Median Effective Volume of 0.25% Ropivacaine for Bilateral Transversus Thoracis Muscle Plane Block in Patients Undergoing Open Cardiac Surgery

Background: The median effective dose of ropivacaine required to achieve an effective transversus thoracis muscle plane block (TTP) has not been established. This study aimed to determine the EV50 of 0.25% ropivacaine for TTP using Dixon's up-and-down method. Methods: This prospective study involved 21 patients with American Society of Anesthesiologists physical status II to III, aged 20 to 60 years, with a BMI of 18 to 30 kg/m², who were scheduled for open cardiac surgery. Bilateral TTP was performed preoperatively, and 30 minutes post-block, the cold sensory decrease area (CSDA) was marked and recorded. The initial volume of 0.25% ropivacaine administered was 20mL. The effective block was defined as the CSDA encompassing thoracic nerves 2 to 6, which dictated a 2 mL increment or decrement in the subsequent patient's dosage. The study concluded when volumes reached 5 mL or 40 mL for 7 patients, or after 7 turning points. Results: A total of 21 cases were evaluated. Twelve (57.1%) of 40 received an effective block, while nine (42.9%) patients received a non-effective block. Analysis using SPSS 26.0 revealed the median effective volume (EV50) of 16.36mL for 0.25% ropivacaine and the 95% effective volume (EV95) of 19.81mL. Conclusion: In open cardiac surgery, the median effective volume of 0.25% ropivacaine for TTP is 16.36 mL (95% CI 14.35 to 18.43), while the 95% effective volume is 19.81 mL (95% CI 18.60 to 22.75). Trial Registration Number: ChiCTR2300073449.