Shoulder Transcutaneous Electric Nerve Stimulation Decelerates Heart Rhythm of Subjects Via Potentiating Vagal Tone

Study design This study complied with the Declaration of Helsinki, and all protocols were approved by the ethics committee of Taoyuan General Hospital, Ministry of Health and Welfare, Taoyuan, Taiwan. All the participants gave written informed consent before experiments. 32 subjects (20-51 years old) were included in the statistical analysis. Participants were eligible if they have cardiovascular illnesses, major mental conditions, or severe inflammation. Participants were allocated to groups that received sham stimulation (TENS-S) and TENS on the right and left shoulder (TENS-R and TENS-L, respectively).

ECG and HR recordings Using a monitor lead, electrocardiogram (ECG) was recorded through electrodes connected to a recording system with a sampling rate of 5,000 samples/sec. The ECG and the HR calculated by a built-in rate meter were continuously recorded and displayed on a monitor. For TENS caused marked artifacts in ECG tracings, HR derived from ECG were confirmed off-line by manual examination.

TENS stimulation To mimicking clinical scenarios, a commercial TENS equipment was used throughout this study. A built-in waveform recommended for treating capsulitis in the user's manual was used for stimulation (triple pulses with 1 ms pulse durations separated by a 1 ms intervals were ramps up and down between 2 to 10 Hz within 20 sec for 3 min and then ramps up from 10 to 200 Hz within 20 sec and kept at 200 Hz for 2 min; supplementary data 1). The current intensity was adjusted to the maximal tolerance level below the pain threshold. A pair of stimulating electrode was placed in the front of the should at the level of the sternal notch at about 2 finger-width from the mid-clavicle line, and a pair of dispersive electrode was placed at the back of shoulder opposite to the stimulating electrodes. Participants kept a stationary supine position and was asked to refrain from moving during recording. Before the stimulation, there was an equilibrium period for at least 10 min, and then the recording started. TENS was turned on for 5 min at 5 min after the baseline recording, and the recording continued until 10 min after the offset of stimulation. Parameters of the sham stimulation was identical to TENS stimulation on the right shoulder excepting the equipment was left un-powered.