Anesthetic procedure
No premedication was administered, and parturients were required to abstain from food and liquid intake for at least 6 h and 2 h before surgery, respectively. Intravenous access was established and nasal oxygenation at a rate of 3 L/min was administered upon entering the operating room. As a preload prior to anesthesia, 10 mL/kg of Ringer's lactate was administered. The parturient was placed in a left lateral position, and an 18-gauge cannula needle was used to puncture the epidural space, followed by the insertion of an epidural catheter. After administering 3 mL of 2% lidocaine as a test dose via the epidural catheter, an initial bolus of 6 mL 0.5% ropivacaine was delivered, followed by a supplemental dose of 6–9 mL of the same local anesthetic solution to ensure the upper sensory block level of T6. The local anesthetic solution could be administered as needed, up to a safe maximum dose of 200 mg. If the sensory block level failed to reach T6 at 30 min after epidural administration, intravenous analgesics as rescue agents or general anesthesia would be considered and the parturient would be excluded from the study. The patient-controlled epidural analgesia package containing 150 mg ropivacaine and 3 mg morphine in 100 mL normal saline, with a bolus of 2 mL, a background flow of 2 mL/h, and a lockout interval of 15 min, was initiated for postoperative analgesia before transfer to the post-anesthesia care unit (PACU).