Diffusing Capacity Lab Report

Diffusing Capacity Tests
Diffusing capacity test it's done to measure the lung's ability to exchange gases through alveolar-capillary membrane by using gas mixture. In this test they assess 3 things to see the efficacy of the lung;
1- Lungs' surface area with contact diffusing alveoli which is "alveolar volume".
2- Thickness of alveolar-capillary membrane.
3- blood volume available in the pulmonary capillaries.
Physiology:
In normal lung the gas exchange occur by passive diffusion of the gas via the alveolar-capillary membrane following Fick's law of diffusion which is : D = (∆P x A x S ) ÷ (√MW x T)
 ∆P = the pressure gradient of partial pressure of CO in the alveoli (PACO) - partial pressure of CO in the capillaries.
 A= surface area
Factors affecting gas diffusion :
1. Thickness of alveolar-capillary membrane:
When the thickening of AC membrane increase the diffused gas will decrease. So, thickening of the membrane inversely proportional to the diffusion capacity. There are some cases that the reduction in the diffusion capacity is due to increase in membrane thickness such as:
 Pulmonary edema.
 Pulmonary fibrosis.
2. Surface area of the alveolar- capillary membrane:
When the surface area decreased the diffusion capacity will decreased " directly proportional" e.g.;
 Pnemonactomy : it is a surgical procedure to remove one of the lungs.
 Lobactomy: it is a surgical procedure to remove lung lobe.
 Emphysem; maybe sometimes students will be confused with emphysema because they may think there will be increase in the surface area because of the enlargement of the alveolar sac but with this situation the alveolar wall will be damaged and will impair the diffusion.
3. Diffusion coefficient for the gas:
It is affected by two factors which are :
 solubility " directly proportional ". CO 20X more soluble than oxygen.
 molecular weight of the gas " inversely
Pressure gradient through the alveolar-capillary membrane: which is the difference of partial pressure of CO in the alveoli and partial pressure of CO in the pulmonary capillaries. Greater the difference , greater the diffusion capacity " directly proportional".
The direction of the diffusion also it is controlled by the pressure gradient, the gas will diffuse from the highest pressure of CO "in alveoli" to the lower pressure of CO "in capillaries".
Characteristics of the gas that used for diffusion capacity test:
1. Carbon monoxide 0.3%
2. Helium 12.0%; Tracer gas
3. Oxygen 21.0%
4. Balance N2
Which gas can be used to measure the diffusion capacity?
If it have these two features :
 It diffuse across alveolar capillary membrane
 It binds with hemoglobin.
Why we use CO in this test instead of other gases?
Because of two factors:
 Carbon monoxide diffuse very rapidly across the alveolar-capillary membrane. So, there will not be a back pressure.
 Carbon monoxide has high affinity 230 times more than oxygen to bind with hemoglobin. So, there will not be CO dissolved in plasma and that will not affect the partial pressure.

Equipments:
Spirometer, He/CO analyzer, end tidal sampler, gas mixture and one way