Effective Dose Range of Intrathecal Isobaric Bupivacaine to Achieve T5-T10 Sensory Block Heights for Elderly and Overweight Patients: An Observational Study.

Background and Objectives: The dose selection for isobaric bupivacaine determines the success of spinal anesthesia (SA). A dose higher than the optimal dose causes high SA, whereas an underdose leads to inadequate spread of cephalad. As it involves anatomical and physiological alterations, the dosing should be reduced with advancing age and body mass index values. Therefore, this study aimed to demonstrate the association between the isobaric bupivacaine dose and block height, and to determine the dose intervals of bupivacaine to achieve the T5-T10 sensory block with a low probability of high SA in elderly and overweight patients. Material and Methods: This retrospective observational study recruited 1079 adult patients who underwent SA with 0.5% isobaric bupivacaine from 2018 to 2021. The patients were divided into four categories: category 1 (age < 60, BMI < 25), category 2 (age < 60, BMI ≥ 25), category 3 (age ≥ 60, BMI < 25), and category 4 (age ≥ 60, BMI ≥ 25). The bupivacaine dose and sensory block height (classified into three levels: high (T1-T4), favorable (T5-T10), and low (T11-L2)) were recorded. Results: The sensory block level increased significantly with increasing doses of bupivacaine for patients in categories 1 and 2. The suggested dose ranges for the favorable block heights were 15-17 and 10.5-16 mg in patient categories 1-2 and 3-4, respectively. In these dose ranges, the probability range of high SA was 10-15%. Conclusions: The sensory block height following SA was associated with the bupivacaine dose in patients aged <60 years. Regardless of the BMI, the suggested dose ranges of 0.5% isobaric bupivacaine are 15-17 mg (3.0-3.4 mL) and 10.5-16 mg (2.1-3.2 mL) for patients aged <60 and ≥60 years, respectively.