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Abstract
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Background and Aims Effective blinding in clinical trials is crucial to avoid bias. We collected secondary data to assess the adequacy of investigators’ blinding to study drugs during five prospective randomised, double-blind studies to estimate optimal local anaesthetic doses for axillary, interscalene and supraclavicular brachial plexus blocks (BPB).
Methods Following an ethics committee-approved study protocol, an investigator divided a local anaesthetic’s study dose into four or six 10 mL syringes, covering their barrels entirely with an opaque tape. An expert anaesthetist performed the required ultrasound-guided BPB using these syringes. We asked this anaesthetist to guess the drug volume used at the end of the injection. Importantly, we did not inform them of the accuracy of their guess and chose patients randomly. We recorded this data and analysed it using SPSS 29.
Results We collected data for 20 out of 81 axillary blocks, 11 out of 46 interscalene blocks, and 22 out of 73 supraclavicular blocks. The error in the guesses ranged from -55% (underestimate) to 100% (overestimate), and the median of errors was –11%. Figure 3 shows the variability in the guess accuracy at different local anaesthetic volumes.
The table shows the percentage of errors in local anaesthetic volumes guessed. The negative numbers mean an underestimate, while positives indicate an overestimate
A box plot of the percentage of errors in local anaesthetic volumes guessed. The negative numbers mean an underestimate, while positives indicate an overestimate
A plot showing the relationship between the volume injected and variability in the guess accuracy at different local anaesthetic volumes with a best fit (linear regression) line
Conclusions We demonstrate that robust blinding of drug syringes is possible using a simple method of carefully placing opaque tape covering their barrels entirely. The blinded investigators cannot correctly guess the volume of a drug injected, e.g., by looking at the position of the syringe plunger or the ultrasound machine’s screen, preserving the data integrity. Reference: Day SJ, Altman DG. Blinding in clinical trials and other studies. BMJ.2000;321(7259).