This means that a pathological process is affecting the barrier which is normally present between alveolar gas and the capillary blood. In health, this barrier is very thin and is made of the alveolar epithelial cell, the interstitial space and the capillary endothelium, fig 2.
Figure 2a. A schematic representation of the alveolar-capillary unit. The normal diffusion barrier is made of an alveolar epithelial cell, the interstitial space and a capillary endothelial cell.
The most common acute cause is interstitial pulmonary oedema.
Figure 2b. In pulmonary oedema, fluid accumulates in the interstitial space increasing resistance to gas exchange. In the more advanced stages, fluid accumulates in the alveolus further impeding gas exchange.
The most common chronic cause is pulmonary fibrosis.
Figure 2c. Pulmonary fibrosis. The diffusion barrier is fibrotic and gas exchange is impaired.
Diffusion deficit from fluid or fibrosis often contributes to respiratory failure in combination with other conditions, but rarely causes it on its own in ICU. When chronic fibrosis causes respiratory failure, patients do not benefit from ventilatory support. Acute fibrosis (e.g. non specific interstitial pneumonitis) presents in a very similar fashion to pneumonia and may respond to immunosuppression.