Microscopic crystals have been reported when local anaesthetics are mixed with each other or with adjuvants such as sodium bicarbonate. An Austrian laboratory study measured the concentration of eight commonly used mixtures over about 13 hours and analysed the precipitate from lidocaine–bicarbonate and lidocaine–ropivacaine–bicarbonate mixtures.
The local anaesthetic concentration in solution stayed constant over time, and the crystals were composed of the local anaesthetic itself rather than a new compound.
This is reassuring about dose delivery but leaves the clinical question open: whether injecting crystals near nerves or into tissue matters. That needs in-vivo study. Until then, mixing immediately before use and inspecting the syringe remain sensible.
- Prepare local anaesthetic–bicarbonate mixtures immediately before injection rather than in advance.
- Inspect mixtures for cloudiness or precipitate before use.
- Avoid unnecessary mixtures of different local anaesthetics where a single agent will do.
- Note that dose in solution was not reduced by crystal formation in this laboratory study.
Why it matters
It answers a chemistry question about common mixtures, while the clinical consequence of injecting crystals remains unknown.
Don't overread it
This was a bench study; it says nothing about nerve injury or block quality in patients.
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