Suspension of Mechanical Ventilation in Ureteroscopic Lithotripsy Under General Anesthesia

For some upper ureteral and renal stones,ureteroscopic lithotripsy is often perdurbed by the patient's respiratory movement either under epidural anesthesia or general anesthesia with mechanical ventilation. So intermittent suspension of mechanical ventilation under general anesthesia can ensure the surgery to be performed continuously and successfully. But this could result in hypercapnia and hypoxia. So far, there have been few studies focused on the change of the internal environment in blood as result of suspending breath. In this study, the investigators hypothesize that 10 cmH2O PEEP added to the respiratory circuit with 1 L/min 100% oxygen might extend the time of ventilation suspension but have no effect on carbon dioxide retention in blood.