Diffusion Capacity Test

Question 1

What is the purpose of Diffusion (gas transfer) tests?

Answer 1

To measure the lung’s ability to permit the transfer of gas across the A/C membrane

Question 2

Why is CO used?

Answer 2

1. Easily crosses the A/C membrane due to pressure gradient (relatively no CO in plasma)
2. Combines very rapidly with Hgb so that plasma pressure gradient is maintained throughout RBC transit time

Question 3

What is the definition of DLco?

Answer 3

Amount of CO (ml/min) that diffuse across the A/C membrane due to a pressure gradient (mmHg)
Indicates diffusion of A/C membrane and V/Q mismatching

Question 4

What are the structural components the determine gas transfer?

Answer 4

Lung gas volumes
Thickness of A/C membrane
Airway closure effect
Volume of blood in capillaries

Question 5

What are the functional aspects that determine gas transfer?

Answer 5

Uniformity of gas distribution
V/Q relationships
Composition of alveolar gas
HBg concentration and body position
PO2 of pulmonary capillaries

Question 6

How does Hgb concentration effect DLco

Answer 6

A higher Hgb level means a higher DLco because there is more affinity

Question 7

What is the technique for performing a DLco?

Answer 7

1. Patient exhales to RV
2. Quickly inhales to TLC inspiring a mixture gas of CO, tracer gas, and air
3. Patient holds breath for approximately ten seconds
4. Rapidly exhales
5. Deadspace gas exhaled (not collected)
6. Alveolar gas sample collected and anzlysed for final tracer gas concentration and final CO concentration

Question 8

Describe VA

Answer 8

Lung volume or alveolar volume
Estimate of the lung gas volume into which CO distributed and then transferred across the A/C membrane
Is calculated from a single breath dilution of tracer gas

Question 9

What is the significance of DLco?

Answer 9

Normal DLco ~ 25 mlCO/min/mmHg
Abnormal DLco occurs before abnormal PaO2
If DLco normal and PaO2 is low diffusion isnt the cause

Question 10

What are the reasons for a abnormal DLco?

Answer 10

1. Changes in A/C membrane conductivity (Dm)

2. Changes in blood related factors (Vc)

Question 11

What are the DLco classifications?

Answer 11

Mild >60%
Moderate 40-60
Severe <40

Question 12

Describe DL/VA

Answer 12

Allows analysis of individual components: DLco and VA as well as comparison of the two (determines whether the decreased DLco is due to volume loss or V/Q mismatching)
Normal DLco/VA= 4-5 ml/min/mmHg or 85% predicted
Normal DLVA + Decreased DLco = volume loss
DEcreased DL/VA + decreased DLco = V/Q mismatching