Clinical Aspects of Diffusion Capacity

Question 1

What causes decreased DL_CO?

Answer 1

Anemia

• Must be corrected for:

DL_COcorrected = DL_{COuncorrected} x [10.22+Hb]/[1.7xHb]

Question 2

1. Why is CO used to measure diffusion capacity?
2. Compare it to N₂O:

Answer 2

• Hb has high affinity for CO
• Partial pressure stays low
  
  • Therefore, gradient is always maintained irrespective of perfusion
• Purely diffusion limited….

Compared to N₂O

• N₂O: No affinity to Hb
• Partial pressure rises rapidly
• Gradient wiped out if perfusion low
• Purely perfusion limited

Question 3

• What is the average DL_CO?
• What happens to DL_CO during exercise?
• What causes DL_CO adjustment?

Answer 3

• Average DLco of resting adult patient by ‘single breath’ method is ~ 25ml of CO/min/mmHg
• DLco increases during exercise by 2-3 times.
• DLco depends on lung volumes so they have to be adjusted for age, sex and height

Question 4

What are the major factors that cause change in the DL_CO?

Answer 4

1. Surface area
2. distance (thickness)
3. Pressure gradient

• Loss of surface area is the more common than thickening of alveolar-capillary membrane

Question 5

How will a pneumonectomy affect DL_CO?

How do you account for alveolar ventilation?

Answer 5

Pneumonectomy = one lung is surgically removed

• DL_CO = 50% of predicted
• DL_CO corrected for alveolar ventilation?
  
  • DLco/VA = 100%

Question 6

How would you treat an emphysematous bullae? Why is this type of treatment done?

Answer 6

Pneumonectomy/lobectomy

• = Lung volume reduction surgery
• Removal helps in expansion of the compressed lungs
• Improves elastic recoil of lungs
• Diaphragm moves to more normal position.

Question 7

How will a restrictive defect (like muscular dystrophy) affect the DL_CO and ventilation?

Answer 7

• Reduced capacity to take a deep breath
• Single breath DLco and vital capacity will be low
• Clinical significance of DL_CO/V_A is low

Question 8

What spirometry values would be expected for a patient with emphysema?

Answer 8

• ↓ FEV₁
• FVC normal
• ↓ FEV₁/FVC ratio

Question 9

Why is the DL_CO reduced in patients with emphysema?

Answer 9

**↓ surface area ** ⇒ ↓ D (diffusion rate) ⇒ ↓ DL_CO

Question 10

What spirometry values would be expected for a patient with asthma?

Answer 10

• ↓ FEV₁
• FVC normal
• ↓ FEV₁/FVC ratio

Question 11

• Why does the DL_CO normal or increased in patients with asthma?

Answer 11

**↑ DL_CO **

• ↑ negative intrathoracic pressure
• ↑ thoracic blood volume

Question 12

Define silicosis:

Answer 12

Interstitial lung disease caused by chronic exposure to silica dust

Question 13

What spirometry values would be expected for a patient with silicosis?

Answer 13

• ↓ FEV₁
• ↓ FVC
• normal FEV₁/FVC ratio

Question 14

Why is DL_CO decreased in patients with silicosis?

Answer 14

↑ d (thickness) ⇒ ↓ D (diffusion rate) ⇒ ↓ DL_CO

Question 15

What causes increased distance (d)?

Answer 15

Thickening of alveolo-capillary membrane

• Late stages of Congestive heart failure
• Interstitial lung disease/ Pulmonary fibrosis
  
  1. Asbestosis
  2. Sarcoidosis
  3. Collagen vascular disease
  4. Hypersensitivity pneumonitis
  5. Histiocytosis X
  6. Pulmonary alveolar pneumonitis

Question 16

How does usual interstial pneumonitis affect the DL_CO? What syndromes/diseases can cause this?

Answer 16

**↓ surface area & **↑ distance ⇒ ↓ diffusion rate ⇒ ↓ DL_CO

Causes:

1. rheumatoid arthritis
2. SLE
3. scleroderma

• ​drugs like methotrexate can also cause this

Question 17

How does pulmonary HTN affect diffusion rate (D)?

Answer 17

↓ surface area & ↑ distance ⇒ ↓ diffusion rate ⇒ ↓ DLCO

Question 18

How is pulmonary HTN clinically different from usual interstitial pneumonitis?

Answer 18

• Usually a normal FEV₁ & normal FVC
• normal FEV₁/FVC ratio
• Worsening dyspnea
• Elevated right heart pressures with poor function of right ventricle
• Reduced amount of capillary bed is being perfused
• Classical lesion: Plexiform lesion

Question 19

How does CO poisoning affect the DL_CO?

Answer 19

**↓ DL_CO **

• ​Smoking (so no smoking before 24 hours of test)
• Smokers can have up to 10% or more of carboxyhemoglobin levels
• Each 1% increase in carboxyhemoglobin levels result in 1% decrease in DLco measurements

Question 20

• What is the effect of the Valsalva maneuver?
• Why does this effect happen?

Answer 20

**Decrease DL_CO **

• ↑ intrathoracic pressure with closed glottis
• ↓ intrathoracic blood volume

Question 21

What happens to DL_CO if patient does not take in good breath during breath hold?

Answer 21

**Low DL_CO **

• Thus, patient should achieve inspired volume > 90% of vital capacity

Question 22

What spirometry values would be expected for a patient with usual interstial pneumonitis?

Answer 22

• ↓ FEV₁
• ↓ FVC
• normal/high FEV₁/FVC ratio

Question 23

How should you interpret the DL_CO if:

1. Is DL_CO increased →
2. Is DL_CO decreased →
   
   • If DL_CO/VA is normal →
   • If DL_CO/VA is low →
     
     • if spirometry is normal →
3. If DL_CO < 50% →

Answer 23

1. Is DL_CO increased → increased pulmonary blood volume/ hemorrhage
2. Is DL_CO decreased → look at DLco/VA
   
   • If DL_CO/VA is normal → pure restriction
   • If DL_CO/VA is low → obstruction or increased dead space
     
     • if spirometry is normal → think about pulmonary hypertension
3. If DL_CO < 50% → **consider blood gases, oxygen **evaluation with exercise