Investigation of TVNS Administration on Postpartum Mental Health

tVNS-induced changes in postpartum depression symptoms

To evaluate the effect of transcutaneous vagus nerve stimulation (tVNS) on depressive symptoms, as measured by the Edinburgh Postpartum Depression Scale (EPDS), which ranges from 0 to 30, with higher scores indicating more severe depressive symptoms. A standard cut-off score of 12-13 is typically used to indicate probable depressive symptoms. The total EPDS score will be assessed at multiple time points throughout the study, allowing for the comparison of changes in depressive symptom trajectories across conditions (tVNS vs. control). This design aims to capture differences in the development and progression of depressive symptoms between groups over the course of pregnancy and postpartum.

tVNS-induced changes in the daily mood

The study will track and analyze the daily trajectory of mood changes-both positive and negative-from a baseline period prior to the 6-week transcutaneous vagus nerve stimulation (tVNS) intervention and throughout the intervention period, with assessments concluding on the final day of the intervention. Daily mood changes and stability will be measured using the Positive and Negative Affect Schedule (PANAS), administered through app-based ecological momentary assessment (EMA). Each item on the PANAS is rated on a 5-point Likert scale, ranging from 1 ("gar nichts" / "not at all") to 5 ("äußerst" / "extremely"), with higher scores reflecting a greater intensity of the specified mood. Positive affect (PA) scores represent more positive mood states (higher scores indicating better mood), while negative affect (NA) scores indicate the severity of negative mood states (higher scores representing worse mood). The analysis will focus on identifying trends, patterns, and fluctuations in mood

Intervention acceptance analysis

Using a mixed-method approach, including compliance analysis (tracking the frequency and duration of tVNS device use), retention analysis (comparing retention and dropout rates between groups at different time points), and post-intervention questionnaire-based data on participants' satisfaction with the intervention (assessing the burden of the intervention, side effects, and participants' theoretical willingness to recommend it to others).

Heart-rate variability changes

HRV will be assessed through smartwatches, with analyses focusing on tVNS-induced changes compared to controls using RMSSD (Root Mean Square of Successive Differences), SDNN (Standard Deviation of NN intervals), and frequency-domain methods.

Subjective sleep quality changes

Subjective sleep quality will be assessed monthly using the Pittsburgh Sleep Quality Index (PSQI), along with an ecological momentary assessment (EMA) question about sleep quality. Group comparisons will determine whether tVNS induces changes in sleep quality compared to control conditions. In scoring the PSQI, seven component scores are derived, each rated from 0 (no impairment) to 3 (severe impairment). The component scores are summed to produce a global score (range 0 to 21), with higher scores indicating poorer sleep quality.

Objective sleep quality

Sleep quality (duration, depth, and regularity) will be measured objectively using smartwatch data, focusing on tVNS-induced changes compared to the control group. Sleep duration will be measured in hours and minutes, sleep depth will be assessed based on time spent in different sleep stages (light sleep, deep sleep, and REM) in minutes, and sleep regularity will be evaluated by the variability in sleep and wake times, also in hours and minutes. Those submeasures will be further analysed to an overall sleep quality outcome Note: While smartwatches provide estimates, their limitations in accuracy are acknowledged, and subjective data will be triangulated with these measures.