9- Diffusion Capacity

Question 1

What is the respiratory unit composed of?

Answer 1

• respiratory bronchiole, alveolar duct, atria and alveoli
• 300 million alveoli in 2 lungs, each with extremely thin walls and are about .2mm in diameter
  *

Question 2

What are alveolar walls like in terms of thickness?

Answer 2

extremely thin

Question 3

gas exchange occurs through the membranes of ____________

Answer 3

gas exchange occurs through the memebranes of all terminal portions of lungs-respiratory/ pulmonary membrane

solid interconnecting capillaries

Question 4

What is diffusion? What law describes it? What is diffusion directly/inversely proportional to?

Answer 4

• transfer of gas across the blood-gas barrier described by Fick’s law
• rate of transer of a gas through a sheet of tissue
• directly proportional: to tissue area, the difference in gas partial pressure between the two sides, and the diffusion property of the gas
• inversely proportional: to the tissue thickness

Question 5

What 3 things affect the diffusing capcaity of a gas?

Answer 5

surface area

thickness

pressure gradient

Question 6

What is diffusing capacity of a respiratory memebrane? What does this mean for mean oxygen pressure? use an average young man as an example.

Answer 6

• diffusing capcaity is the volume of a gas that will diffuse through the memebrane each minute for a partial pressure difference of 1mmHg
  
  • average young man- diffusing capcaity for oxygen under resting conditions is 21ml/min/mmHg
  • mean oxygen pressure difference across the respiratory memebrane under normal resting conditions is 11mmHg
• 230ml of oxygen diffuses through the respiratoy membrane each minute (21x11=230)

Question 7

How do we assess the adequecy of effort for a PFT?

Answer 7

the volume inspired (VI) should be>90% of the largest vital capacity (VC) to show that any reduction in VA is not due to poor inspiratory efort

VI is within 85% of the individual’s largest VC, and the VI is withitn 200ml or 5% of the largest VA

Question 8

WHat 3 things does a Pulmonary Function Test (PFT) assess?

Answer 8

spirometry/FV loop

lung volumes

diffusing capcaity

Question 9

How long do RBCs have in the capillary for oxygen uptake (transit time)? How long do they need? How does this relate to hypoxemia symptoms that occur only with exercise?

Answer 9

RBCs have .75 seconds

they onyl need .25

this is why at rest anemic people usually aren’t symptomatic because their RBCs have plenty of time for transfer. when there is less transit time due to exercise, then your body will notice the decreased amount of hemoglobin because there isnt enough time for the RBCs to exchange oxygen and there arent enough hemoglbin to grab the oxygen

Question 10

What are the 3 determining facotrs for diffusion? Why is CO used instead of N2O to measure diffusion?

Answer 10

membrane thickness

partial pressure/gradient

surface area

• N2O: no afinity to O2
  
  • partial pressure rises rapidly
  • gradient wiped out is perfusion is low
  • purely perfusion limited
• CO: Hb has high afiinity
  
  • partial pressure stays low
  • no appreciable back pressure
  • purely diffusion limited **therefore good measure of diffusion**

Question 11

How do you analyze a case for restrictive vs. obstructive? what are the enxt steps in deciding your differnetial?

Question 12

What are your two possible diagnosis based on DL,CO when:

FEV1/FVC> 70% (or LLN)

TLC> 80% (or LLN)

Answer 12

FEV1/FVC> 70 means not obstructive

TLC> 80 means not restrictive

DL, CO > 80 (or LLN)= healthy and normal!

DL, CO < 80 (or LLN) = Pulmonary vascular disroders (lungs are fine its a vascular problem) or early ILD

Question 13

What are the 2 possible differentials based on DL,CO if:

FEV1/FVC >70% (or LLN)

TLC< 80% (or LLN)

Answer 13

FEV1/FVC >70 means not obstructive

TLC< 80 (or LLN) means restrictive

DL,CO > 80= CW and Neuromuscular disorders

DL,Co< 80= ILD Pneumonitis

Question 14

What are the 2 possible differentials based on DL,CO if:

FEV1/FVC< 70

TLC>80

Answer 14

FEV1/FVC<70 means obstructive

TLC> 80 seconds obstructive

DL,CO >80= Asthma, Chronic Bronchitis

DL, CO<80= COPD (Emphysema)

remember obstrutive disroders are COPD, Chronic bronchitis, CF

Question 15

In a patient with emphysema would you expect the DL,CO to be high or low? why?

Answer 15

LOW!

• structural changes
• abnormal and permanent enlargement of the air spaces
• destruction of the airspace walls (decreasing surface area)

Question 16

In a patient with pulmonary HTN would you expect the DL,CO to be high or low? Why?

Answer 16

LOW!

remodeling and progressive loss of pulmonary arterial vasculature

results in decreased capillary blood volume available for gas exchange

(decreased SA)

Question 17

In a patient who had a pneumonectomy would you expect the DLCO to be high or low?

Answer 17

low! but DLCO/VA will increase because the same CO will be flowing through less space, meaning more blood being deleivered to each functioning alveoli.

