Comparison of Two Different Ultrasound Guided Techniques for Saphenous Nerve Block

Efficacy of Ultrasound Guided Nerve Block

Saphenous nerve blocks can be technically challenging. Recently described ultrasound techniques have improved the success rate of saphenous nerve blocks, but randomized controlled trials comparing these techniques are lacking. The investigators compared two common ultrasound guided approach for saphenous nerve block: saphenous nerve block at the adductor canal (ACSNB) vs. block by the distal trans-sartorial (DTSNB) approach.

Patients received either ACSNB or DTSNB in this prospective randomized, blinded, non-inferiority clinical trial. The primary objective was to show the non-inferiority of ACSNB to DTSNB in terms of block success. Secondary outcome measures were time required to perform the block, time to onset of successful block, and the visibility of the nerve using ultrasound. Sample size calculation: To address the primary objective of the study--to show non-inferiority of the ACSNB to DTSNB in terms of success as defined above- the investigators used the test of non-inferiority of two proportions. DTSNB is the primary technique of doing saphenous nerve block at the investigators' institution. From chart review of the electronic records from 2011 to 2012, the investigators found there was a 3% incidence of rescue blocks among saphenous nerve blocks (19 out of 667 blocks) at their institution. This assumed a success rate of approximately 97%. A study by Saranteas et al reported a success rate of 95.6% in healthy volunteers undergoing saphenous nerve block just after it exited from the adductor canal. Based on these two observations, the investigators expected the success rate of DTSNB to be 95%. To establish the non-inferiority of ACSNB to DTSNB, the investigators pre-determined that the success rate of ACSNB should not be lower than 85% (the expected difference in proportions should be less than (95% - 85%) 10%). Under these assumptions, to achieve 80% power, with a one-sided type I error rate of 0.05, the study needed 59 subjects per group; 118 patients total. 120 patients were enrolled for this study., randomized to receive either ACSNB (n=58) or DTSNB (n=62).To show the non-inferiority of the ACSNB to DTSNB, the upper 95% confidence bound of the success rates (πDTSNB - πACSNB) should be less than 10%. The normality of continuous variables was tested by the Shapiro Wilk's test. Based on the normality assumption, two independent samples t-test or Mann-Whitney U test was performed for continuous variables. Chi-square test or Fisher's exact test was performed for categorical variables.

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