Square and 4-7-8 breathing are popularly promoted by psychotherapists but have little empirical support. We hypothesized that breathing at 6 breaths per minute (bpm) would improve HRV, reduce blood pressure, and improve mood more than either square or 4-7-8 breathing. We also hypothesized square and 4-7-8 breathing would increase end-tidal CO2 (PETCO2). Eighty-four college students (60% female) had HRV measured at baseline and during 1) square breathing, 2) 4-7-8 breathing, 3) 6 bpm with a 4:6 inhale-to-exhale ratio, and 4) 6 bpm with a 5:5 ratio. We used a 3-lead electrocardiogram to measure HRV and a capnometer to measure PETCO2. We measured mood using the Scale of Positive and Negative Experience (SPANE) after each breathing condition. We used a repeated measures mixed effects model to examine the effects of each breathing technique on HRV, blood pressure, mood, and PETCO2 levels. Breathing at 6 bpm increased HRV measures more than square or 4-7-8 breathing with small to medium effects. None of the breathing conditions resulted in meaningful changes in blood pressure or mood. Unexpectedly, breathing at 6 bpm led to mild over-breathing. While various breathing techniques are used in psychotherapy, their relative efficacy remains unclear. This study found that breathing at 6 bpm is likely more effective at increasing HRV than square or 4-7-8 breathing, albeit with a higher risk of over-breathing. Future studies could examine whether these effects hold over multiple weeks of breathing practice and attend to the effects of different breathing techniques on CO2 levels.
