DSA - Ventilation and Diffusion

Question 1

Which lung volumes can be measured with spirometry?

Answer 1

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.

Question 2

What is tidal volume?

Answer 2

Amount of air that enters or leaves the lungs in a single cycle, ~500 mL for a normal breath.

Question 3

What is functional reserve capacity?

Answer 3

Volume of gas that remains in the lung at the end of a passive expriation (equilibrium point for lung).

Question 4

What is inspiratory capacity?

Answer 4

Maximal volume of air that can be inhaled from FRC

Question 5

What is inspiratory reserve volume?

Answer 5

Volume of air that can be inhaled after a normal inspiration

Question 6

What is expiratory reserve volume?

Answer 6

Volume that can be exhaled after a normal expiration

Question 7

What is residual volume?

Answer 7

Volume of air that remains in the lungs after macimal expiration - cannot be measured by spirometry.

Question 8

What is vital capacity?

Answer 8

Maximal volume that can be expired after maximal inspiration.

Question 9

What is total lung capacity?

Answer 9

Amount of air in the lung after maximal inspiration.

Question 10

What is spirometry?

Answer 10

A way of testing lung volumes and pulmonary functioning. Can measure:

inspiratory and expiratory reserve volumes

tidal volumes

inspiratory capacity

vital capacity

Question 11

How is a capacity measurement different from a lung volume measurement?

Answer 11

Capacity refers to a combination of 2 or more volumes

Question 12

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

Answer 12

Remember, residual volume, functional residual capacity, and total lung volume can’t be measure by spirometry.

Question 13

What is anatomical dead space?

Answer 13

Represents volume of conducting airways, not involved in gas exchange. Generally ~150 mL.

Question 14

What is alveolar dead space?

Answer 14

Alveoli containing air but not participating in gas exchange.

Question 15

What is physiological dead space?

Answer 15

Volume of gas that does not eliminate CO₂.

Question 16

What is alveolar ventilation? How does dead space affect alveolar ventilation?

Answer 16

Alveolar ventilation: room air delivered to the respiratory zone per minute

Va = (Vt-Vd)f

Vt = tidal volume

Vd = dead space

f = respiratory rate

Question 17

How can you determine functional reserve capacity if you can’t use spirometry?

Answer 17

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.

Question 18

What can be used to measure alveolar ventilation and why?

Answer 18

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.

Question 19

What is alveolar ventilation? What is the relationship between alveolar ventilation and PCO₂?

Answer 19

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.

Question 20

What is the Bohr method? What does it measure and what is it predicated on?

Answer 20

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.

Question 21

Draw out Fick’s law. What does it explain?

Answer 21

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

1. Pressure gradient
2. Thickness or diffusion distance
3. Area of barrier
4. Diffusion constant

Question 22

What is perfusion limited in regards to gas?

Answer 22

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.

Question 23

What is diffusion limited?

Answer 23

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.

Question 24

Why is CO diffusion limited?

Answer 24

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.

Question 25

How would an individual with impaired gas diffusion handle exercise? What would happen to Hb saturation with O₂?

Answer 25

Normally, O₂ would saturate depleted heme as it travels past alveoli in RBCs travelling through the capillary. If diffusion across the alveolar membrane is impaired, it could lead to heme not getting as fully saturated as it needs, and tissues distal to the lungs might not get the O₂ that they need.

CO₂ is highly soluble, and might still be able to diffuse out of the blood despite the impaired barrier. Individual might be hypoxic, but wouldn’t be hypercapnic.

Question 26

What is the uptake of O₂ in the pulmonary capillary in a normal situation?

Answer 26

Partial pressure between alveoli and artery is nearly equal by the time an RBC is 1/3 of the way through a pulmonary capillary bed. Since there is a large difference in the PO₂ in the alveoli versus the PO₂ in the RBC, this is plenty of time to achieve saturation. This means that lungs/alveoli have high diffusion reserves of O₂.

RBCs can have suboptimal saturation if blood gas barrier is thickened, and RBCs might leave capillary before becoming fully saturated.

Question 27

What is the diffusion rate for CO₂? What affects this? What are high levels of CO₂ called?

Answer 27

The diffusion for CO₂ is ~20x higher than the rate for O₂.

The concentration gradient for CO₂ is lower than O₂, and reaction with blood is buffered - and complex.

High CO₂ levels are called hypercapnia.

Question 28

What is diffusing capacity (D_L)?

Answer 28

The distance that a gas travels across membranes into the blood, and the time it takes to react with hemoglobin

Measured by the uptake of CO in the lung measured in mL/min/mmHg, normally ~25

Changes in Hb concentration will alter diffusion capacity - i.e. Fe-deficient anemia will have a reduced diffusing capacity.

Question 29

What are some pathological changes that reduce D_L?

Answer 29

Diffuse interstitial pulmonary fibrosis - thickening of the interstitium, alveolar wall, and destruction of capillaries

COPD - loss of lung elastic tissue and pulmonary capillaries - decreases surface area and total Hb content

Loss of functional lung tissue - decreases surface area and Hb content

Anemia - fall in Hb content

(Hb = hemoglobin)

Question 30

What are some impairments to the diffusion process?

Answer 30

Normally, the diffusion process is O2 gas diffusing past the blood-gas barrier to saturate RBCs. This can be impaired by 3 things:

1. Exercise - normally not a big deal. Most RBCs are fully saturated 1/3 of the way through their pulmonary capillary course. Maximal exercise will shorten the time RBCs spend in the pulm capillary beds to 1/3 of normal, but there is still enough time for saturation.
2. Alveolar hypoxia - less O2 gas in alveoli means that there isn’t the gradient needed to establish adequate perfusion, and RBCs might take longer in their course through the pulm capillaries to get fully saturated. Can lead to exercise intolerance or tissue hypoxia in extreme situations.
3. Thickening of the blood gas barrier - will lead to less diffusion of O2, and less opportunity for RBCs to get saturated.

Question 31

What is a major limiting factor for the saturation of Hb by O2?

Answer 31

Diffusion of gas across the blood gas barrier

• O2 has to cross membranes, diffuse across capillary lumen, diffuse past RBC membrane, etc.
• even though this is a quick process, this isn’t negligible when considering diffusion time - add in parallel. RBCs don’t spend a long time in the capillary, so this rate can become a limiting factor