Airway Closure, More Harmful Than Atelectasis in Intensive Care?
Highlights
- Airway closure increases with increasing age, a consequence of loss of elastic tissue in the lung that may produce a positive pleural pressure in dependent regions (View Highlight)
- Factors that in combination reduce FRC, like anesthesia, age, obesity and head down positions for surgery will all increase likelihood for airway closure (View Highlight)
- Interestingly while auto-PEEP (or intrinsic PEEP) is easily eliminated by prolonging expiratory time, the level of airway opening pressure is not influenced by the duration of expiration and remains at a very reproducible level (View Highlight)
- a continuously closed or briefly open airway will promote atelectasis because of absent or poor ventilation, alveolar gas being sucked up by the capillary blood. Indeed, the atelectasis during anesthesia is almost always caused by airway closure [1]. The use of high inspired oxygen concentration speeds up the atelectasis formation because of rapid absorption of oxygen in the alveoli, whereas nitrogen in the alveoli will prevent or slow down atelectasis formation (View Highlight)
- the impaired ventilation by cyclic airway closure causes a ventilation/perfusion mismatch with impairment of oxygenation of blood. In addition, continuous airway closure promotes alveolar collapse, as said above, that causes a shunt and impairment of arterial oxygenation. (View Highlight)
- there are animal experiments suggesting that the beginning of an inflammatory reaction in the lung is not in dependent, collapsed/airless regions, nor in the uppermost, possibly over-aerated regions, but in a zone somewhere in the middle of the lung [14]. This zone is mostly poorly aerated, suggesting cyclic airway closure. Such cyclic closure results in a physical stress to the airway wall, opening and closing where surface forces have to be overcome and where the wall may suffer from damage [14]. Importantly, whereas atelectasis results in alveolar injury mostly in aerated regions, the repetitive opening and closing of distal airways occur also in lung regions that are atelectatic (View Highlight)
- the presence of airway closure makes measurement of alveolar pressure at the airway opening impossible or unreliable (View Highlight)
- A simple technique to assess complete airway closure is by using the low-flow inflation pressure–volume curve pattern (Fig. 1). The presence of low inflection point associated with, in the initial part of the curve, the absence of cardiac oscillations and very low compliance, close to the 2.5 mL/cm H2O of an occluded breathing circuit, is suggestive of complete airway closure (View Highlight)