The Effect of Sex on median effective concentration of ropivacaine for ultrasound-assisted caudal block in the elderly undergoing Anorectal Surgery

Technical Summary: The Effect of Sex on Median Effective Concentration of Ropivacaine for Ultrasound-Assisted Caudal Block in the Elderly

Problem Statement
Caudal block is a widely utilized anesthetic technique for anorectal surgeries due to its operational simplicity and safety profile. However, determining the optimal dosage of local anesthetics, specifically ropivacaine, in elderly patients remains challenging. While previous studies have established gender differences in ropivacaine requirements for caudal blocks in adult populations, it is unclear whether these differences persist in the elderly demographic. Age-related physiological changes, including altered sacral canal anatomy, reduced muscle mass, and modified drug metabolism, complicate the extrapolation of data from younger cohorts. Currently, dosing in geriatric patients often relies on empirical judgment, increasing the risk of local anesthetic toxicity or inadequate blockade. This study addresses the gap in knowledge regarding whether sex influences the median effective concentration (EC50) of ropivacaine for ultrasound-assisted caudal blocks specifically in patients aged 60–80 years.

Methodology
The study was a prospective, double-blind, single-center randomized controlled trial conducted between September 2022 and April 2024 at Chongqing Traditional Chinese Medicine Hospital.

• Participants: 40 elderly patients (20 male, 20 female) aged 60–80 years with ASA physical status I–II scheduled for elective hemorrhoidectomy. Exclusion criteria included severe comorbidities (e.g., uncontrolled hypertension, cardiac conduction abnormalities, BMI >30), local anesthetic allergy, and history of central nervous system disorders.
• Intervention: Patients received a single 20 mL injection of ropivacaine via an ultrasound-assisted caudal block. The needle was advanced into the caudal epidural space under ultrasound guidance to ensure precise placement.
• Study Design: The study employed Dixon's up-and-down sequential allocation method to determine the EC50.
  • Stratification: Patients were stratified by sex into Group C-m (male) and Group C-f (female).
  • Dosing Protocol: The initial concentration was set at 0.35%. Subsequent concentrations were adjusted by 0.025% increments or decrements based on the analgesic response of the preceding patient in the same group.
  • Efficacy Assessment: Analgesic effectiveness was evaluated 20 minutes post-injection via pinprick testing of the perineal anal area. A block was deemed "effective" if pain was absent and "ineffective" if pain was perceived despite numbness. Technical failures (e.g., vascular puncture, unilateral block) were excluded, and the concentration was repeated for the next patient.
• Statistical Analysis: Probit regression was used to calculate the EC50 and EC95 values with 95% confidence intervals (CI) for each group. Demographic and intraoperative data were analyzed using t-tests, Mann-Whitney U tests, or chi-square tests as appropriate.

Key Results

• Demographics: The two groups were well-matched regarding age, BMI, surgical duration, blood loss, and analgesia duration, with no statistically significant differences.
• EC50 Values:
  • Elderly Females (Group C-f): The EC50 was 0.281% (95% CI: 0.161%–0.353%).
  • Elderly Males (Group C-m): The EC50 was 0.263% (95% CI: 0.179%–0.311%).
  • Comparison: The EC50 for elderly females was approximately 6.4% higher than that for elderly males, a difference that was statistically significant (P = 0.003).
• EC95 Values: The EC95 for females was 0.362% (95% CI: 0.282%–0.562%) compared to 0.338% (95% CI: 0.288%–0.543%) for males.
• Safety: No cases of drug-requiring bradycardia or severe hypotension were observed in either group.

Key Contributions and Claims
The paper claims to be the first study to systematically evaluate gender differences in ropivacaine requirements for caudal anesthesia specifically within an elderly population.

• Quantification of Sex Differences: The study provides quantitative evidence that elderly female patients require a higher median effective concentration of ropivacaine (0.281%) compared to elderly male patients (0.263%) for successful caudal blockade.
• Age-Related Attenuation: The authors note that while the gender disparity exists, the magnitude of the difference in the elderly (6.4%) is less pronounced than in younger adult populations (where differences have been reported as significantly larger, e.g., 0.296% vs. 0.389% in a cited study by Li et al.). The paper attributes this attenuation to age-related declines in sex hormone levels, particularly estrogen, which modulates pain thresholds.
• Physiological Context: The study reinforces that age is a critical determinant of anesthetic requirements, noting that the EC50 in the elderly is generally lower than in younger adults due to factors such as increased dura mater permeability, reduced nerve fiber density, and altered drug clearance.
• Methodological Validation: The use of ultrasound guidance is highlighted as a method to improve procedural accuracy and safety, reducing the risk of complications associated with blind puncture techniques.

Significance
The findings offer evidence-based guidance for precision anesthesia in the geriatric population. By establishing that elderly females require slightly higher concentrations of ropivacaine than males for caudal blocks, the study suggests that clinical dosing strategies should account for sex-specific variations even in older adults. This contributes to optimizing anesthetic efficacy while minimizing the risk of local anesthetic systemic toxicity (LAST) in a vulnerable demographic with reduced physiological reserve. The authors conclude that while sex differences persist in the elderly, they are modulated by the aging process, necessitating tailored dosing strategies that consider both age and sex.