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

Background. Spread of intrathecal local anaesthetics is determinedprincipally by baricity and position of the patient. Hypobaricsolutions of bupivacaine are characterized by an unpredictablespread of sensory block whereas addition of dextrose 80 mg ml^–1provides a predictable spread but to high thoracic levels. Incontrast, dextrose concentrations between 8 and 30 mg ml^–1have 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 atboth 23 and 37°C before embarking on clinical studies. Methods. Density (mg ml^–1) was measured using the methodof mechanical oscillation resonance, accurate to five decimalplaces on 1250 samples. 500 density measurements were performedin a randomized, blind fashion at 23 and 37°C on 10 plainsolutions of bupivacaine (2.5, 5, and 7.5 mg ml^–1) levobupivacaine(2.5, 5, and 7.5 mg ml^–1) and ropivacaine (2, 5, 7.5,and 10 mg ml^–1). Following this, 750 density measurementswere taken at 23 and 37°C on the 5 mg ml^–1 solutionsof bupivacaine, levobupivacaine, and ropivacaine with addeddextrose (10, 20, 30, 50, and 80 mg ml^–1). Results. There was a linear relationship between density anddextrose concentration for all three local anaesthetics (R²=0.99)at 23 and 37°C. The mean density of levobupivacaine 5 mgml^–1 was significantly greater than the densities of bupivacaine5 mg ml^–1 and ropivacaine 5 mg ml^–1 after adjustingfor dextrose concentration using analysis of covariance. Thisdifference existed both at 23 and 37°C. The mean (SD) densityof levobupivacaine 7.5 mg ml^–1 was 1.00056 (0.00003) mgml^–1, the lower 0.5% percentile (1.00047 mg ml^–1)lying above the upper limit of hypobaricity for all patientgroups. Conclusions. The density of local anaesthetics decreases withincreasing temperature and increases in a linear fashion withthe addition of dextrose. Levobupivacaine 5 mg ml^–1 hasa significantly higher density compared with bupivacaine 5 mgml^–1 and ropivacaine 5 mg ml^–1 at 23 and 37°Cboth with and without dextrose. Levobupivacaine 7.5 mg ml^–1is an isobaric solution within all patient groups at 37°C. Br J Anaesth 2004; 92: 547–51