Diffusion Capacity - Dagar

Question 1

Name (7) external variables that must be compensated for in a DLco test

Answer 1

1. Alveolar ventilation
2. Hemoglobin
3. Carbon monoxide levels
4. Altitude
5. Age
6. Sex
7. Height

Question 2

Why is CO so useful for the DLco test?

Answer 2

CO has a high affinity for Hb, so during normal breathing its partial pressure in the blood remains low (purely diffusion limited). Thus, there is always a gradient between the alveoli and alveolar capillaries regardless of perfusion. Thus, its concentration in the alveoli and blood is related to the rate of diffusion only.

Question 3

What is the average DLco of a resting adult patient assessed by the single-breath method?

How much does DLco change during exercise?

Answer 3

~25 ml/min/mmHg

exercise: DLco increases 2-3X

Question 4

Which is a more common cause of decreased DLco: loss of surface area or thickening of the alveolar-capillary membrane?

Answer 4

Loss of surface area

Question 5

A man undergoes a left-sided pneumonectomy due to a mass in this lower left lobe.

What is his DLco? Is this significant?

Answer 5

His DLco should be ~50%

This is not significant because we must correct for alveolar ventilation (DLco/V_A = 100%). Normal lung is normal.

Question 6

A patient has an FEV1/FVC ratio of 56%, indicating an obstructive lung condition. How might we differentiate emphysema vs. asthma in this situation (forgetting drug-challenge methods)?

Answer 6

Emphysema: DLco reduced due to decreased surface area

Asthma: DLco normal or increased due to increased negative intrathoracic pressure increasing thoracic blood volume

Question 7

What is silicosis? How does it affect lung diffusing capacity (DLco)?

What is the treatment?

Answer 7

Silicosis: interstitial lung disease caused by chronic exposure to silica dust

DLco reduced due to thickening of the diffusion barrier (increased diffusion distance)

No treatment currently available (supportive only)

Question 8

Give several examples of conditions that cause thickening of the alveolo-capillary membrane (i.e. increased the diffusion distance -> decreased DLco)

Answer 8

• Late-stage CHF
• Interstitial lung disease / Pulmonary fibrosis
  
  • asbestiosis
  • sarcoidosis
  • collagen vascular disease
  • hypersensitivity pneumonitis
  • histiocytosis X
  • pulmonary alveolar pneumonitis

Question 9

Name (3) conditions associated with normal spironmetry and reduced DLco

Answer 9

1. pulmonary vascular disease
2. compined emphysema and ILD
3. anemia

Question 10

Decribe the numerical relationship between DLco and carboxygemoglobin levels

Answer 10

Each 1% increase in carboxyhemoglobin levels results in a 1% reduction in DLco

Question 11

Does the valsalva meneuver increase or decrease DLco? Why?

Answer 11

Decreases DLco

Less intrathoracic blood volume

Question 12

What is the V/Q ratio of a shunt?

What is the V/Q ratio of dead space?

Answer 12

zero

infinity

Question 13

Describe the 6 steps required to properly interpret a DLco

Answer 13

1. Verify the maneuvers were performed acceptabily
2. Verify all appropriate corrections were made (altitude, Hct%, COHb)
3. Verify correct reference values (age, sex, height)
4. Is DLco increased? -> increased pulmonary blood volume or hemorrhage
5. Is DLco decreased? -> look at DLco/VA
   
   1. if DLco/VA normal -> pure restriction
   2. if DLco/VA low -> obstruction or increased dead space
   3. if spirometery is normal, think pulmonary hypertension
6. If DLco < 50%, consider ABG and oxygen evaluation with exercise