Acute Effects of Two Slow-Paced Breathing Protocols With and Without Inspiratory Breath-Holding on Heart Rate Variability: A Randomized Controlled Trial

Background/Objectives: Slow-paced breathing (SPB) is associated with changes in heart rate variability, although these responses are strongly influenced by respiratory patterns; the effects of incorporating an inspiratory breath-hold into an SPB protocol remain insufficiently characterized. This study evaluated the immediate effects of two SPB protocols on heart rate variability (HRV) and cervical pressure pain thresholds (PPT). Methods: A single-blind, randomized, controlled trial was conducted with 60 healthy adults (aged 18–65) stratified by age and sex, and allocated equally (n = 20 per group) to: Protocol 1 (6 s inhalation, 6 s hold, 6 s exhalation), Protocol 2 (6 s inhalation, 6 s exhalation), or a sham control (general respiratory education). Interventions lasted 5 min. Primary outcomes were time-domain HRV indices, root mean square of successive differences (RMSSD) and standard deviation of normal-to-normal (SDNN), recorded using a Polar H10 chest strap. The secondary outcome was PPT at the fourth cervical vertebra (C4) measured via digital algometry. Results: Repeated measures ANOVA revealed a significant time-by-group interaction for heart rate, RMSSD, and SDNN. Both active breathing protocols significantly increased RMSSD and SDNN compared to control, with high effect sizes (η2). Notably, Protocol 1 (with inspiratory breath-holding) produced the largest changes in HRV, RMSSD, and SDNN. Conversely, changes in cervical PPT were modest and reached only borderline significance with small effect sizes. Conclusions: Both SPB protocols were associated with acute increases in time-domain HRV indices. Protocol 1 (combining inspiratory breath-holding and lower respiratory frequency) produced a larger acute autonomic response, suggesting its potential utility in non-pharmacological interventions for autonomic dysregulation. However, the isolated contribution of breath-holding cannot be separated from the lower respiratory frequency inherent to this protocol.: