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.