RESUMO
OBJECTIVE: To assess the effects of lipid on ropivacaine-induced convulsion and LD50 in rats and compare with those of the traditional anticonvulsants midazolam and propofol. METHODS: Protocol 1: A total of 120 SD rats (60 males, 60 females), weighing 200-300 g, were randomly assigned into 4 groups with equal males and females: lipid (L), midazolam (M) and propofol (P) and control (C). Rats were pretreated with 10 ml/kg lipid intravenously in group L, saline and 0.23 mg/kg midazolam (10 ml/kg in volume) sequentially in group M, saline and 4 mg/kg propofol (10 ml/kg in volume) in group P and saline 10 ml/kg in group C. Then ropivacaine 44 mg/kg (0.75%) was injected intraperitoneally into each rat. The convulsion rate in each group and the time of convulsion after ropivacaine injection were observed. Meanwhile, the plasma concentration of ropivacaine at the time of convulsion was measured. Protocol 2: Additional 100 male SD rats were used for the measurements of ropivacaine LD50 with different pretreatments including lipid, midazolam, propofol and saline through the up-and-down method. Rats were randomly assigned into 4 groups similarly as protocol 1. The doses of ropivacaine in each group were determined according to our pilot study and 6 dosage levels with the same interval ratio 8.5 was applied in each group. The doses of these pretreatment drugs and administration methods were similarly as protocol 1. RESULTS: The convulsion rate after 44 mg/kg ropivacaine ip injection was 43.3% in group C, 0% in group M, 13.3% in group P and 70% in group L. Lipid increased the convulsion rate significantly. The plasma concentration of ropivacaine at the time of convulsion was 1.65 ± 0.30 µg/kg in group C, 1.73 ± 0.14 µg/kg in group P and 3.45 ± 0.26 µg/kg in group L. The LD50 of ropivacaine in group C was 64.39 mg/kg, 88.40 mg/kg in group M and 90.20 mg/kg in group P and 55.45 mg/kg in group L. CONCLUSIONS: Midazolam and propofol not only decrease the convulsion rate of ropivacaine, but also increase its LD50. Lipid not only increases the convulsion rate of ropivacaine, but also decreases its LD50. The application of lipid for the prevention of local anesthetic toxicity has potential risks.