Expansion
Nitrogen splints alveoli open; replacing it with rapidly absorbed oxygen allows collapse
Expansion
Alveolar gas is normally about 79 per cent nitrogen, which is poorly soluble and is absorbed very slowly. It therefore acts as an internal splint, maintaining alveolar volume even when ventilation is poor.
Breathing high concentrations of oxygen washes nitrogen out. In an alveolus with a low V/Q ratio or behind a partially obstructed airway, oxygen is then absorbed into the blood faster than it can be replaced, and the alveolus collapses.
The collapsed units are still perfused, so they become shunt, and oxygenation paradoxically worsens.
Practical consequences:
- Atelectasis develops in most patients within minutes of induction of anaesthesia with high inspired oxygen
- Preoxygenation with 100 per cent oxygen before intubation is still correct, since the benefit of a full oxygen store outweighs the risk, but PEEP and recruitment help
- Avoiding unnecessarily high inspired oxygen during maintenance, and using PEEP, reduce postoperative atelectasis