Impact of changes of positive end-expiratory pressure on functional residual capacity at low tidal volume ventilation during general anesthesia

Purpose: Several reports in the literature have described the effects of positive end-expiratory pressure (PEEP) level upon functional residual capacity (FRC) in ventilated patients during general anesthesia. This study compares FRC in mechanically low tidal volume ventilation with different PEEP levels during upper abdominal surgery. Methods: Before induction of anesthesia (awake) for nine patients with upper abdominal surgery, a tight-seal facemask was applied with 2 cmH₂O pressure support ventilation and 100 % O₂ during FRC measurements conducted on patients in a supine position. After tracheal intubation, lungs were ventilated with bilevel airway pressure with a volume guarantee (7 ml/kg predicted body weight) and with an inspired oxygen fraction (FIO₂) of 0.4. PEEP levels of 0, 5, and 10 cmH₂O were used. Each level of 5 and 10 cmH₂O PEEP was maintained for 2 h. FRC was measured at each PEEP level. Results: FRC awake was significantly higher than that at PEEP 0 cmH₂O (P < 0.01). FRC at PEEP 0 cmH ₂O was significantly lower than that at 10 cmH₂O (P < 0.01). PaO₂/FIO₂ awake was significantly higher than that for PEEP 0 cmH₂O (P < 0.01). PaO₂/FIO₂ at PEEP 0 cmH₂O was significantly lower than that for PEEP 5 cmH ₂O or PEEP 10 cmH₂O (P < 0.01). Furthermore, PEEP 0 cmH₂O, PEEP 5 cmH₂O after 2 h, and PEEP 10 cmH ₂O after 2 h were correlated with FRC (R = 0.671, P < 0.01) and PaO₂/FIO₂ (R = 0.642, P < 0.01). Conclusions: Results suggest that PEEP at 10 cmH₂O is necessary to maintain lung function if low tidal volume ventilation is used during upper abdominal surgery.