[Airway pressure settings during general anaesthesia].
In volume-controlled mechanical ventilation with constant inspiratory gas flow, which is most often used during general anesthesia, only the end-expiratory airway pressure (PEEP) has to be set by the user. Inspiratory airway pressures result from the combination of tidal volume and respiratory compliance as well as instantaneous gas flow and respiratory resistance. Given a normal respiratory compliance of 50-60 ml/mbar in mechanically ventilated patients, a driving pressure of 7-10 mbar is necessary for a tidal volume of about 6 ml/kg predicted body weight. The profile of airway pressure over time offers valuable information about respiratory mechanics. However, since the respiratory settings have a tremendous influence on the airway pressure-time profile, its interpretation has to be performed knowing the specific ventilator settings like inspiratory gas flow, presence or absence of an inspiratory pause and respiratory rate. Surprisingly, PEEP does not improve gas exchange in an unselected group of patients, although PEEP decreases the size of atelectasis and atelectasis can be detected after induction of anesthesia in 90% of adult patients. However, in obese patients gas exchange improves significantly when PEEP is applied. Furthermore, PEEP increases the apnoea interval tolerated without desaturation and thereby increases the safety margin during induction of anesthesia. If atelectasis shall be completely recruited, an airway pressure of 40 cm H2O is needed for 40 seconds. In order to avoid a severe drop in arterial blood pressure which may be accompanied by cardiac arrhythmia, such a recruitment manoeuvre should only be performed in normovolemic patients.