Gas Exchange Flashcards
What is the fraction of O2 in air? Nitrogen? Co2?
21% O2
78% nitrogen
0.3% CO2
What is the eqn for minute ventilation?
Tidal volume*(Respiration rate)
T or F. All inspired air reaches the alveoli
F. There are anatomic (and functional) dead spaces
What is alveolar dead space?
alveoli that are ventilated but NOT perfused with blood so they cannot participate in gas exchange
What is the physiologic dead space?
the anatomic dead space + alveolar dead space
What is the eqn for alveolar ventilation (aka effective ventilation)?
minute ventilaton - dead space
Physiologic dead space represents what fraction of tidal volume?
25-30%
T or F. Dead space is static
F. Dead space increases with lung volume (deep inspiration)
Does dead space increase or decrease in exercise?
decrease because the increase in tidal volume is overshadowed by a huge increase in perfusion recruitment
Can increased alveolar dead space ever be a good thing?
No, always pathologic (e.g. pulmonary embolism)
R= Respiratory Exchange Ratio.
= Vco2 (co2 production)/ Vo2 (O2 consumption)
normal = 200/250= 0.8
What is the eqn for alveolar gas exchange?
PAO2= PiO2- (PaCO2/R)
where R = Vco2 (co2 production)/ Vo2 (O2 consumption)
What does hyperventilation do to PacO2?
decreases it (you are pushing it off).. so PAO2 increases
How can you calculate arterial oxygen tension (PaO2)?
measure directly with arterial blood gas test
How can you calculate alveolar oxygen tension (PAO2)?
alveolar gas eqn
What does a widened A-a difference suggest? What is normal?
intrinsic lung disease
Eqn for PiO2?
(Pb-Ph20)*FiO2
T or F. A-a gradient increases with age
T. We cant diffuse gas as well
What is the formula for a normal A-a gradient?
(Age+4)/4
What is hypoxemia?
decreased PaO2 in blood
Hypoxemia with a normal A-a difference suggests what?
No lung disease, so either decreased PiO2 or hypoventilation
Hypoxemia with a widened A-a difference suggests what?
LUNG DISEASE, so either:
- diffusion limitation
- R-to-L shunt
- Ventilation/perfusion (V/Q) mismatch
When can decreased PiO2 occur?
-altitude
Rule of thumb for A-a gradient
an increase in A-a gradient of 10% isn’t that indicative of lung disease.. play it case by case
Why would a diffusion limitation become evident when walking/exercising?
Because normally RBCs take up to 0.75s to transmit a pulmonary capillary while full o2 binding only requires 0.25s but during exercise CO increases and blood moves faster so PaO2 decreases
T or F. 100% FiO2 infusion ELIMINATES effects of diffusion limitations
T. Normalizes A-a ratio and PaO2
What is a shunt?
a fraction of venous blood bypasses alveoli and enters systemic circulation de-oxygenated
What is a normal physiologic shunt?
5%
T or F. 100% FiO2 infusion ELIMINATES effects of shunts
F. Only cause that doesn’t correct with 100% O2
What is the overall V/Q of the lung?
0.8
T or F. 100% FiO2 infusion ELIMINATES effects of V/Q mismatch
T.
What are some causes of low V/Q?
- obstructive disease (COPD, asthma)
- shunt (V/Q=0)**
- pulmonary edema
What are some causes of high V/Q?
- pulmonary embolism
- extreme high is DEAD SPACE
T or F. Most lung disease cause V/Q mismatch
T. V/Q Mismatch is the most common cause of hypoxemia
What is a normal PaO2?
100- age*0.3
Is FEV1:FVC decreased in asthma?
Yes
How can you differentiate between COPD and asthma with PFTs?
both low FEV1:FVC but only EMPHYSEMA has a low DLCO, normal in asthma and chronic bronchitis
How does PAH/PE present on PFT?
NORMAL PFT but low DLCO
How can you differentiate between interstitial disease and chest wall disease with PFTs?
both have normal FEV1:FVC interstitial disease has a low DLCO and chest wall disease is normal
What are some things that cause low DLCO?
- anemia
- interstitial lung disease
- emphysema
- pulmonary vascular disease (PAH, pulmonary emboli)
What things can produce high DLCO (120+%)?
- Early CHF
- Obesity
- Pulmonary Alveolar hemorrhage
Eqn for arterial oxygen content of blood
= 1.39HbO2sat% + 0.003*PaO2
