Sample Size Determination Preliminary data have shown that nasopharyngeal ventilation may improve applied tidal volumes compared to mask ventilation. Sample size analysis: a within-group difference of 50 ml tidal volume will be considered to be clinically significant in our study, because such difference will be equivalent to a 10% increase in tidal volume during naso-pharyngeal ventilation. Accordingly, per our a priori assessment, in order to detect an increase of 10% of tidal volume 16 patients will be required with 90% power at a significance level of 0.05. We propose to enroll 20 patients for this cross-over trial.