Effect of Recording Duration on Heart Rate Variability During Transcutaneous Auricular Vagus Nerve Stimulation: A Randomized, Sham-Controlled Crossover Study

The vagus nerve plays a central role in autonomic regulation, and heart rate variability (HRV) is a widely used noninvasive marker of autonomic nervous system activity. Transcutaneous auricular vagus nerve stimulation (taVNS) has increasingly been used in experimental and clinical research as a noninvasive method to modulate vagal activity. Although previous studies suggest that taVNS may influence parasympathetic activity, HRV measurement duration remains an important methodological issue. Standard short-term HRV analysis is commonly based on 5-minute recordings, but ultra-short and longer recordings are also used, and their validity during taVNS has not been sufficiently clarified.

This study is designed to determine whether different HRV recording durations provide comparable and reliable information during taVNS. The study will use a randomized, sham-controlled crossover design. Healthy adult participants will attend two sessions separated by a washout period of 48 to 72 hours. In one session, participants will receive active taVNS; in the other, they will receive sham stimulation. The order of sessions will be randomized.

During each session, HRV data will be collected continuously for 10 minutes using a Polar H10 chest strap. From this single uninterrupted recording, 1-minute, 5-minute, and 10-minute analysis windows will be derived. HRV parameters from time-domain and frequency-domain analyses will be calculated. Blood pressure and pulse will be measured before and after the intervention using an automated upper-arm blood pressure monitor.

Active taVNS will be applied to the cymba conchae region of the ear using commonly reported stimulation parameters. Sham stimulation will be delivered with the same device and similar sensory conditions, but at a non-vagal ear location to minimize physiologic vagal effects. This design is intended to control for nonspecific sensory and expectation-related effects.

The primary objective is to assess whether the standard 5-minute HRV recording during taVNS detects autonomic effects compared with sham stimulation. Secondary objectives are to examine the agreement between 1-minute, 5-minute, and 10-minute HRV measurements and to evaluate whether shorter or longer recordings provide additional methodological value. The relationship between HRV findings and hemodynamic responses, including blood pressure and pulse, will also be explored. This study may help identify the most appropriate HRV recording duration for future taVNS research and contribute to methodological standardization in this field.