11
Diffusing capacity of the
lungs for CO (D
L
CO)
• Increased blood flow
▶ e.g., left-to-right shunt,
asthma, obesity
• Alveolar hemorrhage
• Erythrocytosis
• Pulmonary vascular
abnormality
▶ e.g., pulmonary
hypertension, pulmonary
embolism, vasculitis
• Emphysema with preserved
lung volume
• Anemia
>ULN (95th percentile)
Abnormally high
Normal
First determine if the D
L
CO is low or high based on the lower and upper bounds defined by the
5th and 95th percentiles of the reference values. A high D
L
CO is almost always due to increased
pulmonary blood volume, as in a le to right shunt, increased hemoglobin, as in erythrocytosis,
or free hemoglobin in any component of the airway, as in alveolar hemorrhage. To further
understand the cause of a low D
L
CO, next examine its components, V
A
and K
CO
. If V
A
is normal
then this is consistent with pulmonary vascular abnormalities, emphysema with preserved lung
volume, or anemia. If V
A
is low and K
CO
is low or normal, then there is typically loss of alveolar
capillary structure, such as in ILD, or emphysema with loss of lung volume. If V
A
is low and K
CO
is high, then there is a low lung volume state, either due to localized loss of lung volume, such
as from lung resection, which may raise K
CO
somewhat, or incomplete lung expansion, such as
failure to fully inspire, which can increase K
CO
substantially.
