Tolerability of large-dose intravenous levobupivacaine in sheep

In preclinical pharmacological studies of levobupivacaine (S-bupivacaine), we determined its tolerability, cardiovascular actions, and pharmacokinetics, and we estimated its margin of safety compared with bupivacaine in conscious sheep. Levobupivacaine HCl.H₂O was infused IV for 3 min into 10 previously instrumented ewes (approximately 50 kg). On subsequent days, the doses were increased by 50 mg from 200 or 250 mg until fatality occurred. All doses produced convulsions, QRS widening, and cardiac arrhythmias. With incremental doses, 4 of 4 animals survived 200 mg, 7 of 10 survived 250 mg, 3 of 7 survived 300 mg, but 0 of 3 survived 350 mg. Death resulted from sudden onset ventricular fibrillation (n = 3, within 2-3 min), electro-mechanical dissociation-pump failure (n = 5, within 4-5 min), or ventricular tachycardia-induced cardiac insufficiency (n = 2, > 10 min). The estimated fatal dose (mean ± SD) was 277 ± 51 mg for levobupivacaine (compared with 156 ± 31 mg found previously for bupivacaine). Pharmacokinetic analysis indicated initial and total distribution volumes = 4.5 (±1.6) and 97 (±22) L, total clearance = 1.7 (±0.4) L/min, and slow half life = 70 (±29) min; these values did not differ from those found previously with smaller doses. Heart and brain tissue levobupivacaine concentrations were approximately 3 times those in arterial blood. The doses of levobupivacaine survived were larger than found previously for bupivacaine, indicating its greater margin of safety.