Expiratory flow limitation causes hyperinflation and increased resting end expiratory pressure. This is termed auto or intrinsic PEEP,53 and it acts as a load during inspiration as the patient must generate a negative pressure equal to the level of auto-PEEP in order to generate gas flow at the mouth that triggers inspiration. Asynchrony with the ventilator may be caused by excessive auto-PEEP and may be resolved by matching the external applied PEEP to balance the system.54 In normal subjects, such a load would be easily borne. For example, an average level of intrinsic PEEP of 11 cm H2O is a small fraction of the total pressure generating ability. However, many intubated patients generate a maximum of -30 cm H2O. In this context, overcoming the threshold load effect of intrinsic PEEP uses 33% of available pressure generation and may contribute to fatigue.

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