Heart rate variability
The two-electrode heart rate recorder Firstbeat Bodyguard 2 (Firstbeat Technologies Ltd., Jyväskylä, Finland) was used to monitor heart rate. The device was attached across the chest by a research nurse on the morning the day before the first treatment day. After this visit, the participants wore the device until the next day, also during the night, and returned it when they came back for their first treatment session. The same procedure was repeated the day after the last treatment, 2 weeks after baseline. Sampling rate was 1,000 Hz with a resolution of 1 ms. Since the aim of this study was to assess the potential lasting effects on HRV, an hour of data were extracted during the night (midnight to 5 am). This hour was chosen from a period where heart rate seemed stable, when inspected visually. The chosen hour varied between subjects depending on visual inspection of where there were fewest artifacts. This hour could also vary between each participant’s baseline and follow-up data, but was aimed to be the same. Within the chosen time period, the following HRV metrics were calculated: the standard deviation of the RR interval (SDNN), the root mean square of successive RR interval differences (RMSSD), high frequency HRV (HF), low frequency HRV (LF), LF/HF ratio, and the RR triangular index (In a subset of patients (