Inclusion Criteria:  As per the American Society of Anesthesiologists (ASA) classification I-I physical condition children undergoing thoracic surgery with OLV. From one day old to 12 years old. There is no prejudice in terms of gender. This study practically does not exclude any child who will undergo OLV even in the presence of cardiac, renal or hepatic diseases.  Or even in the presence of cardiac stimulants and supports (dopamine or dopamine), provided that the general condition is stable, which allows surgery and OLV.  Exclusion Criteria:  Premature infants: due to immaturity of the lung and insufficient formation of the surfactant. Cystic Fibrosis: The depressor for surfactant and lung immaturity is not permitted to perform the OLV technique There is no contraindication for the administration of dexmedetomidine in children except those who show signs of allergy to the dexmedetomidine. This study does not exclude practically any child who will undergo OLV. During operating surgery: The child is excluded from the study if hypoxia occurs (SpO2 <90%) and did not respond to maneuvers and anesthetic techniques (Increase PEEP Increase the FLOW, Tidal volume, increase Vt, Pmax increased pressure, FiO2 increased) Then the lungs are periodically ventilated with positive pressure, and the OLV technique is switched off and the child is excluded from the pilot study. During operating surgery: If there is a drop-in heart rate less than 60 beats / minute and it does not respond to atropine or an increased dose of Cardiac tonics (dopamine).  At this time the infusion of dexmedetomidine is stopped, it is assumed that there is no very slow pulse, and if it does, it is likely that the primary cardiac lesion is the cause.