Effects of thoracic epidural anesthesia with and without autonomic nervous system 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. This is primarily because of the lack of appropriate experimental models. In most of the studies performed thus far, TEA was used in anesthetized animals. Baseline anesthesia itself may have modified the effects of TEA. This study investigates right atrial and ventricular repolarization by recording monophasic action potentials after TEA in awake dogs. The authors hypothesized that an antiarrhythmic role of TEA exists, which may be related to a direct effect of TEA on myocardial repolarization.
METHODS: The hypothesis was tested in an in vivo canine model, in which atrial and ventricular myocardial action potential duration and refractoriness are recorded by means of monophasic action potential catheters.
RESULTS: Thoracic epidural anesthesia significantly increased ventricular monophasic action potential duration for cycle lengths shorter than 350 ms. Changes in monophasic action potential duration were paralleled by a concomitant prolongation of effective refractory period (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 repolarization and arrhythmogenicity. Because lengthening of repolarization and prolongation of refractoriness may, in some circumstances, be antiarrhythmic, TEA may be protective against generation of ventricular arrhythmias mediated, e.g., by increased sympathetic tone. The results also imply that the beneficial role of TEA might be stronger at the ventricular site as compared with the atrium. At atrial sites there was only a trend toward prolongation of repolarization even at short cycle lengths.