What is Gas Transfer?

Question 1

What is Gas Transfer?

Answer 1

• Primary role of the lungs is gas exchange between the atmosphere and the pulmonary circulation -Oxygen and Carbon Dioxide
• Required for metabolism
• Certain lung diseases can impair transfer of gases across lung membrane
• Therefore important that gas exchange can be measured in lung function laboratories

Question 2

Review of Gas Transport in the blood

Answer 2

1) Oxygen - carried in 2 forms:

—> in combination with Hb in the RBC

• –> O2 dissolved in plasma
• Low capacity

2) Carbon Dioxide -

—-> As HCO3- ions (90%)

CO2 + H2O H2CO3

                (CA)

H2CO3 H+ + HCO3 -

—> Dissolved in solution, Bound to Proteins (e.g Hb)

Question 3

Which gas has the greatest affinity for Haemoglobin?

Answer 3

CO

Question 4

equations

Answer 4

TLco = VA X Kco

Kco = TLco / VA

Question 5

CO

Answer 5

• -> High affinity for Haemoglobin
• -> Membrane diffusion coefficient and rate of reaction of CO and Hb are similar to O2
• -> CO is safe to inspire at low concentrations (IR & E)
• -> CO binds to same sites as Oxygen on Hb molecule
• -> Low [CO] present in bloodstream at start of test usually facilitates CO uptake

Question 6

Inert Gas Component

Answer 6

• Commonly methane (CH4) or Helium (He) (Not BOTH)
• Inert/ Relatively insoluble
• Does not cross the lung membrane

Uses:

• Gives a measure of alveolar volume (VA)
• Calculation gas transfer per unit volume (Kco)
• Quality Assurance

Question 7

Gas Analysers

Answer 7

CO --> Infra Red/ Electrochemical
He --> Thermal Conductivity
CH4 --> Infra Red
Acetylene --> Infra Red
Oxygen --> Paramagnetic AND Zirconium

Question 8

Patient Preparation

Answer 8

Smoking –> 24 hours
Consuming Alcohol –> 4 hours
Vigorous Exercise –> 30 minutes
Wearing Restrictive Clothing –> No time to discontinue
Eating a substantial meal –> 2 hours
Short acting bronchodilators –> > 4 hours

Question 9

Gas Transfer Techniques

Answer 9

• -> Single Breath Method
• -> Rebreathing Method
• -> Intrabreath Method

Question 10

Measurement Procedure

single breath - TLco, sb

Answer 10

1) Test is performed seated, wearing nose-clips
2) Patient connected to mouthpiece/breathes normally
3) Small breath in (optional)
4) Blow out as far possible to RV (relaxed)
5) Maximally inhale test gas (to TLC) as fast as possible
6) Hold breath for 10s
7) Breathe out as far as possible (relaxed)
8) Disconnect patient from mouthpiece
9) Repeat test after a minimum of 4 minutes
10) A maximum of 5 manoeuvres should be attempted

Question 11

BHT (Breath Hold Time)

Answer 11

• -> 2 methods for calculating BHT:
• Jones-Meade
• Ogilvie
• -> Ogilvie gives similar results except in presence of airflow obstruction
• -> BHT should be set at 10 seconds
• -> Should not be < 6 seconds
• -> Technical comments