Gas Distribution Tests

Question 1

Describe the distribution of ventilation

Answer 1

Rate if gas distribution is dependent upon time constants (TC = Raw x C)
Cl different throughout lung due to different Ppl
Raw is different throughout lung due to different Pl (transpulmonary pressure)
Because of different TC air is not distributed uniformly throughout lung

Question 2

What are the 3 factors that affect movement of gas into terminal lung units?

Answer 2

1. Dimension of Airways (Size, integrity of lung tissue)
2. Transairway pressure
3. Compliance of the lung unit (gravity affects this)

Question 3

Describe gas distribution at FRC

Answer 3

At FRC: basal alveoli are more compliant than apical alveoli and therefore receive most of the gas entering the lungs

Question 4

Describe gas distribution at TLC

Answer 4

At TLC: both apical and basal alveoli are fully open therefore equal distribution of gas bin both basal and apical alveoli

Question 5

Describe gas distribution at RV

Answer 5

At RV: Basal airways are compressed since Ppl is positive therefore when taking in a breath from RV basal alveoli don’t receive initial gas, rather apical alveoli get the preferential gas

Question 6

What is the purpose of measuring distribution of ventilation?

Answer 6

To determine evenness/unevenness of gas distribution in the lung
Early detection of small airway disease

Question 7

Describe single breath N2 washout

Answer 7

Measures distribution of ventilation by analyzing %N2 expired
Patient exhales to RV
Then slowly inhales to TLC breathing in 100% O2
THe Patient then exhales slowly to RV and the %N2 exhaled is analyzed and the volume exhaled is measured
Normal = less than 1.5%

Question 8

What are the causes of uneven ventilation?

Answer 8

Regional changes in elasticity
Regional partial obstruction
Regional changes in elasticity and obstruction
Regional changes in expnasion

Question 9

What is the closing volume (CV)?

Answer 9

The volume from the start of phase IV to maximum exhalation

Normal CV is 10-20% of the VC

Question 10

Describe the four phases

Answer 10

Phase I: From the upper airway and contains no N2 (only 100% O2)
Phase II: Abrupt rise in N2 concentration and alveolar gas mixes with dead space gas (transitional gas)
Phase III: true alveolar gas (mixture of alveolar gas from apices, middle and base) and is the alveolar plateau portion of the curve
Phase IV: Ppl exceeds Paw and the basal airways are compressed and do not contribute and exhaled gas; expired gas now comes from apical alveoli