Effect of ropivacaine concentration on the distribution of neural blockade in continuous thoracic paravertebral block

INTERVENTION: Postoperative continuous thoracic paravertebral block of ropivacaine 0.2% 6mL/h for 48 hours Postoperative continuous thoracic paravertebral block of ropivacaine 0.5% 6mL/h for 48 hours CONDITION: Patients scheduled for elective unilateral thoracotomy (lung lobectomy or pulmonary segmentectomy) under general anesthesia and continuous thoracic paravertebral block PRIMARY OUTCOME: Number of anesthetized dermatomes 24 hours after surgery SECONDARY OUTCOME: 1.Pain score (Numerical Rating Scale) assessed at 1 hour, 6, 24, and 48 hours after surgery; 2.The consumption of additional analgesics; 3.Patient satisfaction score; 4.Sedation score (Richmond Agitation Sedation Scale); 5.Incidence of nausea and vomiting INCLUSION CRITERIA: 1.Patients undergoing unilateral lung lobectomy or pulmonary segmentectomy at Niigata University Medical and Dental Hospital 2.American Society of Anesthesiologists Physical Status Classification I‐III 3.Written informed consent is provided