Effects of thoracic epidural anesthesia with and without autonomic nervoussystem blockade on cardiac monophasic action potentials and effective refractoriness in awake dogs

Background: The effects of thoracic epidural anesthesia (TEA) on myocardial repolarization and arrhythmogenicity are only incompletely understood, Thi s is primarily because of the lack of appropriate experimental models. In m ost of the studies performed thus far, TEA was used In anesthetized animals . Baseline anesthesia itself may have modified the effects of TEA. This stu dy investigates right atrial and ventricular repolarization by recording mo nophasic action potentials after TEA in awake dogs, The authors hypothesize d that an antiarrhythmic role of TEA exists, which may be related to a dire ct effect of TEA on myocardial repolarization. Methods: The hypothesis was tested in an in vivo canine model in which atri al and ventricular myocardial action potential duration and refractoriness are recorded by means of monophasic action potential catheters. Results: Thoracic epidural anesthesia significantly increased ventricular m onophasic action potential duration for cycle lengths shorter than 350 ms. Changes in monophasic action potential duration were paralleled by a concom itant prolongation of effective refractory pet-led (ERP) at higher rates so that the ratio of ERP to action potential duration was unaffected. Conclusions: This model helps to study the role of TEA on ventricular repol arization and arrhythmogenicity, Because lengthening of repolarization and prolongation of refractoriness may, in some circumstances, be antiarrhythmi c, TEA may be protective against generation of ventricular arrhythmias medi cated, e.g., by increased sympathetic tone. The results also imply that the beneficial role of TEA might be stronger at the ventricular site as compar ed with the atrium, At atrial sites there was only a trend toward prolongat ion of repolarization even at short cycle lengths.