Abstracts

RATIONALE: Heart period variability (HPV) patterns change, in normal subjects, depending upon sleep stage. We previously found that patients with intractable epilepsy have lower state differentiation of heart period variability patterns than control subjects. Vagal nerve stimulation may alter autonomic control based upon the chronic stimulation of brainstem afferents. METHODS: To evaluate the possible alteration in state dependent HPV, we examined seven patients prior to and 3 months following vagal nerve stimulator implantation. Subjects underwent polysomnography and had ECG data extracted for periods of awake, stage 2, stage 3 & 4 and REM sleep. Following three months of stimulation, the stimulator was set for the sleep study to a duty cycle of maximal tolerated current, signal frequency 30 HZ, pulse width 500 microseconds, on time 150 seconds and off time 5 minutes. HPV power spectra were estimated for pre-implant and post-implant (divided into 2.5 minutes pre-stimulation, stimulation and 2.5 minutes post-stimulation)for each sleep stage. Findings were compared using a two way ANOVA (p<0.05). RESULTS: We found that in both the pre-implant and post-stimulation measures, HPV output was highly state dependent (p=0.005). We also found trends showing that following 3 months of stimulation, average power in the high frequency band (0.18-0.4Hz) increased in Stage 2 and Stage 3 & 4 sleep, and decreased in REM sleep, when compared to pre-implant state. Average power in the low frequency band (0.05-0.15Hz) decreased in Stage 2 and Stage 3 & 4 sleep and increased in REM sleep compared to the pre-implant state. These changes in power were most evident during the actual period of stimulation. However, these changes were also seen in the nonstimulation portions of the duty cycle. CONCLUSIONS: These results show that in patients with intractable epilepsy, HPV patterns change with sleep state, and that these changes trend toward increased state dependence following chronic vagal nerve stimulation. Further study is required to determine the long term effects of vagal stimulation on the autonomic output.