BACKGROUND: This study was designed to assess the effects of rilmenidine on the autonomic nervous system, and to evaluate whether it prevents bupivacaine-induced cardiovascular toxicity during intravenous bupivacaine infusion in anesthetized cats.

METHODS: Thirty male cats were randomly divided into a control group (n = 15) and a rilmenidine group (n = 15). Following the injection of rilmenidine (10µg/kg), systolic blood pressures (SBP) and R-R intervals (RRI) were recorded for 5 minutes. Then power spectral analyses of the SBP and RRI, and transfer function analysis were conducted to evaluate the autonomic nervous system. During the infusion of bupivacaine (0.5 mg/kg/min), blood pressures, heart rates, times to reach each events, and bupivacaine doses were measured at the first QRS modification, the first dysrhythmia, at 25% (HR25) and 50% reductions in baseline heart rate, and at 25% and 50% reductions in baseline mean arterial pressure and at final systole.

RESULTS: The high frequency (HF) power of heart rate variability (HRV) was significantly elevated in the rilmenidine group versus the control group. Magnitude HF was significantly higher in the rilmenidine group than in the control group. The onset of dysrhythmia correlated significantly with the HFs of HRV and baroreflex sensitivity (BRS). Except for HR25, the rilmenidine group showed significantly higher bupivacaine doses and delayed event onsets versus the control group.

CONCLUSIONS: We suggest that pretreatment with rilmenidine delays the onset of dysrhythmia by increasing vagal tone and BRS and by reducing cardiovascular toxicity when bupivacaine is infused continuously to isoflurane anesthetized cats.