Sensing and stimulation of the vagus nerve for artificial cardiac control

This thesis focuses on sensing cardiovascular signals from the vagus nerve and electrically stimulating the vagus nerve for cardiovascular effects. Sensing cardiovascular signals was attempted on both spontaneous and evoked neural activity. A cardiac-modulated vagus nerve activity pattern was found in spontaneous vagus nerve activity. This pattern may reflect cardiac receptor activity, as it is affected by increased blood pressure and contractility brought about by dobutamine. In evoked vagus nerve responses a component was found that had a long latency, a low stimulation threshold and a high velocity along the nerve. These results led to the hypothesis that the component was an indirect component of a burst of single fibre action potentials, emanating with varying delays from the neural network on the heart. The component was strongly reduced with a tripolar derivation or with the average reference configuration on the centre electrode in a cuff with three rings. It was proposed that this was due to the second order spatial derivative, which reduces a dispersed component when it creates a close to linear field in the cuff. A computer model was used to show that it is possible to record an evoked dispersed component as a result of a burst of single fibre action potentials with varying delays. To further elucidate the origin of the evoked component, two additional experiments were performed in which the recurrent laryngeal nerve was cut. The evoked component disappeared after cutting of the recurrent laryngeal nerve. Arguments are presented for a muscular origin from the larynx or a neuronal origin from the neural network on the heart or the recurrent laryngeal nerve. The physiological origin of the evoked late component remains however as yet unclear. Evoked responses may also originate from the brain stem, but that these are difficult to record. This thesis also described cardiovascular effects of vagus nerve stimulation. A segmented electrode cuff for improving selectivity of vagus nerve stimulation for cardiovascular effects is presented