Thoracic epidural anaesthesia combined with enflurane anaesthesia reduces atrioventricular conduction in dogs

Cardiac electrophysiological variables during thoracic epidural lidocaine (TEL) were compared with those during continuous intravenous lidocaine (IVL) infusion in 14 mongrel dogs anaesthetized with enflurane in order to investigate the combined effects of thoracic epidural anaesthesia (TEA) and enflurane anaesthesia on intracardiac conduction. Thoracic epidural lidocaine suppressed intracardiac conduction. Sinus cycle length (SCL) and Atrium-His (AH) interval increased by 9 and 11 per cent respectively (P less than 0.05), 30 min after TEL. Intravenous lidocaine did not increase either SCL or AH. The functional refractory period of the atrioventricular node increased five per cent above the control value 15 min after TEL (P less than 0.05), while it was unchanged in the IVL group. The mean plasma concentrations of lidocaine ranged from 0.48 +/- 0.07 to 1.00 +/- 0.14 micrograms.ml-1 in the TEL group and from 0.98 +/- 0.13) to 1.21 +/- 0.15 micrograms.ml-1 in the IVL group. There were no significant differences in plasma concentrations of lidocaine in both groups during the observation period. Therefore, it is concluded that the depressant effects of TEA on intracardiac conduction were caused by blocking of the sympathetic efferent activity. Caution may be advised in administering TEA when cardiac conduction is already compromised.