Findings
There is a need for a deeper understanding of biomarker dynamics in response to tcVNS for real-time physiological monitoring.
PPG amplitude is identified as a superior biomarker compared to heart rate for predicting responses to tcVNS.
Current digital health technologies may benefit from exploring beyond standardized waveforms for tcVNS applications.

Recommendations
Focus on PPG amplitude as a primary biomarker for real-time tcVNS systems.
Consider latency in clinical monitoring7 and closed-loop system design.
Explore multimodal closed-loop systems utilizing signals other than ECG and PPG.

Regulatory Considerations
Use of FDA-approved devices like gammaCore for tcVNS.
Ensure user safety during stimulation by varying parameters appropriately.