Slow breathing using pursed-lip exhales or a resistance device both potentially improve cardiovascular health, but pursed-lips breathing is free, more pleasant, and provides a better sense of control.
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.