<qa>  What is the effect of pressure support ventilation on anesthetized spontaneous breathing subjects?The aim of this prospective study is to determine the effect of pressure support ventilation PSV on minute alveolar ventilation MAV and end-tidal carbon dioxide ETCO2 in anesthetized spontaneously breathing subjects by observing alterations of respiratory rate RR, expiratory tidal volume, MAV and ETCO2 at variable levels of pressure support. 