Density of spinal anaesthetic solutions of bupivacaine, levobupivacaine, and ropivacaine with and without dextrose

Background. Spread of intrathecal local anaesthetics is determined principally by baricity and position of the patient. Hypobaric solutions of bupivacaine are characterized by an unpredictable spread of sensory block whereas addition of dextrose 80 mg ml provides a predictable spread but to high thoracic levels. In contrast, dextrose concentrations between 8 and 30 mg ml have shown reliable and consistent spread for surgery. Hence, the aim of this study was to determine the density of bupivacaine, levobupivacaine, and ropivacaine with and without dextrose at both 23 and 37°C before embarking on clinical studies. Methods. Density (mg ml) was measured using the method of mechanical oscillation resonance, accurate to five decimal places on 1250 samples. 500 density measurements were performed in a randomized, blind fashion at 23 and 37°C on 10 plain solutions of bupivacaine (2.5, 5, and 7.5 mg ml) levobupivacaine (2.5, 5, and 7.5 mg ml) and ropivacaine (2, 5, 7.5, and 10 mg ml). Following this, 750 density measurements were taken at 23 and 37°C on the 5 mg ml solutions of bupivacaine, levobupivacaine, and ropivacaine with added dextrose (10, 20, 30, 50, and 80 mg ml). Results. There was a linear relationship between density and dextrose concentration for all three local anaesthetics (R=0.99) at 23 and 37°C. The mean density of levobupivacaine 5 mg ml was significantly greater than the densities of bupivacaine 5 mg ml and ropivacaine 5 mg ml after adjusting for dextrose concentration using analysis of covariance. This difference existed both at 23 and 37°C. The mean (SD) density of levobupivacaine 7.5 mg ml was 1.00056 (0.00003) mg ml, the lower 0.5% percentile (1.00047 mg ml) lying above the upper limit of hypobaricity for all patient groups. Conclusions. The density of local anaesthetics decreases with increasing temperature and increases in a linear fashion with the addition of dextrose. Levobupivacaine 5 mg ml has a significantly higher density compared with bupivacaine 5 mg ml and ropivacaine 5 mg ml at 23 and 37°C both with and without dextrose. Levobupivacaine 7.5 mg ml is an isobaric solution within all patient groups at 37°C. © The Board of Management and Trustees of the British Journal of Anaesthesia 2004.