Auto-PEEP: how to detect and how to prevent--a review.

Auto-positive end expiratory pressure (auto-PEEP) is a physiologic event that is common to mechanically ventilated patients. Auto-PEEP is commonly found in acute severe asthma, chronic obstructive pulmonary disease, or patients receiving inverse ratio ventilation. Factors predisposing to auto-PEEP include a reduction in expiratory time by increasing the respiratory rate, tidal volume or inspiratory time. Auto-PEEP predisposes the patient to increased work of breathing, barotrauma, hemodynamic instability and difficulty in triggering the ventilator. Failure to recognize the hemodynamic consequences of auto-PEEP may lead to inappropriate fluid restriction or unnecessary vasopressor therapy. Auto-PEEP can potentially interfere with weaning from mechanical ventilation. Many methods have been described to measure the Auto-PEEP. Although not apparent during normal ventilator operation, the auto-PEEP effect can be detected and quantified by a simple bedside maneuver: expiratory port occlusion at the end of the set exhalation period. The measurement of static and dynamic auto-PEEP differs and depends upon the heterogeneity of the airways. The work of breathing can be decreased by providing external PEEP to 75-80% of auto-PEEP in patients who are spontaneously breathing during mechanical ventilation but there is no evidence such external PEEP would be useful during controlled mechanical ventilation when there is no patient inspiratory effort. Ventilator setting should aim for a prolonged expiratory time by reducing the respiratory rate rather than increasing inspiratory flow. Routine monitoring for auto-PEEP in patients receiving controlled ventilation is recommended.