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Flashcards in Diffusion Capacity - Dagar Deck (13)
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Name (7) external variables that must be compensated for in a DLco test

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


Why is CO so useful for the DLco test?

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.


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

How much does DLco change during exercise?

~25 ml/min/mmHg

exercise: DLco increases 2-3X


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

Loss of surface area


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

What is his DLco? Is this significant?

His DLco should be ~50%

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


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)?

Emphysema: DLco reduced due to decreased surface area

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


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

What is the treatment?

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)


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

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


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

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


Decribe the numerical relationship between DLco and carboxygemoglobin levels

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


Does the valsalva meneuver increase or decrease DLco? Why?

Decreases DLco

Less intrathoracic blood volume



What is the V/Q ratio of a shunt?

What is the V/Q ratio of dead space?




Describe the 6 steps required to properly interpret a DLco

  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

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