Title: Actions of Halothane and Isoflurane on Purkinje Fibers in the Infarcted Canine Heart
Abstract: The actions of halothane (HAL) and isoflurane (ISO) on conduction and regional refractoriness were studied in infarcted canine hearts to compare their effects on re-entry in vitro. In two anesthetic groups of 8 hearts, high and low dose effects were assessed using action potentials recorded from Purkinje fibers located in the nonischemic and ischemic regions. An extrastimulus technique was used to determine the relationship between delay of conduction of premature impulses into the more refractory ischemic region and induction of reentrant responses. At high doses (HAL 0.60 mM and ISO 0.64 mM, approximately 2.3 minimum alveolar anesthetic concentration [MAC]) both anesthetics decreased (P ≤ 0.05) the effective refractory period for direct intracellular stimulation of nonischemic fibers (local ERP, initial control: 294 ± 8 ms); the decrease with HAL (−29 ± 6 ms) was smaller (P ≤ 0.05) than with ISO (−50 ± 7 ms). HAL and I s 0 also decreased (P ≤ 0.05) the coupling interval of the earliest premature impulse which conducted into the infarct (system effective refractory period [SEW], control: 301 ± 7 ms) by −31 ± 11 and −44 ± 8 ms, respectively. In contrast, the functional refractory period (FRP) in the ischemic region (control: 354 ± 4 ms) was increased by HAL (26 ± 8 ms, P 20.05) but decreased by ISO (−14 ± 4 ms, P ≤ 0.05). Although both agents increased the difference (FRP - SERP) between regional refractory periods (HAL, 43 ± 6 to 100 ± 15 ms; ISO, 63 ± 11 to 93 ± 12 ms), HAL tended (P ≤ 50.06) to slow conduction of premature impulses initiated at the SERP (from 103 ± 19 to 173 ± 23 ms) more so than ISO (from 105 ± 16 to 126 ± 13 ms). HAL increased the incidence of reentry (from 33% to 69%, P ≤ 0.01) within its group and widened the range of premature impulses inducing reentry. ISO also tended to increase the incidence of reentry (from 25% to 37%, P ≤ 0.06) and shifted the reentrant zone to shorter coupling intervals in association with its actions decreasing refractoriness in both regions. The results indicate that the actions of both HAL and ISO on conduction and regional refractoriness influence reentry in this in vitro model and that the more prominent effects of HAL on conduction of premature impulses into the infarct are more “proarrhythmic” than those of ISO. (Anesth Analg 1993;76:718–25)