DLCO (75% - 125%)

Diffusing Capacity

Breath in and out carbon monoxide (CO) → measure the difference
1. Gas diffuse through the membrane
2. Gas binding to Hb (very readily in the case of CO)

Normal: 75% - 125%

Decrease:

Mild	60% - LLN (lower limit of normal)
Moderate	40% - 60%
Severe	< 40%

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Directly proportional to
- Surface Area (A)
- Gas solubility
- Pressure differential on both sides
Inversely proportional to
- Thickness of the membrane
- Molecular weight of the gas
Not affected by V/Q mis-match
- CO readily bind to Hgb after diffusion. It is only limited by diffusion, but not by perfusion (unless there is a near-complete obstruction)

Lower DLCO
- ILD - Interstitial Lung Disease → increased thickness
- Emphysema (’pink puffer’) → decreased surface area
- !ARDS (inflammatory debris in alveoli) → increased thickness
- Pulmonary vascular disease
  - !PE - Pulmonary embolism → obstruction of blood (zero Hb)
  - PH - Pulmonary hypertension (only for Group 1) → Arterial wall remodelling → gas need to diffuse through the thickened arterial wall
- Non-membrane causes
  - Anemia (↓Hb) → low Hb
  - Carboxyhemoglobinemia (CO poisoning - no Hb left) → low Hb
  - high altitude, small lung volume
High DLCO
- Increased pulmonary capillary blood volume
- No gas exchange membrane → Diffused alveolar hemorrhage