Diffusing Capacity

1. See Also
   1. Pulmonary Function Test
   2. Lung Volumes
   3. Inhalation Challenge Test
   4. Exercise Spirometry
   5. Functional Residual Capacity
   6. Vital Capacity
   7. Peak Expiratory Flow Rate (PEFR)
   8. Spirometry Lower Limit of Normal (PFT LLN)
   9. Obstructive Lung Disease
   10. Restrictive Lung Disease
2. Background
   1. Diffusing Capacity (DLCO) is capacity of lung to transfer gas across alveoli
   2. Alveoli exposed to Carbon Monoxide
      1. Measure disappearance of Carbon Monoxide from the lung
      2. Carbon Monoxide (CO) has very high affinity for Hemoglobin And Hemoglobin binds almost all CO inhaled
      3. DLCO = VCO/PACO
         1. Where VCO is volume of CO gas per unit time
         2. Where PACO is alveolar pressure of Carbon Monoxide (CO)
   3. Key Factors affecting DLCO
      1. Thickness and area of alveolar membrane
         1. Alveolar thickness plays a minor role in DLCO
      2. Reaction time of gas with Hemoglobin
         1. Capillary bed integrity and volume
         2. Hemoglobin concentration
3. Technique: Single-Breath DLCO for patients at rest
   1. Patient takes a full Vital Capacity breath
   2. Inhalation mixture
      1. Carbon Monoxide 0.3% (Used to determine gas transfer)
      2. Helium 10% (Used to determine Lung Volume, to calculate alveolar CO)
      3. Oxygen 21%
      4. Nitrogen 68%
   3. Breath held to Total Lung Capacity for 10 seconds
   4. Alveolar gas sample taken after washout
      1. First 750 ml of exhaled gas is discarded (contaminated with dead space)
      2. Next liter of exhaled gas is collected and analyzed
   5. Requires baseline Hemoglobin
      1. Results are adjusted based on the Hemoglobin level
      2. Corrected DLCO = DLCO / (0.07 x Hemoglobin)
   6. Result
      1. DLCO is volume of CO taken up per minute per mmHg PACO
4. Causes: Decreased DLCO
   1. Restrictive Lung Disease
      1. Idiopathic Pulmonary Fibrosis
      2. Asbestosis
      3. Sarcoidosis
      4. Berylliosis
      5. Hypersensitivity pneumonitis
      6. Langerhans Cell Histiocytosis X
      7. Miliary Tuberculosis
      8. Silicosis (advanced stage)
      9. Pneumonectomy
      10. Bronchiolitis Obliterans with Organizing Pneumonia
   2. Obstructive Lung Disease
      1. Cystic Fibrosis
      2. Emphysema
      3. Silicosis (early stages)
   3. Normal lung function tests
      1. Chronic Pulmonary Embolism
      2. Congestive Heart Failure
      3. Interstitial Lung Disease prior to PFT restrictive lung changes
      4. Primary Pulmonary Hypertension
      5. Connective Tissue Disease with lung involvement
         1. Dermatomyositis or Polymyositis
         2. Inflammatory Bowel Disease
         3. Rheumatoid Arthritis
         4. Systemic Lupus Erythematosus
         5. Systemic sclerosus
         6. Granulomatosis with Polyangiitis (previously known as Wegener's Granulomatosis)
5. Causes: Increased DLCO
   1. Asthma
   2. Left-to-right cardiac shunt
   3. Polycythemia Vera (see below)
   4. Pulmonary Hemorrhage
6. Causes: Normal DLCO
   1. Restrictive Lung Disease
      1. Cystic Fibrosis
      2. Emphysema
      3. Silicosis (early stages)
   2. Obstructive Lung Disease
      1. Alpha-1-Antitrypsin Deficiency
      2. Asthma
      3. Bronchiectasis
      4. Chronic Bronchitis
7. Causes: Falsely Abnormal DLCO (if Hemoglobin not 15 g/dl)
   1. See corrected DLCO for Hemoglobin Above
   2. Falsely elevated DLCO
      1. Polycythemia Vera
      2. Early Congestive Heart Failure
   3. Falsely reduced DLCO
      1. Anemia
8. References
   1. Johnson (2014) Am Fam Physician 89(5): 359-66 [PubMed]
   2. Pellegrino (2005) Eur Resp J 26(5): 948-68 [PubMed]
   3. Weinberger (1980) Chest 78(3): 483-8 [PubMed]