Intermittent vagus nerve stimulation allows preventing the “escape” effect of heart rhythm changes

Background.Vagus nerve stimulation has been proposed for the treatment of a number of diseases. The positive effects of vagus nerve stimulation on ischemic and reperfusion myocardial injury has been tested in experimental models. However, the escape effect of vagus activation on heart rate and the methodology to overcome this effect have not been reported properly. Objective. The purpose of the study is to evaluate parameters of prolonged stimulation that decrease heart rate and allow overcoming the escape effect of vagus nerve activation. Design and methods. We used Wistar rats (n = 9). Cervical section was performed under general anesthesia. Left vagus nerve isolated from adjacent tissue was contacted with custom stimulation electrodes and a custom pulse generator. Blood pressure was measured in the right common carotid artery. Limb electrocardiogram was continuously recorded. First, stimulation parameters repeatedly evoking vagal reaction (decrease in heart rate) without nerve damage were evaluated. Second, parameters of intermittent stimulation that allowed repeat and consistent heart rate decrease were assessed. Results. During experiments, in 5 animals the following parameters leading to sustained 30 ± 20 % heart rate reduction were found: rectangular pulse, 30 Hz, 0,5 ms, 1–2 V (0,6– 0,8 mA). Stimulation with 50 Hz frequency led to nerve damage in 1 case. Stimulation with 20 Hz frequency led to heart rate over-suppression of heart rate and blood pressure. Intermittent nerve stimulation was tested in 4 animals and led to repeated heart rate decrease by 38 ± 15 %. The parameters which helped to avoid escape effect on heart rate change were the following: the length of stimulation episode of 45 s and interruption of stimulation for 15 s. Conclusion. Intermittent electrical stimulation evokes vagal reactions on heart rate and allows overcoming the escape effect of vagal activation.