Harnessing non‑invasive vagal neuromodulation: HRV biofeedback and SSP for cardiovascular and autonomic regulation (Review)

The autonomic nervous system plays a central role in maintaining physiological homeostasis, with the vagus nerve as a crucial component. Heart rate variability (HRV) is a non-invasive biomarker of vagal tone and autonomic flexibility. A reduced HRV has been associated with cardiovascular diseases, hypertension, inflammation and mental health disorders. The present narrative review discusses two promising non-invasive interventions designed to enhance vagal activity: HRV biofeedback (HRV-B) and the safe and sound protocol (SSP). HRV-B utilizes paced breathing at an individual's resonance frequency combined with real-time HRV monitoring to strengthen baroreflex sensitivity, improve autonomic balance, reduce systemic inflammation and enhance emotional regulation. Extensive research supports the efficacy of HRV-B in improving cardiovascular outcomes, managing hypertension, reducing depressive and anxiety symptoms, and fostering resilience. By contrast, SSP is a relatively newer auditory-based intervention grounded in the polyvagal theory. It targets the ventral vagal complex through filtered music to promote social engagement, emotional regulation, and parasympathetic activation. Preliminary studies suggest SSP may benefit individuals with autism spectrum disorder, post-traumatic stress disorder and cardiovascular disease; however, large-scale controlled trials are needed to validate its clinical efficacy. By bridging cardiovascular, neurological and psychological domains, both HRV-B and SSP offer innovative non-invasive strategies for promoting health and resilience. The review highlights the mechanisms, clinical applications, and outcomes associated with each approach, while also discussing current limitations, such as individual variability, adherence challenges, and the need for further research. Future directions include integrating wearable HRV monitoring, AI-driven adaptive biofeedback, and expanding investigation across diverse populations to optimize clinical protocols. Together, HRV-B and SSP represent promising avenues for enhancing vagal neuromodulation in both preventive and therapeutic settings.