9 - Diffusion Capacity Flashcards
What is the respiratory unit composed of?
- respiratory bronchiole, alveolar duct, atria and alveoli
- 300 million alveoli in 2 lungs, each with extremely thin walls and are about .2mm in diameter
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What are alveolar walls like in terms of thickness?
gas exchange occurs through the membranes of ____________
gas exchange occurs through the memebranes of all terminal portions of lungs-respiratory/ pulmonary membrane
solid interconnecting capillaries
Describe the respiratory membrane in terms of thickness and surface area. How does this relate to the size of RBCs?
Resiratory membrane is on average .6um
total surface area= 70 sq m
amount of blood in pulmonary capillaris= 60-140 ml
diameter of pulmonary capillaries=5um and RBCs are 6um. this allows the RBCs to squish through the capillaries directly touching the walls. this makes oxygen exchange easier bc it doesnt have to travel through plasma to reach the RBC
What is diffusion? What law describes it? What is diffusion directly/inversely proportional to?
- 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
What 3 things affect the diffusing capcaity of a gas?
surface area
thickness
pressure gradient
What is diffusing capacity of a respiratory memebrane? What does this mean for mean oxygen pressure? use an average young man as an example.
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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)
How do we assess the adequecy of effort for a PFT?
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
WHat 3 things does a Pulmonary Function Test (PFT) assess?
spirometry/FV loop
lung volumes
diffusing capcaity
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?
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
What are the 3 determining facotrs for diffusion? Why is CO used instead of N2O to measure diffusion?
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**
How do you analyze a case for restrictive vs. obstructive? what are the enxt steps in deciding your differnetial?
What are your two possible diagnosis based on DL,CO when:
FEV1/FVC> 70% (or LLN)
TLC> 80% (or LLN)
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
What are the 2 possible differentials based on DL,CO if:
FEV1/FVC >70% (or LLN)
TLC< 80% (or LLN)
FEV1/FVC >70 means not obstructive
TLC< 80 (or LLN) means restrictive
DL,CO > 80= CW and Neuromuscular disorders
DL,Co< 80= ILD Pneumonitis
What are the 2 possible differentials based on DL,CO if:
FEV1/FVC< 70
TLC>80
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
In a patient with emphysema would you expect the DL,CO to be high or low? why?
LOW!
- structural changes
- abnormal and permanent enlargement of the air spaces
- destruction of the airspace walls (decreasing surface area)
In a patient with pulmonary HTN would you expect the DL,CO to be high or low? Why?
LOW!
remodeling and progressive loss of pulmonary arterial vasculature
results in decreased capillary blood volume available for gas exchange
(decreased SA)
In a patient who had a pneumonectomy would you expect the DLCO to be high or low?
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.
What is DLCO/VA (KCO)? what is it commonly confused as? What kind of relationship exists between KCO and lung volume?
- 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
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)
- 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
What problems lead to a reduced DLCO due to decreased area? What conditions will you see this in?
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
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What would you expect DLCO to change for a patient who has silicosis from chronic exposure to silica dust? why?
DLCO will decrease beacuse of decreased SA and increased distance due to scarring
How does scarring influence DLCO?
decreases!
loss of surface area and increased distance
what conditions are associated with increased thickness of alveolar capillary memebrane? how does this influence DLCO?
decreases due to increased distance
interstitial lung disease/pulmonary fibrosis
asbestosis
sarcoidosis
collagen vascular disease
hypersensitivity pneumonitis
histiocytosis X
What happens to DLCO in a patient with SLE? Why?
increases! increased thoracic blood due to alveolar hemorrhage
what conditions have increased blood flwo to the lungs?
early stages of heart failure
supine position
left to right shunts
exercise
polycythemia
asthma
alveolar hemorrhage
How does oxygen diffusing capacity change during exercise?
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
What are some characteristics of the upper lung compared to the lower lung?
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
What happens in the lower lung rather than the upper?
physiologic shunt
slightly too little ventilation compared to blood flow
what is the main change in the lung that occurs during exercise
far less physiologic dead space
abnormal V/Q in the upper and lower normal lung
How does CO poisoning impact DCLO?
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
How does anemia influnce DLCO?
decreases! But thats why we always correct for it!
Why do we check DCLO beofre a lung resection surgery?
- 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
basically low DLCO=high risk
How is DLCO used in disability evaluations? What about assessing need for oxygen therapy?
- 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
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
Why is DLCO increased in goodpasture?
hemorrhage
