The effects of non-invasive neuromodulation on autonomic nervous system function in humans

Neuromodulation, the alteration of nerve activity through the use of targeted electrical stimulation or pharmacology, is a rapidly advancing field with applications in a plethora of conditions e.g. epilepsy. Many of these techniques are invasive and expensive such as vagus nerve stimulation limiting their applicability to patient populations. Autonomic nervous system imbalance is a characteristic of many conditions including heart failure. The ability to favourable alter autonomic function non-invasively would, therefore, offer potential therapeutic benefit to a large number of patients. This thesis investigated the effects of two non-invasive neuromodulatory techniques on cardiovascular autonomic function in humans – transcutaneous vagus nerve stimulation (tVNS) and transcranial direct current stimulation (tDCS).
tVNS was performed using surface electrodes placed on the ear to stimulate the auricular branch of the vagus nerve (ABVN). High pulse width and frequency tVNS was found to alter heart rate variability (HRV) towards parasympathetic predominance in healthy participants (n = 34) and heart failure patients (n = 8). Furthermore, microneurography recordings performed in healthy participants (n = 10) revealed that this effect may have been mediated, at least in part, by a reduction in muscle sympathetic nerve activity (MSNA).
tDCS was performed by placing electrodes over the motor cortex and the contralateral supraorbital region of healthy participants (n = 22). Anodal stimulation (positive electrode over the motor cortex) altered HRV towards sympathetic predominance and increased MSNA whereas cathodal and sham tDCS had no effect.
tVNS and anodal tDCS over the motor cortex had opposite effects on cardiovascular autonomic function. These techniques may be tailored to the needs of individual patients to shift cardiovascular autonomic function towards either parasympathetic (tVNS) or sympathetic (tDCS) predominance. tVNS and tDCS are simple, non-invasive and inexpensive allowing a wide cohort of patients to access these potential therapies.