Explanation for the choice of comparators

Previous studies have only looked at the effects of TaVNS or TENS independently of each other. Therefore, the proposed research will assess the combined effects of TaVNS and TENS regarding pain modulation.

Intervention description TENS electrode placement

A 5 × 5 cm electrode (ValuTrode®) will be placed 1 cm proximal to the elbow crease and 1 cm proximal to the wrist crease on the dorsum of the left upper limb [13]. These self-adhesive electrodes are commercially available and will be purchased for each subject.

TENS parameters

All subjects in all groups will rest in a supine position for the 30-min treatment period. Area of the forearm will be prepared with an alcohol wipe prior to the application of electrodes. High-frequency TENS (100 Hz) [13] (IBRAMED Neurodyn portable TENS/FES, Brazil) will be applied with a pulse duration of 200 µsec [12, 23, 24, 26], waveform of an asymmetrical biphasic square wave, and intensity of maximal tolerable without pain [13]. TENS will be placed for 30 min after baseline testing and then turned off; the electrodes will remain on for the post-assessment test.

TaVNS electrode placement

Both conchas will be swabbed with an alcohol wipe before applying a thin layer of conductive gel. An ear clip electrode (Plafnio, China) will be placed in the concha of both ears [2, 23, 26, 27].

Active TaVNS

An IBRAMED Neurodyn portable system unit will be used to deliver the active TaVNS intervention. A frequency of 25 Hz [19,20,21] will be applied with a pulse duration of 200 µs [12, 23, 24, 26] at the maximum intensity tolerable without pain. Every 5 min, the subject will be asked if the intensity can be increased, decreased, or remain the same to maintain the same level of intensity.

Placebo TaVNS

For placebo TaVNS (IBRAMED Neurodyn portable system, Brazil), the intensity will be increased to a sensory level and then decreased to 0 mA.

Criteria for discontinuing or modifying allocated interventions

There will be no changes in the treatment allocation order. If participants discontinue treatment, recent data will be computed for the analyses according to the intention-to-treat principle, and the reason for the withdrawal from the study will be recorded.

Strategies to improve adherence to interventions

To minimize data loss, all participants will be guided when they sign the informed consent form and commit to attending on the scheduled treatment dates. An evaluator will be responsible for notifying and monitoring the participants weekly (via telephone contact, text message, and/or email) and accompanying them during the research. In cases of abandonment or impossibility of continuing the study, the data will be analyzed according to an intention-to-treat protocol.

Relevant concomitant care permitted or prohibited during the trial

Throughout the trial, participants will be asked not to start/use any other interventions or pain medications within 48 h of assessments as it may influence outcomes.

Provisions for post-trial care

The interventions are designed to inflict minimal to no harm, and no compensation for harm is deemed necessary.

Outcomes

The primary outcome will be pressure pain threshold (PPT). The secondary outcomes will be heat pain threshold (HPT) and autonomic function including heart rate, oxygen saturation, and blood pressure. These outcomes will be measured four times, once as a baseline testing, once at 15 min, once at 30 min, and once at 45 min (15 min after interventions have concluded) (Fig. 1).

Fig. 1

Timeline for when outcome measures will be taken. There will first be a 30-min adaptation time prior to measuring their baseline Heat Pain Threshold, Pressure Pain Threshold, blood pressure, heart rate, and O2 saturation. Measurements will be retaken after 15 and 30 min of treatment and then once again 15 min after treatment is turned off. Orange is when treatment is off, blue indicates that treatment is on

Pressure Pain Threshold (PPT)

A digital pressure algometer (Medoc AlgoMed FPIX, Israel) will measure the pain threshold to deep mechanical stimuli. The intra-rater and inter-rater reliability of measurement of PPT will be performed in 12 asymptomatic subjects by a single evaluator at 48-h intervals. Reliability will be estimated by calculating the intraclass correlation coefficients (ICCs). A 1-cm2 algometer probe will apply pressure at 40 kPa/sec. Subjects will be instructed to activate a button when the sensation of pressure becomes one of painful pressure, and this value will be recorded. Three trials will be done on each region tested. This method will register the mean PPT of the forearm and anterior tibia region.

Measure on the forearm will be done over one point marked on the left arm (primary outcome) with a permanent marker located 3 cm distal to the elbow crease on the extensor mass [13, 14]. Another point will be marked over the ipsilateral tibialis anterior muscle (secondary outcome) that is 5 cm from the tibial tuberosity [28, 29].

Heat Pain Threshold (HPT)

Superficial heat pain sensitivity (secondary outcome) will be assessed using a handheld thermode (Medoc TSA-II, Israel) with a single 40 × 40 mm probe placed on a marked location on the left hand 5 cm distal to the elbow crease on extensor mass. The baseline temperature will be pre-set to 32 degrees Celsius (C). During testing, the temperature will increase at a rate of 1 °C/s until the participant reports the temperature as painful by pressing an indicator button. Maximum temperature will be pre-set at 50 °C, and if no pain had been elicited by then, this will be recorded as the heat pain threshold (HPT) [30]. Three trials will be done on each region tested with the average of these numbers being recorded.

All QST tests (PPT and HPT) will be performed with the patient in a supine position with their arm resting next to their body on the plinth. Before data collection, a test trial of each pain threshold assessment will be performed on the ventral forearm of the non-tested arm.

Autonomic measures

Heart rate and oxygen saturation will be measured using a pulse oximeter (Zacurate/500DL/ USA) on the right index finger. An automatic blood pressure cuff (Omron/BP5100/Japan) will measure the subject’s blood pressure using the brachial artery on the right arm.

Study blinding assessment

Each subject will receive both treatments: (1) TENS with TaVNS and (2) TENS with placebo TaVNS. The treatment order will be randomized via the website (randomization.com). Simple randomization results will be concealed in sealed envelopes with consecutive numbers. Subjects will be tested one time per week for 2 weeks. Importantly, the PPT and HPT assessors will be blinded to the treatment category. A different tester will administer the active or placebo TaVNS. The HPT and PPT assessors will also be asked if they believe it was placebo TaVNS or active TaVNS, and their responses will also be recorded.

Participant timeline

Demographics of the participants will be subjectively collected during the recruitment process: this includes age, sex, weight, and height. Participants will be randomized to treatment group order after providing written consent. It will be decided if the participant receives TENS with active TaVNS or placebo TaVNS during their first session through a randomization process via the website (randomization.com). Simple randomization results will be concealed in sealed opaque envelopes with consecutive numbers and will not be available to the outcomes assessor. The envelopes will be signed, dated, and opened by the TENS and TaVNS allocator before the TENS and TaVNS application and after the outcomes assessor has left the room.

An overview of the study procedures is shown in Fig. 2.

Fig. 2

Schedule of enrolment, interventions, and assessments

Sample size

The sample size was calculated considering a difference of 100 kPa between sessions and a standard deviation of 117 kPa obtained from previous data on pressure pain threshold (PPT) and TENS [15]. At a significance level of 0.05 and a power of 80%, the required sample size in each group was 23 participants (Minitab, v.17, State College, PA, USA). Allowing for attrition, a total of 30 participants will therefore be recruited: 15 men and 15 women.

Recruitment

Participants will be invited to participate in the study through social media sites (e.g., Facebook and Instagram), paper and digital flyers, websites, and online news sources from the University of Hartford. After selecting the participants, a subjective interview will be conducted to assess the sample eligibility.