To measure the distance between the upper edge of the thyroid cartilage to the upper edge of the sternum add the distance from the upper edge of the sternum to the carina was calculated according to the chest CT scans as a guide to the placement of Uniblocker without the aid of FOB.

In previous study, the investigators found Chest computed tomography (CT) image can accurately predicting the optimal insertion depth of Uniblocker,however in some hospital the CT scan performed for the preoperative surgical evaluation without the vocal cords slices. If a more extensive CT scan was needed for obtaining a more reliable image of the vocal cords,the additional exposure dose would be a major disadvantage of the method.So the investigators measure the distance between the upper edge of the thyroid cartilage to the upper edge of the sternum add the distance from the upper edge of the sternum to the carina calculated according to the chest CT scans as a guide to the placement of Uniblocker without the aid of FOB.

Inclusion Criteria:

1. Adult patients undergoing left side thoracic surgery
2. The thoracic surgery requiring a Uniblocker for one-lung ventilation

Exclusion Criteria:

1. Age>70 or <18 years
2. ASA classifications >III
3. BMI >35kg/m2
4. Modified Mallampati classification ≥III
5. Thoracic surgery within the last one month
6. Severe cardiopulmonary disease