Slow breathing with extended exhalations increases vagal activity more than an equal inhale:exhale ratio. Thus, we can extend our exhales to increase parasympathetic nervous system activity.
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.