CAN J ANESTH 54: 3 www.cja-jca.org March, 2007 Purpose: Ropivacaine and levobupivacaine were developed to reduce the risk of occasional toxicity reported with bupivacaine. While the effects of long-acting local anesthetics (LAAs) on myocardial contractility (inotropy) are well described, their effects on relaxation (lusitropy) remain largely unknown. The present study aimed to compare the effects of LAAs on rat myocardium. Methods:Left ventricular papillary muscles of male Wistar rats were used to compare the inotropic and lusitropic responses of increasing concentrations of LAAs (10 -8 to 10 -3 M) under isometric and isotonic conditions. Data are mean % (SD) of baseline value.Results: Long-acting local anesthetics induced a significant impairment of relaxation in isotonic and isometric conditions. As compared to ropivacaine, bupivacaine and levobupivacaine induced greater negative lusitropic effects in isotony [at 10 -3 M, maximum unloaded shortening velocity ( max Vr) = 27 ± 11 vs 13 ± 6 and 8 ± 5%] and isometry (at 10 -3 M, time-to-halfrelaxation: 106 ± 10 vs 127 ± 17 and 133 ± 17%). When the comparison was made with equipotent concentrations, the negative lusitropic effects induced with levobupivacaine were significantly greater than those of bupivacaine and ropivacaine in isometric and isotonic conditions (at 10 -3 M, max Vr = 7 ± 4 vs 13 ± 6 and 17 ± 4 %). As previously described, LAAs also induced concentration-dependent negative inotropic effects that were greater for levobupivacaine compared to equivalent or equipotent concentrations of bupivacaine and ropivacaine.Conclusions: Long-acting local anesthetics induce marked negative inotropic and lusitropic effects. Among LAAs, levobupivacaine exerts the greater depressant effects. Impairment of calcium handling and sarcoplasmic reticulum could explain the differential responses to local anesthetics. Objectif : La ropivacaïne et la lévobupivacaïne ont été dévelop-pées pour réduire le risque d'accidents sévères observés avec la bupivacaïne. Si les effets des anesthésiques locaux (LAAs) sur la contractilité myocardique (inotropie) sont bien décrits, les effets sur la relaxation (lusitropie) restent peu explorés. Cette étude a eu pour objectif de comparer les effets des LAAs chez le rat. Méthode : Les effets de concentrations croissantes de LAAs (10 -8 à 10 -3 M) sur les réponses inotropes et lusitropes ont été comparés à l'aide d'un modèle de muscles papillaires. Les valeurs sont exprimées en % de la valeur de base (moyenne ± ET). Résultats