DSA - Ventilation and Diffusion Flashcards
Which lung volumes can be measured with spirometry?
All lung volumes except those that contain residual volume - the amount of air left after maximal expansion.
Exceptions include residual volume, functional residual capacity, and total lung volume.
What is tidal volume?
Amount of air that enters or leaves the lungs in a single cycle, ~500 mL for a normal breath.
What is functional reserve capacity?
Volume of gas that remains in the lung at the end of a passive expriation (equilibrium point for lung).
What is inspiratory capacity?
Maximal volume of air that can be inhaled from FRC
What is inspiratory reserve volume?
Volume of air that can be inhaled after a normal inspiration
What is expiratory reserve volume?
Volume that can be exhaled after a normal expiration
What is residual volume?
Volume of air that remains in the lungs after macimal expiration - cannot be measured by spirometry.
What is vital capacity?
Maximal volume that can be expired after maximal inspiration.
What is total lung capacity?
Amount of air in the lung after maximal inspiration.
What is spirometry?
A way of testing lung volumes and pulmonary functioning. Can measure:
inspiratory and expiratory reserve volumes
tidal volumes
inspiratory capacity
vital capacity
How is a capacity measurement different from a lung volume measurement?
Capacity refers to a combination of 2 or more volumes
Draw out a graph of lung volumes, and include waves that can represent:
Inspiratory and expiratory reserve volumes
residual volume
tidal volume
inspiratory capacity
vital capacity
functional residual capacity
total lung capacity
Remember, residual volume, functional residual capacity, and total lung volume can’t be measure by spirometry.
What is anatomical dead space?
Represents volume of conducting airways, not involved in gas exchange. Generally ~150 mL.
What is alveolar dead space?
Alveoli containing air but not participating in gas exchange.
What is physiological dead space?
Volume of gas that does not eliminate CO2.
What is alveolar ventilation? How does dead space affect alveolar ventilation?
Alveolar ventilation: room air delivered to the respiratory zone per minute
Va = (Vt-Vd)f
Vt = tidal volume
Vd = dead space
f = respiratory rate
How can you determine functional reserve capacity if you can’t use spirometry?
Use helium dilution method - total volume of helium exhaled should reflect volume of helium and functional reserve capacity.
Use body plethysmography - patient is put in an airtight box and pressure and volume are recorded before and after inspiration. Patient breathes into another box, then change in pressures are evaluated. Using the known volume of the box, FRC is estimated using Boyle’s law.
What can be used to measure alveolar ventilation and why?
CO2 from expired air can be used due to the fact that little CO2 is retained in the anatomical dead space at the end of inspiration - all CO2 is from the alveolar gas. There is rapid equilibration of CO2 across the alveolar space.
What is alveolar ventilation? What is the relationship between alveolar ventilation and PCO2?
Alveolar ventilation is the volume of fresh air entering the alveoli per minute. This can be measured from the expired CO2 concentration, as all expired CO2 is from alveolar gas.
If alveolar ventilation is halved, alveolar and arterial CO2 will double.
What is the Bohr method? What does it measure and what is it predicated on?
It determines physiological dead space.
It’s based on the fact that all expired CO2 derives from the alveolar space and none from the dead space.
Draw out Fick’s law. What does it explain?
A = area
Fick’s law describes movement of O2 across the alveoli space into the capillaries via passive diffusion. More generally, it applies to diffusion of gas through tissue. CO2 generally diffuses faster due to it’s greater solubility.
T = thickness D = diffusion constant P = partial pressure
Factors that influence diffusion rate
- Pressure gradient
- Thickness or diffusion distance
- Area of barrier
- Diffusion constant
What is perfusion limited in regards to gas?
Amount of gas transported is limited by blood flow - partial pressure gradient is not maintained. Oxygen is normally perfusion limited, but can become diffusion limited under certain circumstances.
What is diffusion limited?
Amount of gas that is tranported depends on the diffusion process. Diffusion will continue as long as the partial pressure gradient is maintained. Diffusion can be impaired by pathology - i.e. fibrosis.
Why is CO diffusion limited?
Hemoglobin has a high affinity for CO, so only minimal changes will happen to CO partial pressure in the blood. Pressure gradient for CO is maintained between the alveolar space and capillaries. This means that the amount of CO in the blood is limted by the diffusion properties of the barrier and not the amount of blood available.
CO is an example of a diffusion limited gas.