Fig. 2. (A ) Effect of four concentrations of the pure optical isomers of ropivacaine and bupivacaine on atrioventricular conduction time in 13 and 12 hearts, respectively. Values are mean ± SD of initial and final control and individual concentrations. At the highest S  (−)-bupivacaine concentration, 11 hearts, and for R  (+)-bupivacaine, 2 hearts, remained in sinus rhythm (1:1 conduction). (B ) Effect of five increasing concentrations of the pure optical isomers of ropivacaine (n = 6) compared with a high concentration (10 μm) of the isomers of bupivacaine (n = 12) on atrioventricular conduction time. Displayed are only hearts remaining in sinus rhythm (1:1 conduction). For S  (−)-bupivacaine, 11 of 12 hearts, and for R  (+)-bupivacaine, 2 of 12 hearts, remained in sinus rhythm. For both isomers of ropivacaine, all hearts at 10 μm and 20 μm maintained sinus rhythm. Six and five of six hearts, respectively, for 30 μm R  (+)- and S  (−)-ropivacaine remained in sinus rhythm. At 40 μm, five hearts in the S  (−)-group and four hearts in the R  (+)-group are shown. At 50 μm, three of six hearts treated with R  (+)-ropivacaine and four of six hearts treated with S  (−)-ropivacaine remained in sinus rhythm. AV = atrioventricular; ROP = ropivacaine; BUP = bupivacaine.