Crystal precipitates of local anaesthetic and adjuvant mixtures can be visualised and graded using ultrahigh-frequency ultrasound imaging

Background The use of mixtures of local anaesthetics and/or adjuvants is a common practice in regional anaesthesia, aimed at optimising onset time and prolonging block duration. However, several previous in vitro studies have demonstrated that mixing local anaesthetics may lead to crystallisation. A major barrier to understanding their potential physiological effects has been the lack of imaging techniques capable of detecting crystal formations beyond conventional microscopy. Methods In this in vitro study, ropivacaine 0.75%, lidocaine 2%, chloroprocaine 2% and mepivacaine 2%, as well as six commonly used local anaesthetic or local anaesthetic-adjuvant mixtures, were imaged by high-resolution ultrasound in liquid medium, using two high-resolution probes of 24 MHz and 33 MHz over a maximum duration of 40 min. The sonographic grade of crystallization was assessed on a scale from 0 to 3 based on the number and size of visible precipitates. Results Two pure local anaesthetics—lidocaine and chloroprocaine—demonstrated minimal crystallization. All mixtures produced precipitates immediately after mixing, with mild to moderate increases in crystallization quantities over time. The mixture of ropivacaine, lidocaine, and sodium bicarbonate exhibited the highest tendency to crystallise, reaching a sonographic grade of crystallization of 1 at baseline, increasing to 3 at 20–40 min. Conclusion All investigated mixtures demonstrated precipitation detectable by high-resolution ultrasound; the combination of ropivacaine, lidocaine and sodium bicarbonate showed the most pronounced effect. Ultrasound imaging proved to be a suitable method for detecting and grading crystalline structures in vitro and may represent a possible technology for future in vivo diagnostics.