The role of resonance frequency in slow-paced breathing: Systematic review
Authors: Vukojević Božo , Vater Christian , Laborde Sylvain 
Published: February 2024

Introduction
A healthy heart does not beat like a metronome because it shows characteristics of complex non-linear oscillations and mathematical chaos, together with the existence of resonance in the cardiorespiratory system (Shaffer et al., 2014). On one swing, the inhalation causes an increase of the heart rate, while the opposite effect is during exhaling, thus having variability properties in the inter-beat intervals. By utilizing this effect of respiratory sinus arrhythmia, resonance frequency (RF) gives insight into the respiratory rate at which this resonance is amplified in the heart rate, blood pressure, and respiration systems (Shaffer & Meehan, 2020). The aim of this systematic review was to examine the effects of resonance frequency in Slow-Paced Breathing (SPB) studies, as well as the relation between RF and anthropological measures of gender, height, and age.
Methods
We systematically reviewed the literature regarding the role of resonance frequency in the SPB research using the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) evidence-based reporting checklist and a Participants, Index Test, Comparison, Outcomes, and Study Design (PICOS) framework (PROSPERO Registration Number CRD42021253009). We included SPB studies with experimental and quasi-experimental designs, conducted on healthy populations that determined individual RF in the breathing protocols, with no specific criteria on gender, age restrictions, or publication year. Additionally, interventions combining RF SPB with biofeedback monitoring were included. Outcomes of the included studies included any psychological or physiological findings. Altogether, 17 studies (n = 810 participants) were assessed for risk of bias with The Joanna Briggs Institute Critical Appraisal Checklist (Moola et al., 2015).
Results
Overall, positive effects on psychological constructs like attention control, executive functioning, sleep quality, psychological relaxation, mindful awareness, self-compassion, vigor, self-efficacy, mood, depression, anxiety, and stress have been reported. Physiologically, RF breathing caused a more coherent heart rhythm pattern with high amplitude oscillation, while lowering systolic pressure, decreasing HF power, HR beats per minute, a-amylase and cortisol stress biomarkers, and increasing the average baroreflex gain, SpO2, and LF power. The scarcity of reported data and analysis concerning the effects of anthropological data do not give a strong claim to the existence of height, gender, and age influences, but there is a notion that height and RF are negatively correlated, while men have lower RF than women.
Discussion/Conclusion
We conclude the overall beneficial effects of RF breathing on psychological and physiological systems. Namely, improved attention control, executive functioning, sleep quality with fewer disturbances, and psychological relaxation, increased mindful awareness, self-compassion, vigor, and self-efficacy; less worrying and a decrease in total mood disturbance, depression, anxiety, and stress were observed. On a physiological level, RF breathing caused a lowering of systolic pressure, a decrease of HF power, HR beats per minute, a-amylase, and cortisol stress biomarkers, while increasing the average baroreflex gain, SpO2, and LF power through a more coherent heart rhythm pattern with high amplitude oscillation. In regard to the anthropological measures, scarcely reported data suggests the RF is related to the blood volume and the perfusion of the arterial tree, hence men are supposedly having a lower resonance frequency than women, and RF being negatively correlated with height. While these findings are in overall support for RF breathing, they are not ubiquitous, and thus warrants more experimental evidence to test the aforementioned effects. Besides limitations on technical levels and methodological inconsistencies among reviewed studies (e.g., differences in RF determination protocols, operationalization variables and inhale/exhale ratio), one of the major obstacles in the review process was the insufficient gathering and reporting data on some important RF aspects (i.e. no report on detected RF distribution among participants and their anthropological data, control of transient characteristics and lifestyle choices that influence HRV). Finally, future studies should empirically replicate reviewed studies and test the RF biofeedback model in a laboratory and natural setting.
References
Shaffer, F., McCraty, R., & Zerr, C. L. (2014). A healthy heart is not a metronome: An integrative review of the heart’s anatomy and heart rate variability. Frontiers in Psychology, 5, Article 1040. https://doi.org/10.3389/fpsyg.2014.01040
Shaffer, F., & Meehan, Z. M. (2020). A practical guide to resonance frequency assessment for heart rate variability niofeedback. Frontiers in Neuroscience, 14, Article 570400. https://doi.org/10.3389/fnins.2020.570400
Moola, S., Munn, Z., Sears, K., Sfetcu, R., Currie, M., Lisy, K., Tufanaru, C., Qureshi, R., Mattis, P., & Mu, P. (2015). Conducting systematic reviews of association (etiology): The Joanna Briggs Institute’s approach. International Journal of Evidence-Based Healthcare, 13(3), 163–169. https://doi.org/10.1097/XEB.0000000000000064