What was actually reviewed
Zaccaro and colleagues' 2018 systematic review included 15 studies of slow breathing in healthy participants. The authors examined cardiovascular, brain-activity and psychological measures. They reported changes in heart-rate variability and respiratory sinus arrhythmia, alongside findings related to relaxation and emotional experience. The review explored possible mechanisms; it did not establish a universal treatment for anxiety disorders. [1]
Why the breathing pattern changes the number
HRV describes variation in the intervals between heartbeats. Breathing itself influences that variation. A larger oscillation while you deliberately pace your breath can therefore reflect the measurement conditions, not a permanent improvement in your health. The HRV methods literature stresses the importance of the metric, recording duration and context when interpreting values. [2]
Six breaths is not a universal prescription
Five seconds in and five seconds out makes a ten-second cycle: six breaths per minute. A four-second inhale, seven-second hold and eight-second exhale is a nineteen-second cycle: about three breaths per minute. These are not equivalent protocols. Counting correctly does not establish which is useful or appropriate for a particular person. The review does not justify ranking every named breathing practice as the same intervention. [1]
Separate the outcomes
Feeling calmer, changing HRV during a session and improving a diagnosed condition are different outcomes. A study needs to measure the claim it is being used to support. In particular, HRV should not be presented as a direct reading of thoughts, brain activity or a single 'regulated versus dysregulated' state. A graph can be informative without being a diagnosis. [2]
Safety belongs in the explanation
NCCIH describes relaxation practices as generally safe but notes occasional increased anxiety, intrusive thoughts or fear of losing control, with rare worsening in some people with particular conditions. Do not force a breathing exercise through discomfort or use it to delay appropriate care. Discuss practices intended for a health condition with a qualified clinician. This is an evidence explainer, not a prescribed breathing protocol. [3]