Question 18

What is DLCO/VA (KCO)? what is it commonly confused as? What kind of relationship exists between KCO and lung volume?

Answer 18

• DLCO/VA reflects the alveolar CO uptake efficiency at a given volume
• it is not a correction factor for low lung volume
• the non-linear relationship between KCO and lung colume precludes KCO from being a correction factor fo DLCO when lung volumes are reduced
• the average VA of the tests used to generate the reported DLCO should be reported

NON-LINEAR RELATIONSHIP BETWEEN KCO AND LUNG VOLUME

Question 19

What happens to DLCO, VA, and DLCO/VA (KCO) for the following conditions:

• Incomplete lung expansion ( neuromuscular weakness or inadequate inspiration)
• discrete loss of alveolar units (pneumonectomy)
• diffuse loss of alveolar units (interstitial disease)

Answer 19

• Incomplete lung expansion ( neuromuscular weakness or inadequate inspiration)
  
  • DLCO: decrease
  • VA: decrease
  • DLCO/VA (KCO): increase
• discrete loss of alveolar units (pneumonectomy)
  
  • DLCO: decrease
  • VA: decrease
  • DLCO/VA (KCO): increase
• diffuse loss of alveolar units (interstitial disease)
  
  • DLCO: decrease
  • VA: decrease
  • DLCO/VA (KCO): decrease

Question 20

What problems lead to a reduced DLCO due to decreased area? What conditions will you see this in?

Answer 20

loss of alveoli, loss of alveolar walls, loss of interstitium, loss of blood

• Conditions:
  
  • emphysema
  • lung/lobe resection
  • bronchiol obstruction, as by tumor
  • anemia
  • multiple pulmonary emboli
  • pulmonary HTN
    *

Question 21

What would you expect DLCO to change for a patient who has silicosis from chronic exposure to silica dust? why?

Answer 21

DLCO will decrease beacuse of decreased SA and increased distance due to scarring

Question 22

How does scarring influence DLCO?

Answer 22

decreases!

loss of surface area and increased distance

Question 23

what conditions are associated with increased thickness of alveolar capillary memebrane? how does this influence DLCO?

Answer 23

decreases due to increased distance

interstitial lung disease/pulmonary fibrosis

asbestosis

sarcoidosis

collagen vascular disease

hypersensitivity pneumonitis

histiocytosis X

Question 24

What happens to DLCO in a patient with SLE? Why?

Answer 24

increases! increased thoracic blood due to alveolar hemorrhage

Question 25

what conditions have increased blood flwo to the lungs?

Answer 25

early stages of heart failure supine position left to right shunts exercise polycythemia asthma alveolar hemorrhage

Question 26

How does oxygen diffusing capacity change during exercise?

Answer 26

increases 3X normal (to 65ml/min/mmHg) openeing up of many previosuly dormant capillaries extra dilation of already open capillaries increases surface area of blood into which oxygen cna diffuse improves ventilation-perfusion ration

Question 27

What are some characteristics of the upper lung compared to the lower lung?

Answer 27

physiologic dead space in the upper lung when in an upright position-both ventilation and perfusion are lower in the upper lung compared to the lower lung perfusion is considerably lower in the upper lung

Question 28

What happens in the lower lung rather than the upper?

Answer 28

physiologic shunt slightly too little ventilation compared to blood flow

Question 29

what is the main change in the lung that occurs during exercise

Answer 29

far less physiologic dead space abnormal V/Q in the upper and lower normal lung

Question 30

How does CO poisoning impact DCLO?

Answer 30

smoking (so no smoking 24 hfr before PFT) smokers can have up to 10% more of carboxyhemoglobin levels each 1% incerase in carboxyhemoglobin levels results in 1% decrease in DLCO measurements

Question 31

How does anemia influnce DLCO?

Answer 31

decreases! But thats why we always correct for it!

Question 32

Why do we check DCLO beofre a lung resection surgery?

Answer 32

* to predict which patients are high risk for post-op complications!! * a very low DLCO increases the risk of postoperative morbidity and mortality * a low DLCO (\<20% predicted) suggests diffuse homogenous emphysema and has been identified as a major factor for poor outcomes following LVRS ## Footnote **basically low DLCO=high risk**

Question 33

How is DLCO used in disability evaluations? What about assessing need for oxygen therapy?

Answer 33

* Disability * a DLCO **\<40%** predicted may qualify for total disability according to Social Security standards * AMA and American Thoracic Society says **\<45%** predicted * Need for oxygen therapy * a low DLCO (\<50) is the major predictor of oxygen desaturation during exericise

Question 34

SUMMARY! sorry ashley summary is all one card bc its repeat haha What diseases are associated with the following PFT results: * Low DLCO with obstruction * COPD, CF, Bronchiectasis * Normal DLCO with obstruction * asthma, chronic bronchitis * Low DLCO with restriction * ILD * Normal DLCO with restiction * Neuromuscular disease * Low DLCO with normal spirometry and lung volume * Early ILD * Pulmonary vascular disease

Question 35

Why is DLCO increased in goodpasture?

Answer 35

hemorrhage