Mixing local anaesthetics with each other or with adjuvants such as sodium bicarbonate can produce microscopic crystals, raising questions about what is being injected and whether dosing changes. This laboratory study measured solute concentrations in eight commonly used mixtures known to crystallise, over about 13 hours after mixing, and analysed the precipitate from lidocaine–bicarbonate and lidocaine–ropivacaine–bicarbonate mixtures.
Local anaesthetic concentration in solution stayed constant over the full period. The crystals in the bicarbonate mixtures were composed of pure local anaesthetic.
This is reassuring about dose delivery, but it is a bench study. It says nothing about whether injecting crystals near nerves or into tissue causes harm, which remains untested in patients. Keeping mixtures simple and freshly prepared is still sensible.
- Prepare local anaesthetic–bicarbonate mixtures immediately before use.
- Inspect mixtures for visible precipitate and discard any that are cloudy.
- Avoid unnecessary multi-drug admixtures when a single agent will do.
- Do not assume crystals are harmless in tissue; this study tested chemistry, not patients.
Why it matters
It answers what the crystals are, which is the first question before asking whether they matter.
Don't overread it
A laboratory analysis — it does not show crystal injection is safe in patients.
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