By separately blocking the parasympathetic and sympathetic nervous systems during slow breathing, a 2018 study in Psychosomatic Medicine conclusively found that observed increases in HRV during slow breathing are primarily due to increased vagal activity.
Effects of diaphragmatic deep breathing exercises on prehypertensive or hypertensive adults: A literature review
Effect of rosary prayer and yoga mantras on autonomic cardiovascular rhythms: comparative study
Acute effects of resonance frequency breathing on cardiovascular regulation
Psychophysiological Responses to Various Slow, Deep Breathing Techniques
Influence of a 30-Day Slow-Paced Breathing Intervention Compared to Social Media Use on Subjective Sleep Quality and Cardiac Vagal Activity
Slow-Paced Breathing: Influence of Inhalation/Exhalation Ratio and of Respiratory Pauses on Cardiac Vagal Activity
Effects of voluntary slow breathing on heart rate and heart rate variability: A systematic review and a meta-analysis (2022)
Benefits from one session of deep and slow breathing on vagal tone and anxiety in young and older adults (2021)
Oxygen-induced impairment in arterial function is corrected by slow breathing in patients with type 1 diabetes (2017)
Heart rate variability biofeedback in chronic disease management: A systematic review (2021)
HRVB (and, hence, slow breathing) is a powerful adjunctive therapy for managing a wide variety of chronic diseases. To be effective, it should be practiced for a minimum of 10 min/day (but probably 20+ for best results—see next section). More broadly, this study supports using slow breathing at around 5-6 breaths per minute in people with chronic diseases.