Spontaneously breathing anesthetized patients with a laryngeal mask airway: positive end-expiratory pressure does not improve oxygen saturation

Spontaneous ventilation is a popular mode of ventilation for patients with the laryngeal mask airway (LMA). Studies have shown, however, that spontaneous ventilation impairs gas exchange and that assisting or controlling ventilation results in higher oxygen saturation. Atelectasis during general anesthesia is a well described mechanism which impacts on gas exchange. Positive end-expiratory pressure (PEEP) increases the lung volume available for gas exchange. This study investigated whether the application of PEEP leads to an improvement of oxygen saturation in unassisted spontaneously breathing patients with a LMA. A total of 80 adult patients under general anesthesia were prospectively randomized into two groups. Both groups were left to breathe spontaneously. In group 1 the adjustable pressure limiting (APL) valve was opened resulting in zero end-expiratory pressure. In group 2 the valve was set to a PEEP of +7 cm H₂O. Oxygen saturation was measured by pulse oxymetry at four different phases: pre-induction, after induction and insertion of the LMA, during maintenance and in recovery. The application of PEEP did not improve oxygen saturation. In both groups the mean oxygen saturation was similar (97.2±1.8% in group 1 versus 97.2±1.9% in group 2, p=0.941) during maintenance. No effect on oxygen saturation in recovery could be found either (96.0±1.8% in group 1 versus 96.1±2.0% in group 2, p=0.952) and hemodynamics were unaffected by the application of PEEP. The application of a PEEP of +7 cm H₂O with a LMA under spontaneous ventilation cannot be recommended. Limitations of our study were the selection of healthy patients and omitting pre-oxygenation before induction which might have limited the development of atelectasis. In addition arterial partial pressure of oxygen (p(a)O₂) measurements could have revealed subtle changes in oxygenation.