Every local anaesthetic mixture tested formed crystals, most with ropivacaine, lidocaine and bicarbonate

Design

In vitro ultrasound imaging study

Population

4 plain local anaesthetics and 6 local anaesthetic/adjuvant mixtures

Primary outcome

Sonographic grade of crystallisation (0–3) over 40 minutes

Effect

All mixtures precipitated; ropivacaine + lidocaine + bicarbonate rose from grade 1 to grade 3 by 20–40 minutes

Mixing local anaesthetics and adjuvants is routine in regional anaesthesia, to speed onset or prolong a block. This Austrian in vitro study imaged four plain local anaesthetics and six common mixtures with 24 and 33 MHz ultrasound probes for up to 40 minutes and graded the precipitates seen on ultrasound from 0 to 3.

Plain lidocaine and chloroprocaine showed minimal crystallisation. Every mixture produced precipitates immediately after mixing, increasing modestly over time. Ropivacaine plus lidocaine plus sodium bicarbonate crystallised most, from grade 1 at once to grade 3 at 20 to 40 minutes.

Whether these crystals harm nerves or reduce block quality in patients is unknown. But the finding is a reason to draw up mixtures immediately before injection rather than leaving them in syringes, and to question whether alkalinising ropivacaine adds enough to justify it.

• Draw up local anaesthetic mixtures immediately before injection; do not prepare them in advance.
• Avoid adding sodium bicarbonate to ropivacaine-containing mixtures.
• Crystals here were detected by high-resolution ultrasound; a clear-looking syringe may not exclude them.
• Prefer a single agent where a mixture adds little to onset or duration.

Why it matters

Mixing is so routine that its physical chemistry is rarely considered at the point of injection.

Don't overread it

An in vitro study; it does not show that crystals cause nerve injury or block failure in patients.

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