Term
| How does Fick's law of diffusion describe the alveolar-capillary interface? |
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Definition
Vgas= [A x D x (P1-P2)]/T
The rate at which a gas passes between a gas-liquid interface depends upon the surface area (A), the diffusion coefficient (D), the partial pressure difference (P1-P2) and thickness (T). (A is most important!)
Small gases passing over large, thin areas with large partial pressure differentials will diffuse the best! |
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Term
| What are the major forces driving O2 and CO2 diffusion across the alveolar-capillary barrier, respectively? |
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Definition
Remember, solubility, partial pressure gradient and thickness are major determining factors, by Fick's law.
1) CO2 has a slightly greater MW than O2, but has a 24X greater solubility. SOLUBILITY drives CO2
2) O2 has a much great partial pressure differential (100:40, vs. 46:40 for CO2. PARTIAL PRESSURE drives O2 |
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Term
| How might a disorder that decreases the surface area available for diffusion and/or increases the thickness of the diffusion barrier (fibrosis, for example) create a gradient between alveolar and end-capillary PO2? |
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Definition
Remember, RBCs usually have 0.75s at the capillary endothelium (3X more time then they should need to take up oxygen from alveoli).
However, if SA decreases or it takes longer for O2 to get across, the RBCs may be undersaturated. |
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Term
| Why does it take 0.25 seconds for both PO2 and PCO2 equilibration between capillary blood and alveolar gas, despite the fact that CO2 has a much greater diffusion coefficient? |
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Definition
| CO2 is more soluble, BUT it has a lower partial pressure gradient (balance each other). |
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Term
| How is gas diffusion between the alveoli and the capillary blood assessed in the laboratory? |
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Definition
Measuring diffusing capacity for carbon monoxide (DLco).
CO binds Hb really quickly so diffusion continues constantly |
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Term
| What is the difference between partial pressure and concentration? |
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Definition
1) PP is pressure exerted by gaseous molecules referenced to total gaseous molecules
- Gases move between alveoli and capillaries until PP are equal between spaces
**Rate of equilibration is inversely proportional to solubility**
2) Concentration is reflected by amount of gas in blood.
- Depends on both PP AND solubility.
- So gases with HIGH PP may still have low concentrations in blood if they are insoluble |
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Term
| What determines the rapidity of anesthetic action? |
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Definition
Solubility!
Inversely proportional to rate of partial pressure equilibration, so highly soluble anesthetics act rapidly!
usually want an insoluble anesthetic |
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Term
| What are the 5 major mechanisms capable of generating Hypoxemia? |
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Definition
He Ran Very Slow Indeed
PAO2= (Pb-PH20) FiO2 - (PACO2/R)
1) Hypoventilation
2) Reduced Pb or FiO2
3) Ventilation-perfusion mismatch
4) Shunt
5) Impaired Diffusion
**1 and 2 do not change PA-aO2, and 3-5 do
** 3-5 decrease PaO2, but not PAO2, so PA-aO2 increases |
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Term
| What is the difference between Hypoventilation and Ventilation-perfusion mismatching in terms of PA-aO2? |
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Definition
1) Hypoventilation involves obligatory decrease in PO2, because of increased PCO2, but does not increase PA-aO2
2) Ventilation-perfusion mismatching will create low V/Q and high V/Q regions which will decrease PO2 and increase PA-aO2. |
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Term
| How does Hypercapnia arise? |
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Definition
Hypoventilation (inadequate alveolar ventilation) because of
Remember:
PAO2= (Pb-PH20) FiO2- (PACO2/R)
PaCO2= K X VCO2/ VE- VD
1) Abnormally low Ve
- respiratory center depression
- NM disease
- chronic lung disease
- "restrictive" chest wall disorders
2) Inadequate increase in Ve
- insufficient compensation for increase in dead space or CO2 production. |
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Term
| What would increasing VD, VE and VCO2 do to CO2 balance? |
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Definition
Increased VD and VCO2 will cause hypercapnia
Increased VE will increase CO2 excretion
PACO2= K VCO2/ (VE-VD) |
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Term
| What volume measurements can be obtained by spirometry alone? |
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Definition
Vital capacity (FVC and SVC)
NOT RV, FRC and TLC, because for these you must measure the air remaining in lungs. |
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Term
True or False:
Patient's of European decent tend to have spirometric values that are 10% lower than african americans |
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Definition
False.
AA and asians are 10% lower than whites |
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