Baricity

Abstract: BackgroundInjection of local anesthetic into cerebrospinal fluid (CSF) produces anesthesia of unpredictable extent and duration. Although many factors have been identified that affect the extent of spinal anesthesia, correlations are relatively poor and the extent of spread remains unpredictable. Th

Abstract: Summary Spinal anaesthesia is the preferred anaesthetic technique for elective Caesarean deliveries. Hypotension is the most common side-effect and has both maternal and neonatal consequences. Different strategies have been attempted to prevent spinal-induced hypotension, including the use of low-dose bupivacaine. We conducted a systematic search for randomized controlled trials comparing the efficacy of spinal bupivacaine in low dose (LD ≤8 mg) with conventional dose (CD >8 mg) for elective Caesarean delivery. Thirty-five trials were identified for eligibility assessment, 15 were selected for data extraction, and 12 were finally included in the meta-analysis. We investigated sources of heterogeneity, subgroup analysis, and meta-regression for confounding variables (baricity, intrathecal opioids, lateral vs sitting position, uterine exteriorization, and study population). Sensitivity analysis was performed to test the robustness of the results. In the LD group, the need for analgesic supplementation during surgery was significantly higher [risk ratio (RR)=3.76, 95% confidence interval (95% CI)=2.38–5.92] and the number needed to treat for an additional harmful outcome (NNTH) was 4 (95% CI=2–7). Furthermore, the LD group exhibited a lower risk of hypotension (RR=0.78, 95% CI=0.65–0.93) and nausea/vomiting (RR=0.71, 95% CI=0.55–0.93). Conversion to general anaesthesia occurred only in the LD group (two events). Neonatal outcomes (Apgar score, acid–base status) and clinical quality variables (patient satisfaction, surgical conditions) showed non-significant differences between LD and CD. This meta-analysis demonstrates that low-dose bupivacaine in spinal anaesthesia compromises anaesthetic efficacy (risk of analgesic supplementation: high grade of evidence), despite the benefit of lower maternal side-effects (hypotension, nausea/vomiting: moderate grade of evidence).

Abstract: In a double-blind study of spinal anaesthesia with 0.5% bupivacaine 3 ml with no glucose, 5% glucose or 8% glucose all three solutions gave consistently good nerve blocks. The hyperbaric solutions (5% and 8% glucose) produced a greater cephalad spread and were suitable for lower abdominal surgery, whereas the plain solution (no glucose) seldom affected the thoracic nerves. Cardiovascular changes were more marked with the hyperbaric solutions but only necessitated treatment on two occasions. The duration of block was not affected by baricity and was in the range 140–160 min.

Abstract: 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−1 provides a predictable spread but to high thoracic levels. In contrast, dextrose concentrations between 8 and 30 mg ml−1 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−1) 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−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 measurements were taken at 23 and 37°C on the 5 mg ml−1 solutions of bupivacaine, levobupivacaine, and ropivacaine with added dextrose (10, 20, 30, 50, and 80 mg ml−1). Results There was a linear relationship between density and dextrose concentration for all three local anaesthetics (R2=0.99) at 23 and 37°C. The mean density of levobupivacaine 5 mg ml−1 was significantly greater than the densities of bupivacaine 5 mg ml−1 and ropivacaine 5 mg ml−1 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−1 was 1.00056 (0.00003) mg ml−1, the lower 0.5% percentile (1.00047 mg ml−1) 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−1 has a significantly higher density compared with bupivacaine 5 mg ml−1 and ropivacaine 5 mg ml−1 at 23 and 37°C both with and without dextrose. Levobupivacaine 7.5 mg ml−1 is an isobaric solution within all patient groups at 37°C.