can you approximate anatomic dead space?
it is around lean body weight in pounds (typically 150-180 mL)
True or false: dead space is a static volume.
FALSE!
What will increase the anatomic dead space?
increasing lung volume
What will decrease the alveolar dead space?
exercise (increases blood flow to alveoli at the apex of the lung!)
True or false: increased alveolar dead space is ALWAYS pathologic.
TRUE! (ex. pulmonary embolism)
What does the alveolar gas equation tell us?
It allows us to:
1) estimate the alveolar partial pressure of oxygen
2) Determine if hypoxemia is due to lung disease or not
3) Determine the causes of hypoxemia
CO2 is eliminated from the body solely via what process?
VENTILATION
Alveolar oxygen is a balance of what two processes?
Oxygen delivery Oxygen removal (from alveolus by capillary blood)
What determines the rate of oxygen removal at the alveolus?
tissue oxygen consumption (usually around 250 mL/min
What is the respiratory exchange ratio (R)? What is the normal value of R?
Volume of carbon dioxide produced for volume of oxygen removed (usually 0.8)
What is the alveolar gas equation?
PAO2= PiO2- (PaCO2/R)
What is PAO2 at sea level with normal R and PaCO2 of 40 mmHg (normal)?
PAO2= 150 - (40/0.8)= 100 mmHg
Which gives a greater PAO2: normal breathing, hyperventilation, or hypoventilation?
Hyperventilation (have a drastically lower PaCO2 because you “blow off” carbon dioxide with deep breathing)
How is arterial oxygen tension (PaO2) measured?
arterial blood gas test
How is alveolar oxygen tenxion (PAO2) measured?
NOT measured–calculated from the alveolar gas equation
What is the A-a difference?
Calculation of PAO2-PaO2
What change in A-a will tell you if you have lung disease?
Widened A-a (or larger number) will tell you that your hypoxemia is due to lung problem
What is the formula for a normal A-a gradient?
(Age + 4)/4
True or false: A-a gradient decreases with age.
FALSE: A-a gradient increases with age, because arterial oxygen tension decreases with age due to decreased oxygen transfer to blood (alveolar oxygen tension remains the same!)
What is hypoxemia?
decreased oxygen tension int he circulating blood compared to normal
What are two broad categories of hypoxemia?
Defective exchange of oxygen in the lungs (widened A-a) OR from decreased delivery of oxygen to the alveolus (normal A-a in absence of lung disease)
List the 5 causes of hypoxemia.
Normal A-a
1) Decreased PiO2 (high altitude)
2) Hypoventilation
Widened A-a
3) Diffusion limitation
4) R to L shunt
5) V/Q mismatch
How might you measure hypoventilation?
PACO2 is inversely proportional to alveolar ventilation, so PaCO2 is a good measure of alveolar ventilation.
What is the least clinically significant cause of A-a widening? Why?
Diffusion limitation, because diffusion capacity would have to be SEVERELY decreased (lower than 1/3 of normal) or have extremely decreased transit time (from really high cardiac output)
True or false: diffusion capacity tests can tell you about the mechanism (decreased surface area, increased membrane thickness, etc.) of diffusion limitation.
FALSE: if can’t separate the mechanisms
True or false: abnormal diffusion is more likely to cause problems during exercise than during rest.
TRUE: it may lead to a lower PaO2 during exercise due to lowered transit time
How might you treat hypoxemia due to diffusion limitation?**
Give 100% supplemental oxygen
Name 2 types of R-to-L shuts.
Cardiac Shunts Pulmonary shunts (unventilated alveoli with preserved perfusion)
How might you treat hypoxemia due to R-to-L shunt?**
TRICK QUESTION! Does not respond to supplemental oxygen like the other causes of A-a widening!
What is R when FiO2 is 100%?
1
What is the most common cause of hypoxemia in hospitalized patients?
V/Q mismatch
True or false: shunts have very large V/Q ratios.
FALSE! They are at the extreme of LOW V/Q
How might you treat hypoxemia due to V/Q defect?
supplemental oxygen
What diseases may be associated with low V/Q?
- Obstructive diseases (asthma, COPD)
- Pulmonary edema
What diseases may be associated with high V/Q?
- Pulmonary embolism
- Dead space (extreme)
True or False: must lung diseases cause V/Q mismatch.
TRUE!
How is water cleared from alveoli (after edema)?
Sodium channels! (water follows sodium)
